
Editor's Choice
On Planckian limit for inelastic relaxation in metals
In recent years, a number of interesting papers appeared [1,2], where from the analysis of experiments on rather wide range of compounds, it was shown that in the $T$  linear region of resistivity growth, the scattering rate of electrons (inverse relaxation time) with rather high accuracy is described as $\Gamma=\frac{1}{\tau}=\alpha \frac{k_BT}{\hbar}$, where $\alpha\sim 1$ and is weakly dependent on the choice of the material. In connection with these results the notion of the universal (independent of interaction strength) "Planckian'' upper limit of inelastic scattering rate in metals was introduced as $\frac{1}{\tau_P}=\Gamma_P=\frac{k_BT}{\hbar}$ [3]. To explain this "universality'' a number of relatively complicated theoretical models were proposed [4, 5], including some rather exotic, based on the analogies taken from the black hole physics, cosmology and superstring theory (e.g. see Refs. [69]). It is shown here that the "Planckian'' limit for the temperature dependent relaxation rate actually follows from a certain procedure used in Refs. [1, 2] to derive $\frac{1}{\tau}$ from experimental data on resistivity, using the effective electron mass, determined from low  temperature experiments. Thus, the "experimentally'' observed universal "Planckian'' relaxation rate in metals, independent of interaction strength, is nothing more than a kind of delusion.
[1] J.A.N. Bruin, H. Sakai, R.S. Perry, A.P. MacKenzie. Science 339, 804 (2013) M.V. Sadovskii JETP Letters 111, issue 4 (2020)
Phase diagrams of iron hydrides at pressures of 100400 GPa and temperatures 05000 K
Discovery of high critical temperatures of superconductivity in sulfur [1, 2], lanthanum, and yttrium hydrides [35] led to the active search for stable structures of hydrides of other elements, including iron. Iron hydrides are characterized by a critical temperature of ~50 K and can conditionally be classified as hightemperature superconductors. On the other hand, hydrogen is considered as one of the possible light elements of the Earth’s and planets core, which causes interest in phase relationships for the FeH system over a wide range of pressures and temperatures. In this work, within the density functional theory, the thermodynamic stability of iron hydrides Fe_{4}H, Fe_{2}H, FeH, Fe_{3}H_{5}, FeH_{2}, FeH_{3}, FeH_{4}, Fe_{3}H_{13}, FeH_{5} and FeH_{6} at temperatures up to 5000 K in the pressure range of 100400 GPa was estimated and the corresponding phase PTdiagrams were calculated. We performed a topological analysis of all stable iron hydrides. The regularity of the formation of dumbbellshaped hydrogen molecules with increasing hydrogen concentration in iron hydrides was established. [1] A. Drozdov, M. Eremets, I. Troyan, V. Ksenofontov, S. Shylin, Nature 525, 73 (2015) [2] D. Duan, Y. Liu, F. Tian, D. Li, X. Huang, Z. Zhao, H. Yu, B. Liu, W. Tian, T. Cui, Scientific Reports 4, 6968 (2014). [3] A. Drozdov, P. Kong, V. Minkov, S. Besedin, M. Kuzovnikov, S. Mozaffari, L. Balicas, F. Balakirev, D. Graf, V. Prakapenka, Nature 569, 528 (2019). [4] H. Liu, I. I. Naumov, R. Hoffmann, N. Ashcroft, R. J. Hemley, Proceedings of The National Academy of Sciences 114, 6990 (2017). [5] M. Somayazulu, M. Ahart, A. K. Mishra, Z. M. Geballe, M. Baldini, Y. Meng, V. V. Struzhkin, R. J. Hemley, Physical Review Letters 122, 027001 (2019). D.N. Sagatova et al. JETP Letters 111, issue 3 (2020)
The role of the chiral phase transition in modelling the kaon to pion ratio
The search of a quarkgluon plasma (QGP), where hadrons dissolve and quarks are supposed to be free and deconfined, is difficult due to the short QGP lifetime. Various signals were proposed for detection of the QGP phase, and the ''horn'', which appears in the ratio of positive charged kaon to pion, was supposed be one of them [1]. Nowdays the picture of this peak becomes more clear on the experimental side: the peak appears in the ratio of positive charged kaons and pions at the collision energy $\sqrt{s_{NN}}\sim$ 710 GeV for the largesize systems in Au+Au and Pb+Pb collisions. With decreasing system size, the sharp peak becomes lower and for Be+Be, p+p collisions the ratio demonstrates smooth behaviour [2].
On the theoretical side, the quick increase in the $K^+/\pi^+$ ratio and its decreasing and flattering with further energy increasing is interpreted as a sequence of the chiral symmetry breaking and subsequent deconfinement effect.
In our works [3, 4], including the present one, we discussed the chiral phase transition, deconfinement transition and inmedium behaviour of the pseudoscalar mesons in the framework of the SU(3) Polyakov loop extended NJL model. Using the model it was shown how $K/\pi$ ratio changes as function of $T/\mu_B$, when T and $\mu_B$ are chosen on the phase diagram along the chiral phase transition curve and discussed in this way how the chiral phase transition can affect to the $K/\pi$ behaviour. Several modifications of the model was considered, including the model with vector interaction, where the situation with the absence of the first order transition region can appear when the vector coupling constant is high enough. We can conclude that the peak appears in the range of low temperatures and high baryon chemical potential (which corresponds to low collision energy). The appearance of the peak is weakly sensitive to the type of phase transition in the high density region, as the replacement of the the first order transition to the soft crossover only leads to a changing in the peak hight. The peak structure is more sensitive to the slope of the phase transition curve at low T and the properties of the matter. For example, the hight of the peak is sensitive to the chemical potential of the strange quark. For the case with the zero strange chemical potential ($\mu_S(\mu_K) = 0$), the $K^+/\pi^+$ ratio shows smooth behaviour, and when the strangeness neutrality is introduced, the $K^+/\pi^+$ ratio does not show a clear peak structure.
1. S. V. Afanasiev et al. (NA49 Collabration), Phys. Rev. C 66, 054902 (2002); C. Alt,et al (NA49 Collaboration) Phys.Rev. C 77, 024903 (2008).
2. A. Aduszkiewicz (NA61/SHINE Collaboration) Nucl. Phys. A 967, 35 (2017). 3. A. V. Friesen, Yu. L. Kalinovsky, V. D. Toneev Phys. Rev. C 99, 045201 (2019). 4. A. V. Friesen, Yu. L. Kalinovsky, V. D. Toneev, PEPAN Letters, 16, 681 (2019).
A. V. Friesen, Yu. L. Kalinovsky, V. D. Toneev
JETP Letters 111, issue 3 (2020)
Electronhole liquid in monolayer heterostructures based on transition metal dichalcogenides
Monolayer films of transition metal dichalcogenides (TMD) (in particular, MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$) can be considered an ideal system for studying a hightemperature electronhole liquid (EHL). The quasitwodimensional nature of electrons and holes defines a stronger interaction compared to bulk semiconductors. Screening of the Coulomb interaction in monolayer heterostructures is significantly weakened, because it is determined by permittivity of the environment (e.g., vacuum and substrate), which are much smaller than that of TMD films. The multivalley structure of the charge carriers energy spectrum in TMD many times reduces the kinetic energy. This leads to increase in the equilibrium density and binding energy of EHL. The optical properties of the monomolecular TMD layers are generally determined by excitons and trions. The binding energy of the exciton $E_x$ in the TMD is hundreds of meV. For example, in the monolayers MoS$_2$ $E_x=420$ meV [1]. The binding energy of EHL on one electronhole pair is $\leftE_\text{EHL}\right\sim E_x$, and the critical temperature for the gasliquid phase transition is $T_c\sim0.1\leftE_\text {EHL}\right$ [24]. So, we can expect that EHL will be observed in TMD monolayers even at room temperature. A hightemperature strongly bound EHL with $T_c\simeq500$ K was already observed in the MoS$_2$ monolayers [5]. In this paper, we are theoretically investigating the possibility of the formation of EHL in monolayers of multivalley semiconductors. We consider a thin film of a model multivalley semiconductor on an insulator substrate in vacuum. The semiconductor has a large identical number of equivalent electron $\nu_e$ and hole $\nu_h$ valleys $\nu_e=\nu_h=\nu\gg1$. A large number of valleys can be achieved due to the presence of several monomolecular layers in the film. We found analytically the binding energy of EHL and its equilibrium density and compared the results of calculations with experimental values. [1] Y. Yu, Y. Yu, Y. Cai, W. Li, A. Gurarslan, H. Peelaers, D.E. Aspnes, C.G. Van de Walle, N.\,V. Nguyen, Y.W. Zhang, and L. Cao, Sci. Rep.5, 16996 (2015). [2] E.A. Andryushin, V.S. Babichenko, L.V. Keldysh, T.A. Onishchenko, and A.P. Silin, JETP Lett. 24, 185 (1976). [3] E.A. Andryushin, L.V. Keldysh, and A.P. Silin, JETP 46, 616 (1977). [4] ElectronHole Droplets in Semiconductors ed. C.D. Jeffries and L.V. Keldysh (Amsterdam: NorthHollalnd, 1983). [5] Y. Yu, A.W. Bataller, R. Younts, Y. Yu, G. Li, A.A. Puretzky, D.B. Geohegan, K. Gundogdu, and L.Cao, ACS Nano 13, 10351 (2019). P.L. Pekh, P.V. Ratnikov, and A.P. Silin JETP Letters111, issue 2 (2020)
Nonlinearly enhanced linear absorption of midinfrared pulse undergoing filamentation in high pressure gases
Ten years after recognition of the Nobel Prize, the chirped pulse amplification technique, was first implemented [1] and the unique regime of longrange femtosecond pulse propagation was discovered [2]. This propagation regime without beam divergence, or filamentation, was studied with Ti:Sapphire laser systems centered at ~800 nm with pulse peak power of 10^{10}–10^{13} W [3]. Ultrashort pulse filamentation is accompanied by supercontinuum conical emission [4]. The atmospheric transparency window [5] in the visible range ensures lossless propagation of supercontinuum blue wing in the course of backward propagation after reflection from the cloud [6]. However, the fingerprints of atmospheric molecular pollutants are in the mid and farinfrared range [5]. Besides, the critical power for selffocusing is proportional to the squared wavelength and achieves several hundreds of gigawatts for midinfrared pulse propagating in air. This requires the pulse energy of at least several tens of milliJoules (pulse duration of about 100 fs) to form a filament on an atmospheric path. In order to target the application of femtosecond lidar in the midinfrared part of the spectrum, we suggested the generalized approach for identification of the optimum laser wavelength for supercontinuum remote sensing applications [7,8]. We also developed the gas cell [9] for pressures 10^{–3}–120 bar and temperatures up to 150°C to reach the filamentation with submilliJoule pulses. Our long cell of 75cm length provides the filamentation in highpressure gas in the quasicollimated geometry close to atmospheric path experiments. The gas dispersion in the cell can be continuously tuned from normal to anomalous in the vicinity of water absorption band at 1.35 mm. The reservoir with water is installed into the gas cell and is additionally heated. In our experiments the cell was filled with nitrogen (30 bar) and water vapor (200 Pa). The laser pulses of ~100mJ energy and 1.3mm central wavelength propagate in the cell. The nonlinearly enhanced linear absorption was revealed in the longwavelength part of the supercontinuum spectrum; this observation confirmed the theoretical prediction [7] of launching the pulse on the red (longwavelength) side of the absorption line to ensure the maximum transmission through gases.
[1] D. Strickland and G. Mourou, Opt. Commun. 55, 447 (1985). [2] A. Braun, G. Korn, X. Liu, D. Du, J. Squier, and G. Mourou, Opt. Lett. 20, 73 (1995). [3] S. L. Chin, S. A. Hosseini, W. Liu, Q. Luo, F. Théberge, N. Aközbek, A. Becker, V. P. Kandidov, O. G. Kosareva, and H. Schroeder, Can. J. Phys. 83, 863 (2005). [4] O. G. Kosareva, V. P. Kandidov, A. Brodeur, C. Y.Chien, and S. L. Chin, Optics letters 22, 1332 (1997). [5] L. Rothman et al., J. Quantum Spectrosc. Radiat. Transfer 130, 4 (2013), HITRAN2012 special issue. [6] J. Kasparian et al., Science 301, 61 (2003). [7] N. A. Panov, D. E. Shipilo, V. A. Andreeva, O. G. Kosareva, A. M. Saletsky, H. Xu, and P. Polynkin Phys. Rev. A 94, 041801 (2016). [8] N. A. Panov, D. E. Shipilo, A. M. Saletsky, W. Liu, P. G. Polynkin, and O. G. Kosareva Phys. Rev. A 100, 023832 (2019). [9] V. O. Kompanets, D. E. Shipilo, I. A. Nikolaeva, N. A. Panov, O. G. Kosareva, S. V. Chekalin “Nonlinear enhancement of resonant absorption under filamentation of midinfrared laser pulse in highpressure gas” JETP Lett. accepted for publication, December 2019. V. O. Kompanets, D. E. Shipilo, I. A. Nikolaeva, N. A. Panov, O. G. Kosareva, S. V. Chekalin JETP Letters 111, issue 1 (2020)
Nonquadratic transverse magnetoresistance in the nodalline Dirac semimetal InBi
Quasiparticles with the Dirac spectrum arise in a number of materials. Wellknown examples are graphene, topological insulators, Dirac semimetals. More recently, it has been found that there are also materials in which the vertices of the Dirac cone are not at one or more points of the Brillouin zone, but form a line [1]. A feature of nodalline Dirac semimetals is the much higher density of Dirac states than in materials with Dirac points, which allows us to hope for a more vivid manifestation of the properties due to Dirac fermions.
R(B) = R_{0}+R_{1}(1+η^{2}B^{2})^{1/2}+bB^{2},
[1] A. A. Burkov, M. D. Hook, and L. Balents, Phys. Rev. B 84, 235126 (2011).
S.V. ZaitsevZotov and I.A. Cohn Manifestation of spin superfluidity at room temperature
Superfluid 3He is a wellknown condensed matter whose properties are described by quantum field theory. Upon transition to superfluid states, gauge and spin and orbital rotational symmetries are violated simultaneously, demonstrating the properties of antiferromagnetic superfluid liquid crystals. In these systems, spin superfluidity was discovered  quantum transfer of spins controlled by the gradient of the magnetization precession phase. Spin supercurrents provide coherence during the magnetization precession: the precession becomes coherent even in a strongly inhomogeneous magnetic field. This leads to a longlived signal of free induction, which was observed experimentally, see Review [1]. An even more complex interaction between the spin and orbital degrees of freedom leads to the formation of an extremely long live signal, which was explained in terms of the Coleman Qball model [2]. For a long time, magnetic resonance in solidstate magnets was considered in the limit of small perturbations, which corresponds to a low concentration of no equilibrium magnons. However, at high concentrations, magnons can experience Bose condensation, as in superfluid 3He. Moreover, in the case of a repulsive interaction, magnons can form a superfluid state and exhibit spin superfluidity properties in a solid magnets [3]. In particular, manifestations of a superfluid spin state in yttrium iron garnet (YIG) at room temperature have recently been discovered [4]. This article presents the results of observations of a very longlived induction decay signal obtained in a YIG at room temperature. Its properties are partially similar to the Qball observed in superfluid 3He. Nevertheless, there are some fundamental differences with the Qball, which require the correct theoretical explanation. The formation of this longlived signal can be a manifestation of quantum field theory at room temperature. [1]. Yu. M. Bunkov, G. E. Volovik “Spin superfluidity and magnon BEC” Chapter IV of the book "Novel Superfluids", eds. K. H. Bennemann and J. B. Ketterson, Oxford University press, (2013) . [2]. S. Autti, Yu. M. Bunkov, V. B. Eltsov, et al. “Selftrapping of magnon BoseEinstein condensates in the ground and excited levels: from harmonic to a box confinement” Phys. Rev. Lett. 108, 145303 (2012). [3]. Yu. M. Bunkov, E. M. Alakshin,2 R. R. Gazizulin, et al., “HighTc Spin Superfluidity in Antiferromagnets” Phys. Rev. Lett. 108, 177002 (2012). [4]. Yu. M. Bunkov, A.Farhutdinov A. N. Kuzmichev, et al., “The magnonic superfluid droplet at room temperature” https://arxiv.org/pdf/1911.03708.pdf
Yu.M.Bunkov, P.M.Vetoshko, A.N.Kuzmichev, G.V.Mamin. S.B.Orlinsky, T.R.Safin, V.I.Belotelov, M.S.Tagirov. JETP Letters 111, issue 1 (2020)
Direct measurements of spin subsystem picosecond heating kinetics in diluted magnetic semiconductor nanostructures
In recent years, a rapidly developing field of science and technology  spintronics  has attracted much attention. New principles for the operation of devices have been proposed, in which the electronic spin is used along with its charge to transmit and process information. The main tasks of semiconductor spintronics are the investigations of the carrier spins injection, orientation, accumulation and detection processes and the study of the possibilities of controlling them by optical and electrical methods. Diluted magnetic semiconductors and nanostructures based on II – VI materials with manganese ions are considered as model objects for possible applications in spintronics. In such structures, magnetic Mn^{2+} ions isoelectronically replace metal ions in cationic sublattices. The lowtemperature spectra of magnetooptical photoluminescence provide quantitative information on the temperature and magnetization of the Mn ions subsystem. Indeed, the exciton luminescence line shift in external magnetic fields is directly proportional to the magnetization, which makes it possible to experimentally implement the internal thermometer of the magnetic ions spin temperature, since temperature increase leads to a decrease in the Zeeman shift of the emission band. Measurements of the lowtemperature exciton luminescence spectra with time resolution in external magnetic fields also allow one to study the dynamics of changes in the spin subsystem magnetization and temperature of diluted magnetic semiconductor structures when nonequilibrium magnetization is created in them, for example, using highpower pulsed optical pumping [1]. To determine the real interaction time of carriers with magnetic ions, it is very important to study diluted magnetic semiconductor superlattices with type II band alignment. In such structures based on (Zn,Mn)Se/(Be,Mn)Te the type II band alignment makes it possible to experimentally change the interaction time of photoexcited carriers with magnetic ions. At high levels of optical excitation inside ZnSe/BeTe superlattices, due to the high concentration of spatially separated charges of electrons and holes, strong electric fields arise, which in turn lead to strong band bending [2]. Strong band bending leads to the formation of metastable abovebarrier hole states [3], which increases the hole lifetimes in the ZnSe layer. In the present paper the magnetization kinetics in diluted magnetic semiconductor type II superlattices Zn_{0.99}Mn_{0.01}Se/Be_{0.93}Mn_{0.07}Te in external magnetic fields was studied using an optical technique with a high temporal resolution ~ 2 ps. For the first time, direct measurements of the picosecond kinetics of the process of energy and spin transfer from photoexcited carriers due to the exchange interaction with the localized spins of Mn^{2+} ions were performed and the energy and spin transfer time τ ≈ 17 ± 2 ps was determined. [1] M.K. Kneip, D.R. Yakovlev, M. Bayer, A.A. Maksimov, I.I. Tartakovskii, D. Keller, W. Ossau, L.W. Molenkamp, and A. Waag, Phys. Rev. B 73, 035306 (2006). [2] S.V. Zaitsev, V.D. Kulakovskii, A.A. Maksimov, D.A. Pronin, I.I. Tartakovskii, N.A. Gippius, M.Th. Litz, F. Fisher, A. Waag, D. R. Yakovlev, W. Ossau, and G. Landwehr, JETP Lett. 66, No. 5, 376381 (1997). [3] A.A. Maksimov, S.V. Zaitsev, E.V. Filatov, A.V. Larionov, I.I. Tartakovskii, D.R. Yakovlev, and A. Waag, JETP Lett. 88, No. 8, 511–514 (2008). A.A. Maksimov, E.V. Filatov, I.I. Tartakovskii, D.R. Yakovlev, A. Waag JETP Letters 110, issue 12 (2019)
Optical Properties of $p_x + ip_y$ Superconductors with Strong Impurities
Observation of the polar Kerr effect in $\mathrm{Sr_2RuO_4}$ [1], a layered material considered to realize the chiral $p_x+ip_y$ superconducting state, has lead to extensive theoretical investigations of the anomalous Hall response $\sigma_{xy}(\omega)$ in $p_x+ip_y$ superconductors. These studies consider either multiband superconductor models or effects of potential disorder caused by weak impurities.
[1] J. Xia, Y. Maeno, P. T. Beyersdorf, M. M. Fejer, and A. Kapitulnik, Phys. Rev. Lett. 97, 167002 (2006). Ioselevich P.A., Ostrovsky P.M. JETP Letters 110, issue 12 (2019)
Stimulated Raman scattering in metaldielectric ENZ nanocomposites
Most materials found in nature exhibit negligible nonlinear optical behaviors. To observe them, it is necessary to increase the interaction length (for example, using optical fibers) and/or to amplify the pump intensity with highpowered pulse lasers. It means that the thirdorder nonlinear optical processes, for example, stimulated Raman scattering (SRS), optical Kerr effect, to name a few, do not appear within highly confined media or from single molecules exposed to continuouswave lowpowered laser light. Nonlinear enhancement of light becomes possible due to giant local electric fields and/or changes in higherorder nonlinear susceptibility. The nonlinear optical effects were found to occur in plasmonic and/or epsilonnearzero (ENZ) materials [14]. In paper [5], the authors, for the first time, have succeeded to synthesize a metaldielectric nanocomposite exhibiting the 2ENZ behavior in the visible and nearinfrared region. In such a medium, multiple plasmon resonances at different wavelengths are available. In this paper, we study SRS effects using a percolated 50 nm titanium oxynitride (TiON) thin film that exhibits the 2ENZ behavior in the visible and nearinfrared region. This film was fabricated using dc reactive magnetron sputtering in an argonnitrogen environment at elevated temperature and postoxidation in air. In order to enhance the SRS effect we have patterned the TiON thin film by making squareshaped planar nanoantennas with focused ion beam milling. Using tipenhanced Raman scattering, we have proved that this nanocomposite film can be represented as the mixture of metallic TiN and dielectric TiO_{2 }nanoparticles. The underlying mechanism to observe the SRS is linked to the enhanced effective thirdorder susceptibility due to plasmon resonances at the ENZ wavelengths. Earlier, we have experimentally demonstrated a farfield Raman color superlensing effect by showing a subwavelength resolution of l/6NA (l is the excitation wavelength, NA  numerical aperture) at different SRS overtones using multiwalled carbon nanotubes of 40 nm in diameter directly dispersed on the TiON thin film [6]. This allows one to use this material for developing a multiresonant metalens pushing a spatial resolution beyond the diffraction limit without postrecovery. The metalens serves as a SERS substrate that not only enhances a scattered light but provides the subwavelength resolution. The metaldielectric 2ENZ nanocomposite film can be used as a broadband perfect absorber for thermophotovoltaic cells. [1] Reshef O., De Leon I., Alam M. Z., Boyd R. W. Nat. Rev. Mater. 4, 535 (2019). [2] Caspani, Kaipurath R. P. M., Clerici M.,et al., PRL 116, 233901 (2016) [3] Kharintsev S.S., Kharitonov A.V., Saikin S.K., Alekseev A.M., Kazarian S. G. Nano Lett. 17, 5533 (2017). [4] Kharintsev S.S., Kharitonov A.V., Alekseev A.M., Kazarian S. G. Nanoscale 11, 7710 (2019). [5] Braic L., Vasilantonakis N., Mihai A.,et al., ACS Appl. Mater. Interfaces 9, 29857 (2017). [6] Kharintsev S.S. Opt. Lett. 44 (24), 59095912 (2019).
Tyugaev M.D., Kharitinov A.V., Gazizov A.R., Fishman A.I., Salakhov M.Kh., Dedkova A.A., Alekseev A.M., Shelaev A.V., Kharintsev S.S. JETP Letters 110, issue 12 (2019)
Thermal NiehYan anomaly in topological Weyl materials
In 1982 Nieh and Yan introduced the quantum gravitational anomaly caused by the gravitational torsion field [1, 2]. Since that time the torsional anomaly has been debated, because the coefficient in the NiehYan anomaly term contains the ultraviolet energy cutoff, which is not well defined. In this paper we discuss the temperature correction to the NiehYan anomaly. As distinct from the zero temperature term, the $T^2$ temperature correction does not depend on the ultraviolet cutoff and thus can be universal. Such $T^2$ NiehYan term may exist not only in the relativistic quantum field theories, but also in condensed matter with Weyl fermions. In the topological Weyl semimetals and in the chiral $p+ip$ superfluids and superconductors, this term is fully determined by the quasirelativistic physics in the vicinity of the Weyl nodes. [1] H. T. Nieh and M. L. Yan, J. Math. Phys. 23, 373 (1982). [2] H. T. Nieh and M. L. Yan, Ann. Phys.138, 237 (1982). Nissinen J., Volovik G.E. JETP Letters 110, issue 12 (2019) Superfluid $^3He$ in squeezed nematic aerogel
Nematic aerogels consist of nearly parallel strands. In liquid ^{3}He in such aerogels, the strands lead to anisotropy of ^{3}He quasiparticles scattering that makes favorable new superfluid phases: polar, polardistorted A (PdA) and polardistorted B [1]. A distinctive feature of this work is that experiments were performed with ^{3}He in two samples of nematic aerogel one of which was squeezed by 30% in the direction transverse to the strands. The squeezing leads to anisotropy in a plane perpendicular to the strands that can affect superfluid phases. It was found that the superfluid transition of ^{3}He in both samples occurred into the nonchiral polar phase, where no qualitative difference between properties of nuclear magnetic resonance in ^{3}He in these samples was found. The difference, however, has appeared on further cooling, after a transition to the chiral PdA phase. The results agree with theoretical expectations and provide an additional proof of existence of the polar phase of ^{3}He in nematic aerogels. The obtained quantitative characteristics of the observed phases also agree with recent theoretical paper [2] where it was stated that Anderson theorem for swave superconductors is applicable to superfluid ^{3}He in ideal nematic aerogel.
[1] V.V. Dmitriev, A.A. Senin, A.A. Soldatov, and A.N. Yudin, Phys. Rev. Lett. 115, 165304 (2015). [2] I.A. Fomin, JETP 127, 933 (2018). V.V. Dmitriev, M.S. Kutuzov, A.A. Soldatov, A.N. Yudin JETP Letters 110, issue 11 (2019)
StoneWales graphane: structure, properties and its thermal stability
After the discovery of graphene with its unique mechanical and electronic characteristics, a number of other quasitwodimensional carbon structures were theoretically predicted, including octagraphene [1], pentagraphene [2], ψgraphene [3], StoneWales (SW) graphene [4], as well as their various hydrogenated versions (graphane [5], pentagraphane [6], ψgraphane [7] etc.). In this paper, SW graphane  a new allotropic modification of graphane is proposed. This quasitwodimensional structure is formed upon complete twoside hydrogenation of SW graphene. SW graphene is more thermodynamically stable than most other allotropic modification of carbon. This justifies possibility of the SW graphane formation. Unlike graphane, SW graphane is an anisotropic and soft material. Depending on the direction, its Young's modulus is 194  221 N/m, whereas in isotropic graphane it is 249 N/m. The density of phonon states in SW graphane differs from that in graphane. There are no sharp peaks in the density of phonon states of SW graphane, which are typical for graphane. The densities of electronic states in SW graphane and pristine graphane slightly differ from each other. As well, as for graphane, the main channel of thermal decomposition of SW graphane is the separation of atomic hydrogen. The desorption energies of hydrogen atoms for graphane and SW graphane are also very close. 1. X.L. Sheng, H.J. Cui, et al., J. Appl. Phys. 112, 074315 (2012). 2. S. Zhang, J. Zhou, et al., Proc. Natl. Acad. Sci. U.S.A. 112, 2372 (2015). 3. X. Li, Q. Wang, P. Jena, The J. of Phys. Chem. Lett. 8, 3234 (2017). 4. H. Yin, X. Shi, et al., Phys. Rev. B 99, 041405 (2019). 5. J. O. Sofo, A. S. Chaudhari, and G. D. Barber, Phys.Rev. B 75, 153401 (2007). 6. H. Einollahzadeh, et al., Sci. Technol. Adv. Mater. 17, 610 (2017). 7. X. Huang, M. Ma, L. Cheng, and L. Liu Physica E 115, 113701(2020).
Podlivaev A.I. JETP Letters 110, issue 10 (2019)
Relaxation times and population inversion of the excited states of As donors in germanium
At present the interest to Coulomb impurity centers in semiconductors, particularly in silicon and germanium, is revived due to their natural zerodimensional origin . The specific properties of such centers and advancement in modern technology allow one to create, a qubit with optically controlled coherent states [1], or a source of the THz coherent radiation which utilizes the conventional laser scheme or stimulated Raman scattering [2]. Such applications require accurate knowledge of optical excitation and relaxation processes within an impurity center.
In weakly and moderately doped semiconductors, the lifetime of excited states for a shallow impurity center is controlled by phononassisted relaxation. Recently [3], the relaxation times for arsenic donor states in bulk germanium have been calculated; these values are encouraging and suggest that the population inversion and THz lasing can be realized under The present work is devoted to studying the lowtemperature relaxation of the excited states of As donors in Ge crystal using a pumpprobe technique. We show that the lifetime of lower odd parity 2p states are close to one ns. At the same time, experimental study of the inverse relaxation rate for the first excited state 1s(T_{2}) yields value not longer than 160 ps. The data obtained are compared with the results of theoretical calculations [3] and confirm the possibility to reach THz amplification on the 2p – 1s(T_{2}) transitions of optically excited As donors in Ge.
Zhukavin R. Kh., Kovalevskii K.A., Choporova Yu. Yu. et al. (Collaboration) JETP Letters 110, issue 10 (2019)
Observation of Electron Spin Resonance in the Microwave Induced Photovoltage
Electron spin resonance (ESR) is one of the most fruitful approaches for the exploration of spin physics in a great deal of different materials including twodimensional electron systems (2DES) confined in semiconductor heterostructures [1]. The conventional technique for the observation of spin resonance in a 2DES relies on the high sensitivity of a 2D electron channel resistance to the absorption of microwave radiation in the regime of integer quantum Hall effect. In the presented manuscript we propose the complementary experimental approach for the ESR detection as a sharp peak in the microwave induced photovoltage measured between the ohmic contacts to the 2DES. In the presented manuscript we have demonstrated that the suggested experimental approach works well in different semiconductor heterostructures and in various contact geometries. Detection of ESR in such a way requires no current flow through the sample, thereby protecting 2DES from potential overheating, and from resulting negative impact on subtle physical phenomena like highorder fractional quantum Hall effect [3]. Furthermore, the flow of nonequilibrium charge carriers that is responsible for the generated voltage is at least partly spin polarized, as spin dephasing time in the quantum Hall regime [4] exceeds the transport scattering time. [1] M. Dobers, K. v. Klitzing, and G. Weimann, Phys. Rev. B 38, 5453 (1988). [2] D. Stein, K.v. Klitzing and G. Weimann, Phys. Rev. Lett. 51, 130 (1983). [3] R. Willett, J. P. Eisenstein, H. L. Stoermer, D. C. Tsui, A. C. Gossard, and J. H. English Phys. Rev. Lett. 59, 1776 (1987) [4] A. V. Shchepetilnikov, Y. A. Nefyodov, and I. V. Kukushkin, JETP Lett. 97, 574 (2013).
Destructive quantum interference and exceptional points in highfrequency response of twostate system
Periodic driving transforms the stationary energy spectrum into the Floquet modes spectrum (quasienergies). This can be associated with the socalled synthetic dimension introduced by the Floquet modes [1, 2]. Perturbation frequency in this case becomes an additional degree of freedom, which opens new ways of manipulating the quantum systems spectrum. In this context, periodic driving can introduce phenomena, which are typical for higher dimensional systems, in lower dimensional samples. In a finite system, periodic driving can effectively change its topology (connectivity of tunneling paths). In present letter, we study interference features in the highfrequency conductance of a twostate model system within the Keldysh formalism for nonequilibrium Green functions in tightbinding basis. We provide a clear and illustrative correspondence between highfrequency response and stationary transmittance of spatially symmetric configurations of the model system considered. In particular, we show that the synthetic frequency dimension provides the possibility for effective degeneracy of eigenstates in a simply connected linear quantum conductor, which is impossible in statics. It turns to be the dynamical counterpart of the situation considered in [3] for stationary tunneling. In dynamical transport, this phenomenon manifests itself by the destructive quantum interference and resonance coalescence, described by an exceptional point of a generalized transmission coefficient. As a result, for instance, one can observe a dip in the real part of the conductance at resonant frequency. [1] E. Lustig, S.Weimann, Y. Plotnik et al. Nature 567, 356 (2019). [2] L.Yuan, Q. Lin, M. Xiao et al. Optica 5 (11), 1396 (2018). [3] A. A. Gorbatsevich, G.Ya. Krasnikov, and N. M. Shubin. Scientific Reports 8, 15780 (2018). Gorbatsevich A.A., Shubin N.M. JETP Letters 110, issue 9 (2019)
Electron spectrum and optical properties of transition metal dichalcogenides quantum wires
Specific features of the band structure of transition metal dichalcogenides (TMDCs) monolayers — the presence of two valleys, strong spin – orbit interaction — have recently become the subject of a large number of theoretical and experimental studies. Relatively few papers are available on the spatially inhomogeneous problems with TMDCs – quantum dots and quantum wires (QW). In the present work we consider a QW made of TMDCs monolayer in the form of straight strip. Our analysis is based on the Diractype Hamiltonian with the finite gap and with accounting for the spin splitting both conduction and valence bands [1, 2, 3]. We use the boundary condition for the electron wave function proposed in [4] which is a special case of the more general consideration given in [5]. Our main findings are: 1. There exists a certain critical value of the strip width L= L_{cr} that separates two types of the electron spectrum: for L> L_{cr} there are energy levels (subbands in which energy depends on the momentum along the wire) lying within the band gap of an infinite sample, while at L < L_{cr} such states are absent. Note, that in conventional QW for particles with parabolic dispersion law there are no states in the forbidden gap for any value of width. 2. The optical absorption of the QWs in question differs essentially from the one in conventional QWs. First of all, for the interband transitions there is no strict selection rule Dn=0 where n is the number of the transversal subbands in the valence band and the conduction band (cf. with conventional QWs where only interband transitions at Dn=0 are allowed). However in our case the transitions with Dn=0 are still much more intensive than others. Second, depending on the mutual parity of the numbers of size quantization subbands in the valence and conduction bands, optical transitions are characterized by significantly different threshold behavior of the absorption intensity. Namely, for the transitions even – even or odd – odd types the threshold dependence of the absorption is I µ (ww_{0})^{1/2} while for even – odd and odd – even cases we obtain I µ (ww_{0})^{1/2}.
[1] D.Xiao et al., Phys.Rev.Lett., 108, 196802 (2012). [2] A.Kormanyos et al., 2D Materials, 2, 022001 (2015). [3] V.V.Enaldiev, Phys.Rev.B, 96, 235429 (2017). [4] M.V.Berry and R.J.Mondragon, Proc.R.Soc.Lond., A412, 53 (1987). [5] V. A. Volkov and T. N. Pinsker, Sov. Phys. Solid State 23, 1022 (1981).
R.Z.Vitlina, L.I.Magarill, A.V.Chaplik JETP Letters 110, issue 8 (2019)
Mossbauer spectroscopy study of the superconducting single crystals $FeSe_{0.91}S_{0.09}$
The discovery of superconductivity in ironbased pnictides and chalcogenides with a relatively high transition temperature has attracted considerable interest due to the unusual correlations between magnetism and superconductivity in these compounds [13]. Several theoretical models of superconductivity based on pair interactions associated with magnetic fluctuations have been proposed [37]. Much attention is paid to studying the interaction of superconductivity, nematicity of the electronic structure, and quantum paramagnetism in FeSe and FeSe_{1x}S_{x} compounds [8,9]. The coexistence of ferromagnetism and superconductivity in FeSe crystals doped with Bi_{2}Se_{3} was reported recently [10]. In the present work, the method of Mössbauer spectroscopy on ^{57}Fe nuclei was used to study magnetic correlations and possible structural and electronic transformations that are expected in the temperature range of nematic and superconducting transitions in single crystals of iron selenide doped with sulfur Fe (Se_{0.91 ± 0.01}S_{0.09 ± 0.01})_{1}_{δ}. It was found that at room temperature, FeSe_{0.91}S_{0.09 }samples have a tetragonal βFeSe structure of the PbO type (space group P4/mmm), which transforms into the orthorhombic phase when the crystal is cooled down to Ts ≈ 80 K. The temperature of the superconducting transition is ôÓ = 10.1 ë. The temperature dependence of the hyperfine interaction parameters obtained from the Mössbauer spectra revealed a number of anomalies in the temperature range of the superconducting Tc, structural Ts, and nematic T* phase transitions. It was established that iron atoms are in a nonmagnetic state even in the region of helium temperatures, which is explained by the lowspin state of Fe^{2+} ions (3d^{6}, S = 0). It is shown that this state practically does not change at temperatures of transition to the superconducting state. This means that the lowspin state of iron ions is more likely a structural factor, and is not directly related to superconductivity. Thus, there is no effect of the suppression of magnetism by superconductivity. The electrical resistance and Mössbauer spectroscopy data show that in the Fe(Se_{0.91}S_{0.09})_{1}_{δ} crystal, the temperature of the nematic transition T* is about 200 K and is much higher than the temperature of the structural transition (Ts ≈ 80 K). The Debye temperature, obtained from Mössbauer data for the iron sublattice, is Θ_{M} = 478 K, which turned out to be much higher than in the undoped FeSe_{1}_{δ} compound (Θ_{M} = 285 K).
[1] Y. Kamihara, T. Watanabe, M. Hirano, and H. Hosono, J. Am. Chem. Soc. 130, (2008) 3296. [2] X. H. Chen, T. Wu, G. Wu, R. H. Liu, H. Chen, and D. F. Fang, Nature 453, (2008) 761. [3] M.V Sadovskii. PhysicsUspekhi 59(10), (2016) 947. [4] Y. Mizuguchi, Y. Hara, K. Deguchi, et al., Supercond. Sci. Technol. 23, (2010) 054013. [5] J. Paglione and R. L. Greene, Nat. Phys. 6 (2010) 645. [6] V.A. Gasparov, JETP 111(2), (2010) 313. [7] A.A. Kordyuk, Low Temp. Phys. 38, (2012) 888. [8] K.K. Huynh, Y. Tanabe, T. Urata, et al., Phys. Rev. B 90, (2014) 144516. [9] Q. Wang, Y. Shen, B. Pan, et al., Nature Materials 15 (2016) 159. [10] Y. Liu, X.Y. Pu, K. Zhao, X.S. Yang, Y. Zhao, Solid State Comm. 281, (2018) 27.
K.V. Frolov, I.S. Lyubutin, D.A. Chareev and M. AbdelHafiez JETP Letters 110, issue 8 (2019)
Hall conductivity as the topological invariant in phase space in the presence of interactions and nonuniform magnetic field
The original TKNN invariant [1] responsible for the Hall conductivity has been derived for the uniform magnetic field (constant both as a function of time and space coordinates). The expression for the Hall conductivity discussed in the present paper is an extension of the TKNN invariant to the case of varying (in space) magnetic fields. Therefore, its consideration is important and should be interesting for the wide audience. The non  renormalization of the Hall conductivity (given by the original TKNN invariant) by interactions has been discussed earlier [26]. But this consideration was limited by the case of constant magnetic fields. Now we present the proof that the QHE conductivity (given by our extension of the TKNN invariant) is robust to the introduction of interactions in the case of varying magnetic field. This result has never been obtained in the past, to the best of our knowledge.
In addition, the mathematical form of the topological invariant in phase space discussed here is somehow similar to the one of the topological invariant in momentum space composed of the two  point Green function. The latter topological invariant and its variations are used widely (see G.E. Volovik "Universe in a Helium droplet"). Now the Green function is substituted by its Wigner transformation depending on both space coordinates and momentum. The ordinary products are therefore changed to the Moyal (star) product, thus leading to the beautiful mathematical structure. The resulting expression may be used in the presence of interaction for the calculation of Hall conductivity. One simply has to insert to it the interacting (Wigner transformed) two  point Green function. The Green functions with larger number of legs do not contribute to the Hall conductivity (this also has been proved in the presented paper). We would also like to emphasize, that our proof is valid to all orders in the perturbation theory.
[1] D. J. Thouless, M. Kohmoto, M. P. Nightingale, and M. den Nijs, Phys. Rev. Lett. 49, 405(1982).
[2] Ryogo Kubo, Hiroshi Hasegawa, Natsuki Hashitsume, Journal of the Physical Society of Japan 14(1) (1959) 5674 DOI: 10.1143/JPSJ.14.56
[3] Q. Niu, D. J. Thouless, and Y. Wu, Phys. Rev. B 31, 3372 (1985).
[4] B. L. Altshuler, D. Khmel'nitzkii, A. I. Larkin and P. A. Lee, Phys.Rev.B 22, 5142 (1980).
[5] B.L. Altshuler and A.G. Aronov, Electronelectron interaction in disordered systems (A.L.Efros, M. Pollak, Amsterdam, 1985).
[6] S. Coleman and B. Hill, Phys. Lett. B159 (1985) 184 T. Lee, Phys. Lett. B171 (1986) 247
C.X.Zhang, M.A.Zubkov Modulation of magnetointersubband oscillations in a onedimensional lateral superlattice
Landau quantization in a twosubband Fermi electronic system placed in an external perpendicular magnetic field B leads not only to the wellknown Shubnikov – de Haas (SdH) oscillations, but also to another type of quantum resistance oscillations — the magnetointersubband oscillations (MISO) [1, 2]. MISO are not suppressed by the temperature broadening of the Fermi distribution function and therefore allow one to study quantum transport under conditions when SdH oscillations cannot be used for these purposes [3, 4]. The present work is devoted to the study of MISO in a onedimensional (1D) lateral superlattice (LSL), where 1D periodic potential is applied to a twosubband electronic system. The 1D LSL was created on the basis of a selectively doped GaAs/AlAs heterostructure [5, 6]. The measurements were carried out using Hall bars fabricated by means of optical lithography and wet etching. The 1D LSL of period a = 300 nm was created as an array of metal strips on a planar surface of Hall bars using electron beam lithography and the method of exploding an Au/Ti bilayer metallic film. The potential modulation in the studied LSL arises without applying voltage to the metal strips. One of the reasons for this modulation is elastic mechanical stresses between metal strips and a heterostructure [7]. The measurements were carried out at the temperature T = 4.2 K in magnetic fields B < 2 T. It has been shown that commensurability oscillations (CO) of resistance coexist with MISO in the studied LSL. It has been found that 1D periodic potential in a twosubband electron system leads not only to COs but also to MISO amplitude modulation, which is caused by periodic modulation of Landau level width in a 1D LSL in external inverse magnetic field. It has been shown that increased intersubband scattering time in a twosubband system under 1D periodic potential modulation is one of the reasons of MISO amplitude damping in a 1D LSL. [1] V. M. Polyanovskii, Sov. Phys. Semicond. 22, 1408 (1988). [2] D. R. Leadley, R. Fletcher, R. J. Nicholas, F. Tao, C. T. Foxon, and J. J. Harris, Phys. Rev. B 46, 12439 (1992). [3] A. A. Bykov, A. V. Goran, and S. A. Vitkalov, Phys. Rev. B 81, 155322 (2010). [4] O. E. Raichev, Phys. Rev. B 81, 195301 (2010). [5] K.J. Friedland, R. Hey, H. Kostial, R. Klann, and K. Ploog, Phys. Rev. Lett. 77, 4616 (1996). [6] D. V. Dmitriev, I. S. Strygin, A. A. Bykov, S. Dietrich, and S. A. Vitkalov, JETP Lett. 95, 420 (2012). [7] Ivan A. Larkin, John H. Davies, Andrew R. Long, and Ramon Cuscó, Phys. Rev. B 56, 15242 (1997).
A.A. Bykov, I.S. Strygin, A.V. Goran, D.V. Nomokonov, I.V. Marchishin, A.K. Bakarov, E.E. Rodyakina, A.V. Latyshev JETP Letters 110, issue 5 (2019).
Experimental verification of the principle of microscopic reversibility in the photoluminescence decay kinetics
It has been shown in [1] that any photoluminescent body in thermal equilibrium obeys the following relation: $ P(\lambda_1, T) F(\lambda_1, \lambda_2, t) = P(\lambda_2, T) F(\lambda_2, \lambda_1, t) $ (1) where P(λ, T) is the Planck function, which describes the spectral density of thermal radiation at wavelength λ and temperature T, and F(λ_{1}, λ_{2}, t) is the timeresolved excitationemission matrix, which describes the probability density of emitting a photon with wavelength λ_{2} at time t as a result of absorption of a photon with wavelength λ_{1} at time t = 0. For fixed λ_{1}, the function F(λ_{1}, λ_{2}, t) is the photoluminescence spectrum PL(λ; λ_{0}, t) at time t after a shortpulse excitation at wavelength λ_{0}: PL(λ; λ_{0}, t) = F(λ_{0}, λ, t). For fixed λ_{2}, the function F(λ_{1}, λ_{2}, t) is the photoluminescence excitation spectrum PLE(λ; λ_{0}, t) detected at wavelength λ_{0} at time t after a shortpulse excitation at wavelength λ: PLE(λ; λ_{0}, t) = F(λ, λ_{0}, t). Equation (1) rearranged to $ \frac{ PL(\lambda;\lambda_0, t) }{ PLE(\lambda;\lambda_0, t) } = \frac{ P(\lambda, T) }{ P(\lambda_0, T) } $ (2) is a new universal photoluminescence law stating that for any luminophore in thermal equilibrium, the ratio of the corresponding timeresolved photoluminescence and photoluminescenceexcitation spectra, PL(λ; λ_{0}, t) and PLE(λ; λ_{0}, t), is equal to the ratio of blackbody radiation spectra at wavelengths λ and λ_{0}. For fixed λ_{1} and λ_{2}, the function F(λ_{1}, λ_{2}, t) is the kinetics of decay of photoluminescence excited instantaneously at λ_{1} and detected at λ_{2}. Since the righthand side of equation (2) does not depend on time, the lefthand side is also timeindependent. This means that the photoluminescence decay kinetics is invariant under interchange of the excitation and detection wavelengths up to a timeindependent factor. The aim of the present study is to test the relation (2) experimentally by measuring the photoluminescence decay kinetics with interchanging the excitation and detection wavelengths. This implies that when the forward process is a Stokes photoluminescence, then the reverse process is an antiStokes photoluminescence. Colloidal solutions of InP/ZnS quantumdot nanoclusters, which do not obey the Vavilov law about the independence of the photoluminescent properties of a luminophore of the excitation wavelength, have been used in the study to test the invariance of the decay kinetics under interchange of the excitation and emission wavelengths. [1]. S. A. Tovstun, V. F. Razumov, et all. // J. Lumin. 190, 436 (2017).
Razumov V.F., Tovstun S.A., Kuzmin V.A. JETP letters 109, Issue 5 (2019)
Specific features of magnetointersubband oscillations in HgTe quantum wells
Studies of oscillatory magnetotransport effects are one of the most reliable methods for investigating energy spectrum of 2D carrier systems. A magnetic field normal to the plane of a 2D gas leads to orbital quantization of the spectrum and, as a consequence, to the appearance of oscillations of the magnetoresistance (ρ_{xx}) at low temperatures (Shubnikov de Haas oscillations). These oscillations are periodic in the inverse magnetic field and their frequency f is determined by the carrier concentration. In systems in which two (or several) branches of the spectrum E_{1,2} (k) are filled, the oscillations are observed with frequencies f_{1} and f_{2}, determined by the carrier concentration in its branch. The sum of these oscillations manifests itself as a beating of oscillations of ρ_{xx},, causing nodes and antinodes at certain magnetic fields. In the presence of transitions between the branches, new oscillations arise with a difference frequency, f_{1}  f_{2}. They are called magnetointersubband oscillations (MISO) [1,2]. The simplest qualitative examination shows that the positions of the antinodes in magnetic field must coincide with the ρ_{xx} maxima of MISO. Such mutual positions of the antinodes and the MISOs were investigated for the structures based on widegap semiconductors with double quantum wells, for wide quantum wells where two branches of the spectrum are formed due to the Coulomb repulsion of electrons, and for structures with two filled subbands of the size quantization. Along with the cases described above, the two branches of the spectrum for a single quantum well can arise due to the strong spinorbit (SO) interaction. The large SO splitting can occur for the quantum wells of narrowgap (InAs, InSb) and gapless (HgTe, HgSe) semiconductors, as well as for many new topological insulators. Such MISO oscillations were considered only theoretically [3, 4], but were never observed experimentally. This work reports an experimental study of r_{xx} in the gated structures with HgTe quantum wells of 820 nm widths with an inverted spectrum. It was found that, unlike all other cases and theoretical predictions, the mutual position of the antinodes and MISO is quite opposite. Namely, the positions of the antinodes in a magnetic field coincide with the ρ_{xx} minima of MISO. A possible reason for such unusual behavior is discussed. 1. ÷.í. ðÏÌÑÎÏ×ÓËÉÊ, æôð, 22, 2230. (1988) 2. D.R Leadly, R. Fletcher and R. J. Nicholas, Phys. Rev.B, 46, 12439 (1992) 3. M. Langenbuch, M. Suhrke and U. Ro^¨ssler, Phys. Rev. B 69, 125303 (2004) 4. S. G. Novokshonov, Low Temperature Physics 39, 378 (2013) G.M. Minkov, O.E. Rut, A.A. Shestobitov, S.A.Dvoretski, N.N. Mikhailov JETP Letters 110, issue 4 (2019)
Chiral Estimate of QCD Pseudocritical Line
Precisely mapping the phase diagram of stronglyinteracting matter is a challenging problem. Lattice simulations of QCD, the field theory of strong interactions, are reliable at zero density, but become less precise when the density is finite and at the moment are not capable to map the whole phase diagram of strong interactions.
The most dramatic phenomenon that happens when strongly interacting matter is heated to extreme temperatures is restoration of chiral symmetry, a key symmetry of QCD that largely determines properties of hadrons and interactions among them. Chiral symmetry restoration is a sharp crossover at zero density that happens at temperature $T_{0}\simeq 157\,{\rm Mev}$ accurately known from lattice simulations \cite{Bazavov:2018mes}. Various model estimates predict that at larger baryon densities the crossover becomes sharper and eventually merges into a line of firstorder phase transitions at a critical endpoint whose precise location on the $T\mu $ plane is not entirely known. Model estimates vary by a factor of a few \cite{Stephanov:2004wx} depending on the model assumptions.
The order parameter of the chiral symmetry breaking is the quark condensate $ \bar{\psi }\psi$, which has a nonzero expectation value in the vacuum. The pseudoGoldstone modes that arise from the chiral phase of the condensate: $\bar{\psi }\psi \sim \Sigma\,{\rm e}\,^{\gamma ^5 T^{a}\pi _{a}} $ are identified with pions, kaons and the $\eta $meson, which are substantially lighter than other hadrons. Chiral symmetry restoration is typically associated with melting of the quark condensate, but can also proceed via disordering of the condensate's phase. Strong pion fluctuations, such that $\left\langle \mathop{\mathrm{tr}}\,{\rm e}\,^{iT^{a}\pi _{a}}\right\rangle=0$, will restore chiral symmetry even if condensate's modulus is nonzero.
This paper studied this slightly unconventional scenario of chiral symmetry restoration. Following \cite{Zarembo:2001wr} the shape of the pseudocritical line on the $T\mu $ plane can then be predicted from the lowenergy effective field theory. An interesting consequence of this scenario is an absence of the critical endpoint. The symmetry restoration always proceeds through a crossover which moreover becomes weaker with growing baryon density.
1. A. Bazavov et al., 1812.08235.
2. M. A. Stephanov, Prog. Theor. Phys. Suppl. 153, 139 (2004). 3. K. Zarembo, JETP Lett. 75, 59 (2002). K. Zarembo JETP Letters 110, issue 3 (2019). Layered Superconductor in a Magnetic Field: Breakdown of the Effective Masses Model
Recently, a number of quasitwodimensional (Q2D) hightemperature and intermediatetemperature superconductors have been discovered. The anisotropic upper magnetic critical fields in some of them can be described by the LawrenceDoniach model, which is relevant to Q2D superconductors with high anisotropic properties. On the other hand, there are many Q2D superconductors with intermediate anisotropy of the upper critical magnetic fields, which are usually described by the socalled effective masses (EM) model, partially based on the anisotropic GinzburgLandau equations. The most popular such Q2D compounds are MgB_{2 }and Febased superconductors [1]. It is possible to define anisotropic ratio in Q2D superconductors, g, as the ratio of the parallel and perpendicular upper critical magnetic fields, which is always bigger than 1, g > 1. In accordance with EM model, the ratio g doesn’t have to depend on temperature. Meanwhile recent experiments show strong temperature dependence of anisotropy g, which in the case of superconductor MgB_{2} increases with decreasing temperature. The previous explanations of this phenomenon were based on some approximate manyband calculations of the upper critical magnetic fields and were prescribed to manyband effects. In this Letter, we investigate anisotropy ratio, g, more carefully by using derivation and investigation of an integral equation for the socalled superconducting nucleus, using the Gor’kov equations for nonuniform superconductivity (see, for example, the corresponding derivations for a 3D isotropic case in Ref.[2]) . For the first time, we show that the superconducting nucleus is not of the Gaussian shape for the parallel upper critical magnetic field and even changes its sign with space coordinate. This circumstance breaks down the EM model and predicts a factor of 1.3 increase of the upper critical magnetic fields ratio, g, with decreasing temperature. We prescribe the experimentally observed increase of the parameter g in the superconductor MgB_{2 }[1] to the breakdown of the EM model suggested in the Letter. This issue is an important one since Q2D hightemperature and intermediatetemperature superconductors are good candidates for some scientific and industrial applications in high magnetic fields. [1] See, for example, review V.G. Kogan and R. Prozorov, Rep. Prog. Phys. 75, 114502 (2012). [2] L.P. Gor’kov, Sov. Phys. JETP, 37(10), 42 (1960). Lebed A.G. JETP Letters 110, issue 3 (2019).
Metastable conducting crystalline hydrogen at high pressures
The formation of metallic hydrogen, predicted in [1], was observed experimentally in [2]. It is also assumed that this state of solid hydrogen is a superconductor at room temperature. However, the possibility of practical application of the metallic hydrogen is significantly limited by the pressure of formation of this state. The properties of stability and metastability of metallic hydrogen depend on the structure, which determines the relevance of the theoretical study of this issue. As it was shown in [3  6], atomic metallic hydrogen at zero temperature exists in a metastable state up to normal pressure.
[1] E. Wigner, H. B. Huntington, J. Chem. Phys. 3, 764 (1935).
I.M. Saitov A new type of chargedensitywave pinning in orthorhombic TaS$_3$ crystals with quenching defects
There are a number of physical systems in which, under certain conditions, spatially ordered electronic superstructures are formed. Charge and spin density waves (CDW and SDW), Wigner crystals and vortex lattices in typeII superconductors in a magnetic field are examples of such systems. The interaction of the superstructure with local lattice imperfections (various point defects, impurities, etc.) leads to its pinning. In the simplest case, such a pinning (let's call it local) is divided into collective (weak) and individual (strong).
In the present work, it is experimentally shown that, in the Peierls conductor {\it o}TaS$_3$, a new type of CDW pinning appears as a result of samples quenching. It is characterized by a number of fundamental differences from pinning by local pinning centres, namely:
The presence of these features allows us to conclude that quenching defects are macroscopic (nonlocal) objects, for example, dislocations, which can glide along the crystal. They lead to a previously unknown type of CDW pinning with properties different from local pinning ones. The feature of Peierls conductors is strong CDW interaction with defects. As a result of this interaction, forced diffusion of quenching defects and their exit from the crystal takes place during lowtemperature thermocycling.
V.E.Minakova, A.M.Nikitina, S.V.ZaitsevZotov JETP letters, v. 110, issue1 (2019)
Stimulated terahertz emission from exciton system of photoexcited silicon
In connection with recent report on the first detection of terahertz (THz) emission due to intraexciton radiative transitions in semiconductors [1] and a number of theoretical works predicting the possibility to achieve intraexciton population inversion at intense bandtoband optical excitation of a crystal (see [2] and also [3] and other references therein) it is very important to verify experimentally the possibility of implementing an exciton THz laser. In this work, we studied the THz photoluminescence (PL) from highpurity silicon due to radiative transitions between the energy levels of free excitons under conditions of continuouswave interband photoexcitation with a maximum density of up to 120 W/cm^{2}. The appearance of the superlinear dependence of the intensity of the intraexciton THz emission on the pump intensity at temperatures above 2025 K was found. The transition from the linear to superlinear dependence of the THz PL intensity on the pump intensity occurs at a photoexcitation density of about 7 W/cm^{2}. The observed regular patterns are explained by the appearance of the THz stimulated emission and, accordingly, population inversion in the system of excitons at their high density. The THz gain spectrum was obtained, which shows the lines at 13.7 and 15.5 meV, the gain values of which are 0.5 and 1 cm^{1}, respectively, at 25 K and the photoexcitation density of order of 35 W/cm^{2}. The line at 13.7 meV is due to the population inversion between highly excited exciton states and the ground state of free excitons. The gain line at 15.5 meV possibly corresponds to the population inversion between the twoexciton and biexciton states. The values of the terahertz gain indicate that a new type of THz laser can be created on transitions between energy levels of free excitons in silicon under conditions of interband photoexcitation.
[1] A.V. Andrainov, and A.O. Zakhar’in – Intrinsic Terahertz Photoluminescence from Semiconductors – Appl. Phys. Lett., 112, 041101 (2018). [2] M. Kira, and S.W. Koch  ExcitonPopulation Inversion and Terahertz Gain in Semiconductors Excited to Resonance  Phys. Rev. Lett., 93, 076402 (2004). [3] G.K. Vlasov, and S.G. Kalenkov – Sources of Coherent FarInfrared Radiation on Hot Excitons in Crystals  Int. J. Infrared Millimeter Waves, 4, 955 (1983). Zakhar’in A.O., Andrianov A.V. Petrov A.G. JETP letters, v. 109, issue12 (2019)
Nonlinear Hall effect in threedimensional Weyl and Dirac semimetals
Nonlinear Hall effect has been predicted in a wide class of timereversal invariant materials [1], like topological crystalline insulators, twodimensional transition metal dichalcogenides, and threedimensional Weyl and Dirac semimetals. Recently, the timereversalinvariant nonlinear Hall (NLH) effect has been reported for twodimensional layered dichalcogenides [2, 3]. It stimulates a search for the Berry curvature dipole induced NLH effect in threedimensional crystals, where Dirac and Weyl semimetals are excellent candidates.
In the experiments [2, 3] on twodimensional WTe$_2$, the the secondharmonic Hall voltage depends quadratically on the longitudinal current. On the other hand, topological materials are characterized by strong thermoelectric effects, which also appear as a secondharmonic quadratic signal. For this reason, it is important to experimentally distinguish between the Berry curvature dipole induced NLH effect and a thermoelectric response while searching for the NLH effect in nonmagnetic materials.
We experimentally investigate a nonlinear Hall effect for threedimensional WTe$_2$ and Cd$_3$As$_2$ single crystals, representing Weyl and Dirac semimetals, respectively. We observe finite secondharmonic Hall voltage, which depends quadratically on the longitudinal current in zero magnetic field, as it has been predicted theoretically. We demonstrate that secondharmonic Hall voltage shows oddtype dependence on the direction of the magnetic field, which is a strong argument in favor of currentmagnetization effects. In contrast, one order of magnitude higher thermopower signal is independent of the magnetic field direction. Thus, the magnetic field dependence allows to distinguish the nonlinear Hall effect from a thermoelectric response.
[1] Sodemann and Liang Fu, Phys. Rev. Lett., 115, 216806 (2015).
[2] Kaifei Kang, Tingxin Li, Egon Sohn, Jie Shan, Kin Fai Mak, arXiv:1809.08744 (2018).
[3] Qiong Ma, et al., arXiv:1809.09279 (2018).
Shvetsov O.O., Esin V.D., Timonina A.V., Kolesnikov N.N., Deviatov E.V. JETP Letters 109, issue 11 (2019)
Quantumoptical gate for nonclassical squeezed light
Nonclassical squeezed light is one of the most attractive quantum objects. Squeezed light is in the center of scientific interest nowadays due to its unique features, such as entanglement of large number of photons, twinbeam correlations and suppressed variance of one of the field quadratures. Such light is very important for many applications in quantum information, quantum tomography and measurements with noise reduction beyond the standard quantum limit. It was shown that squeezed light can be presented as superposition of Schmidt modes, which are orthogonal and carry all its nonclassical features [1]. For applications it is necessary to be able to control and manipulate the properties and mode content of squeezed light. To solve this task, the scheme based on the sumfrequency generation process seeded by squeezed light was suggested [24]. The proposed scheme was able to block a certain temporal mode of nonclassical light by converting its photons into the sumfrequency mode with a narrow Gaussian spectral profile. In this work we develop further the idea of the sumfrequency generation with the squeezed light at the input and give detailed theoretical description of this process in the frame of Schmidt modes. We analyze the transformation of spectral properties of squeezed light and predict new effects. We describe quantumoptical gate which provides wide opportunities for managing the spectral signal of squeezed light and allows to control the Schmidt mode weights. The complete blocking of the signal in a certain Schmidt mode is shown to redistribute the weights of other modes and therefore gives the possibility of engineering the spectral and temporal properties of outgoing signal. In the full conversion regime the quantumoptical gate is demonstrated to transfer all the features of nonclassical squeezed vacuum state to the light generated in the sum frequency channel.
[1] P.R. Sharapova, O.V. Tikhonova, S. Lemieux et. al., Phys. Rev. A. 97, 053827 (2018) [2] A. Eckstein, B. Brecht and C. Silberhorn, Optics Express. 19, 13770 (2011). [3] B. Brecht, A. Eckstein, A. Christ et al, New J. Phys. 13, 065029 (2011). [4] V. Ansari, J. Donohue, B. Brecht and C. Silberhorn, Optica 5, 534550 (2018).
V.V. Sukharnikov, O.V. Tikhonova JETP Letters 109, issue 9 (2019)
"Tubular" lanthanum hydride as a new class of hightemperature superconducting material
In 2018, a number of experimental works [13] were published, in which it was shown that lanthanum hydrides at high pressures P = 150¸190 GPa are superconductors with very high critical temperatures T_{c} = 215¸260 K. The detected crystalline phase is considered to have FM3M symmetry and LaH_{10} stoichiometry. However, calculations of the phonon spectrum of this structure show that it is dynamically stable only for pressures of P>210 GPa, which is beyond the pressure range of experimental work. This paper presents the results of a search for new structures of lanthanum hydride, which could correspond to the experimental results [13] and would be dynamically stable at pressures in the range P = 150¸200 GPa. Based on quantum calculations in the framework of the density functional theory, a new structure of lanthanum hydride La_{2}H_{24} was predicted for the first time. This structure is dynamically stable up to pressures of the order of 150 GPa. It is a semimetal and has a low symmetry of crystal lattice P1. An important feature of the structure is the presence of quasimolecular hydrogen chains, which leads to the presence of frequencies of about 420 meV in the phonon spectrum, exceeding the maximum oscillation frequency of the metallic hydrogen FDDD phase (ω~360 meV). These properties allow us to expect to achieve a high superconducting critical temperature for lanthanum hydride La_{2}H_{24}. [1] A. P. Drozdov, V. S. Minkov, S. P. Besedin, P. P. Kong, M. A. Kuzovnikov, D. A. Knyazev, M. I. Eremets – Superconductivity at 215 K in lanthanum hydride at high pressures – arXiv:1808.07039. [2] M.Somayazulu, M.Ahart, A.Mishra, Z.M. Geballe, M.Baldini, Y.Meng, V.V. Struzhkin, and R.J.Hemley – Evidence for superconductivity above 260 K in lanthanum superhydride at megabar pressures – arXiv:1808.07695. [3] A. P. Drozdov, P. P. Kong, V. S. Minkov, S. P. Besedin, M. A. Kuzovnikov, S. Mozaffari, L. Balicas, F. Balakirev, D. Graf, V. B. Prakapenka, E. Greenberg, D. A. Knyazev, M. Tkacz, M. I. Eremets. Superconductivity at 250 K in lanthanum hydride under high pressures – arXiv:1812.01561.
Degtyarenko N.N., Grishakov K.S., Mazur E.A. JETP Letters 109, issue 6 (2019)
Hightemperature superconductivity of graphite particles embedded in polystyrene
To date, a significant number of indirect observations indicating the existence of a superconducting state up to room temperature in some small regions of highly oriented pyrolytic graphite (HOPG) samples have been reported [1]. The main problem was that the superconducting regions included only a small amount of carbon material with an unknown structural nature and, consequently, showed poor reproducibility of the superconductivity effect for different samples of HOPG with the same macroscopic dimensions. Significant progress in controlling the effect of superconductivity was obtained by embedding multilayer multilayered graphene flakes into a polystyrene matrix, so that covalent bonds are formed between the multilayered graphene flakes and the polystyrene [2,3]. In those papers, we reported a current–voltage characteristic of Josephson type up to temperatures higher than room temperature. In the present paper, we show that for the resulting magnetic moment of the same composite a magnetic field dependence typical of superconductors is observed in the same temperature range where previously a Josephson currentvoltage characteristic was observed. In the experiment, we used a vibrating magnetometer of the PPMS9 series (Quantum Design) in the temperature range 2400 K and with magnetic fields of 0 – ± 10 T. The reason for the emergence of superconductivity in multilayered graphene, as was first discussed in [2,3], may be the formation of covalent bonds with the polystyrene, leading to deformation of the graphene. Such deformation can produce a shift or rotation at different angles, including the magic angle [4], of one layer of graphene relative to another in multilayered graphene flakes embedded in a polystyrene matrix. As a result, within the interface regions between the graphene layers, flat energy zones arise, which can lead to roomtemperature superconductivity [5]. [1] P. Esquinazi, N. García, J. BarzolaQuiquia, P. Rödiger, K. Schindler, J.L. Yao, M. Ziese, Indications for intrinsic superconductivity in highly oriented pyrolytic graph. Phys. Rev. B 78(1–8), 134516 (2008) [2] A.N. Ionov, Technical Physics Letters 41(7), 651 (2015) [3] A.N. Ionov, J Low Temp Phys, 185, 515 (2016). [4] Y. Cao, V. Fatemi, S. Fang, K. Watanabe, T. Taniguchi, E. Kaxiras, P. JarilloHerrero, Nature, 556, 43 (2018). [5] G. E. Volovik, JETP Lett. 107, 516 (2018). A.N.Ionov, M.P.Volkov, M.N.Nikolaeva JETP Letters 109, issue 3 (2019)
Folding in twodimensional hydrodynamic turbulence
In the 2D developed hydrodynamic turbulence at high Reynolds numbers the formation of the Kraichnan direct cascade with a constant enstrophy flux is due to the appearance of the vorticity quasi  shocks, because of the compressibility of continuously distributed lines of the divorticity field ${\bf B}=\mbox{rot}\,\mathbf{\omega}$ [1]. This property follows directly from the frozenness equation for ${\bf B}$,
[1] E.A.Kuznetsov, V.Naulin, A.H.Nielsen, and J.J.Rasmussen, Phys. Fluids 19, 105110 (2007).
E.A. Kuznetsov, E.V. Sereshchenko, JETP Letters 109, issue 4 (2019). Terahertz spectroscopy of twodimensional semimetal in threelayer InAs/GaSb/InAs quantum well
The quantum spin Hall insulator state (QSHI) is a twodimensional topological phase of matter with insulating 2D bulk state and a pair of spinpolarized gapless helical edge states. These edge states may have spintronic applications, which are made possible by the allnew demonstration of QSHI state at 100 K performed on a WTe2 monolayer [1]. However, device engineering involving monolayer materials is challenging, often because of structural or chemical instabilities. The realistic candidates for hightemperature QSHI in semiconductor heterostructures with mastered technological process are the threelayer InAs/Ga(In)Sb/InAs quantum wells (QWs) confined between widegap AlSb barriers [2]. Depending on their layer thicknesses, these QWs host trivial, QSHI and semimetal states. A major advantage of the threelayer QWs, compared to the widely studied HgTe QWs, is a temperatureinsensitive inverted bandgap [3], which under certain condition exceeds the value of 45 meV known for WTe_{2} monolayers [1]. This work reports experimental study of 2D semimetal state in InAs/GaSb/InAs QWs. Already observed in inverted HgTe QWs [4,5], these topologically nontrivial states are characterized by a nonlocal overlap between conduction and valence bands. To probe the bulk states of the grown sample, we carried out THz photoluminescence measurements and Landau spectroscopy. By analyzing experimental results, we have demonstrated the existence of a nonradiative recombination channel due to the overlap of the conduction and valence bands. [1] S. Wu, V. Fatemi, Q. D. Gibson et al. (Collaboration), Science 359, 76 (2018). [2] S. S. Krishtopenko and F. Teppe, Sci. Adv. 4, eaap7529 (2018). [3] S. S. Krishtopenko, S. Ruffenach, F. GonzalezPosada et al. (Collaboration), Phys. Rev. B 97, 245419 (2018). [4] Z. D. Kvon, E. B. Olshanetsky, D. A. Kozlov et al. (Collaboration), JETP Lett. 87, 502 (2008). [5] G. M. Gusev, E. B. Olshanetsky, Z.D. Kvon et al. (Collaboration), Phys. Rev. Lett. 104, 166401 (2010).
S.S.Krishtopenko, S. Ruffenach, F. GonzalezPosada et al. (Collaboration) JETP Letters 109, issue 2 (2019)
Light absorption properties related to longliving ensemble spin excitations in an unpolarized quantum Hall system
In connection with recent studies of extremely longliving cyclotron spinflip excitations [13] (CSFE)  actually magnetoexcitons in a quantum Hall electron gas, the contribution to light absorption related to such a magnetoexcitonic ensemble is discussed. The CSFE relaxation found experimentally in the unpolarized quantum Hall system created in a real GaAs/AlGaAs heterostructure reaches 100 $\mu$s [4] at finite temperature $T\!\simeq\!0.5\,$K,
S. Dickmann
JETP Letters 109, issue 1 (2019) Search for highenergy neutrinos from GW170817 with the BaikalGVDneutrino telescope
A gravitational wave signal, GW170817, from a binary neutron star merger has been recordedby the Advanced LIGO and Advanced Virgo observatories on August 17, 2017 [1]. The deep underwater neutrino telescope Baikal Gigaton Volume Detector (BaikalGVD) is currently under construction in Lake Baikal [2].In this work we present results of searches for highenergy neutrinos in coincidence with GW170817 by BaikalGVD. Two different time windowswere used for the search. First, a ±500 s time window around the merger was used to search for neutrinos associated with prompt and extended gammaray emission. Second, a 14day time window following the GW detection, to cover predictions of longerlived emission processes. Since background events from atmospheric muons and neutrinos can be significantly suppressed by requiring time and space coincidence with the GW signal, relatively weak cuts can be used for neutrino selection. For the search for neutrino events within a ±500 s window around the GW event, 731 events were selected, which comprise >5 hit light sensors at>2 hit strings. After applying cascade reconstruction procedures and dedicated quality cuts, two events were selected. Finally, requiring directional coincidence with GW170817y< 20° no neutrino candidates survived.The absence of neutrino candidates associated with GW170817 in the ±500 s window as well as in 14 day window allows to constrain the fluence of neutrinos from GW170817. Assuming an E^{2} spectrum singleflavor differential limits to the spectral fluence in bins of one decade in energy have been derived. In the range from 5 TeV to 10 PeV a 90% CL upper limit is 5.2×(E/GeV)^{2} GeV^{1}cm^{2}for ±500 s time window search. The corresponding upper limit to the spectral fluencefor 14 day search window is 9.0×(E/GeV)^{2} GeV^{1}cm^{2}over the same energy range.
A.D. Avrorin, A.V. Avrorin, V.M. Aynutdinov et.al. (Baikal Collaboration) JETP Letters 180, issue 12 (2018)
Spin diffusion in liquid $^3$He confined in planar aerogel
Transport phenomena in anisotropic porous media are widely discussed in the literature. We investigate the Knudsen regime diffusion in alumina aerogels~~high porosity materials composed of long cylindrical strands. The theory and experimental results for nematic aerogel with nearly parallel strands were reported earlier [1].
In the present paper we explore a different type of anisotropic aerogellike metamaterial, which we call the planar aerogel. Like nematic aerogel, it is a macroscopically uniform system with axial symmetry which consists of strands of diameter $10\,\text{nm}$. The directions of these strands, however, are uniformly distributed in a plane perpendicular to the symmetry axis (rather than parallel to it, as in nematic aerogel). Proposed theory is based on the assumption that elastic collisions with the strands is the most important scattering mechanism. We consider two opposite limits: specular and diffuse scattering (denoted by the subscripts $S$ and $D$). Axially symmetric diffusion tensor has two distinct principal values: $D^{xx}=D^{yy}$ for diffusion in the aerogel plane and $D^{zz}$ along the symmetry axis. From the theory it follows, somewhat surprisingly, that the diffusion anisotropy in the specular scattering model is smaller than that in the diffuse model: $D^{xx}_\text{S}/D^{zz}_\text{S}=1.97$ and $D^{xx}_\text{D}/D^{zz}_\text{D}=2.50$.
In the experiments we used the spin echo technique to investigate the spin diffusion in normal liquid $^3$He confined in the planar aerogel. At very low temperatures $T\sim 1\,\text{mK}$, where the Fermi quasiparticle population is small and the Knudsen regime is achieved, our experimental results are in a good agreement with the theory for the case of the specular scattering.
[1] V.V.Dmitriev, L.A.Melnikovsky, A.A.Senin, A.A.Soldatov, and A.N.Yudin, JETP Lett. 101, 808 (2015).
Dmitriev V.V., Kutuzov M.S., Melnikovsky L.A., Slavov B.D., Soldatov A.A.,Yudin A.N. Chiral torsional effect
The non  dissipative transport effects have been widely discussed recent years. These effects are to be observed in the non  central heavy ion collisions [1]. They have also been considered for the Dirac and Weyl semimetals [2] and in $^3$HeA [3].
Among the other effects their family includes the chiral separation effect (CSE) [4], the chiral vortical effect (CVE) [5], the anomalous quantum Hall effect (AQHE) [2]. All those phenomena have the same origin  the chiral anomaly.
In the present paper we propose the new non  dissipative transport effect  the chiral torsional effect (CTE). Namely, we will discuss the emergence of axial current of thermal quasiparticles in the presence of torsion. It will be shown that this effect is intimately related to the chiral vortical effect [5], i.e. the latter may be considered as the particular case of the CTE. It is well  known that in conventional general relativity torsion vanishes identically, it appears only in its various extensions. However, the background (non  dynamical) gravity with torsion emerges in certain condensed matter systems. For example, elastic deformations in graphene and in Weyl semimetals induce the effective torsion experienced by the quasiparticles [6]. In $^3$HeA torsion appears dynamically when motion of the superfluid component is non  homogeneous.
[1] W. T. Deng and X. G. Huang, \Vorticity in HeavyIon Collisions," Phys. Rev. C 93, no. 6,
064907 (2016) [arXiv:1603.06117 [nuclth]]. [2] A. A. Zyuzin and A. A. Burkov, \Topological response in Weyl semimetals and the chiral anomaly," Phys. Rev. B 86 (2012) 115133 [arXiv:1206.1868 [condmat.meshall]]. [3] G.E. Volovik, The Universe in a Helium Droplet, Clarendon Press, Oxford (2003). [4] \Anomalous Axion Interactions and Topological Currents in Dense Matter",Max A. Metlitski and Ariel R. Zhitnitsky,Phys. Rev. D 72 (2005), 045011 [5] A. Vilenkin, Phys. Rev. D 22, 3080 (1980) [6] G.E.Volovik, M.A.Zubkov, Annals of Physics 340/1 (2014), pp. 352368, arXiv:1305.4665 [condmat.meshall].
Z.V.Khaidukov, M.A.Zubkov
JETP Letters 108, issue 10(2018)
Hidden Fermi surface in $K_xFe_{2y}Se_2 : LDA+DMFT $ study
Investigation of the superconductivity in novel ironbased superconductors is one of the main trends in modern condensed matter physics [1]. Some of iron chalcogenide superconductors [2] have qualitatively different electronic properties from other ironbased superconductors (e.g. iron pnictides) [3]. Among them, the K_{x}Fe_{2−y}Se_{2} compound and the FeSe monolayer on the SrTiO_{3} substrate take quite a special place. Early days angular resolved photoemission spectroscopy (ARPES) experiments practically could not resolve holelike Fermi surface sheets near the Γpoint of the Brillouin zone in contrast to the iron pnictides and some iron chalcogenides (e.g. bulk FeSe). Recently in the work [4] ARPES observation of a “hidden” holelike band approaching the Fermi level near the Γpoint for the K_{0.62}2Fe_{1.7}Se_{2} system and thus proposing a holelike Fermi surface near the Γpoint was reported. Inspired by the work [4] we show by LDA+DMFT [6] study that for K_{0.62}Fe_{1.7}Se_{2} system near the Γpoint there are two holelike bands crossing the Fermi level and forming the Fermi surface near the Γpoint. Its appearance can justify spinfluctuation mechanism of superconductivity in this class of systems [6] with a rather high critical temperature T_{c}∼30K. Good qualitative and even quantitative agreement of the calculated and ARPES Fermi surfaces is obtained. ^{1}M.V. Sadovskii. Usp. Fiz. Nauk 178, 1243 (2008). ^{2}M.V. Sadovskii. Usp. Fiz. Nauk 186, 1035 (2016). ^{3}M.V. Sadovskii, E.Z. Kuchinskii, I.A. Nekrasov, JMMM 324 3481, (2012). ^{4}M. Sunagawa et al., J. Phys. Soc. Jpn. 85, 073704 (2016). ^{5}K. Held et al. Int. J. Mod. Phys. B 15, 2611 (2001). ^{6}P.J. Hirshfeld, M.M. Korshunov, I.I. Mazin. Rep. Prog. Phys. 74, 124508 (2011). I.A.Nekrasov, N.S.Pavlov JETP Letters 108 , issue 9 (2018)
Search for neutrinos with a mass (0.011.0) MeV in beta decays of nuclei $^{144}Ce  ^{144}Pr$
The discovery of solar and atmospheric neutrino oscillations means that at least two of the three mass neutrino states are nonzero. Certain values of the oscillation parameters together with restrictions on the sum of the light neutrino masses obtained from the Planck space telescope data limit the heaviest mass state (ν1, ν2, ν3) of three known types of neutrinos (νe, νμ, ντ) to 70 meV. The measured decay width of the Zboson indicates that the heavier neutrino mass states, if they exist, must be related to the sterile neutrino. The simplest mechanism of mass formation is ensured by the existence of righthanded, sterile neutrino interactions. Such neutrinos can be mixed with three active types of neutrinos. The mixing effect leads to neutrino oscillations, it can manifest itself in the processes of production of active neutrinos and lead to the decay of sterile neutrinos into particles of the Standard Model (SM). Sterile neutrinos, in one form or another, appear in many extensions of the SM, they are wellmotivated candidates for the role of dark matter particles. Although the search for sterile neutrinos has been conducted for many years, convincing results of their existence have not yet been obtained [1]. This paper is devoted to the search for the manifestations of massive neutrinos in the measured electron spectra arising from the decay of nuclei ^{144}Ce – ^{144}Pr. The source of electronic antineutrinos ^{144}Ce – ^{144}Pr is one of the most suitable for studying neutrino oscillations into a sterile state with a mass of about 1 eV. We decided to test the possibility of radiation in these beta transitions of heavy sterile neutrinos with a mass of from 1 keV to 3 MeV. The range of possible studied neutrino masses is determined by the resolution of the spectrometer used [2] and the boundary energy of beta decay of the ^{144}Pr nucleus. A spectrometer consisting of a Si(Li) fullabsorption detector and a transition Sidetector was used for precision measurements of the electron spectrum arising from the beta decays of ^{144}Ce – ^{144}Pr nuclei. The beta spectrum measured during 364 h is analyzed to find the contribution from heavy neutrinos with masses from 10 keV to 1 MeV. For neutrinos with a mass in the range (150–350) keV, new upper limits on the mixing parameter at the level U_{eH}^{2} ≤ 2×10^{–3}  5×10^{−3} for 90% confidence level have been obtained. The achieved sensitivity to U_{eH}^{2} can be increased several times after precision measurement of the response function when using a 4πgeometry spectrometer, in which the response function for monochromatic electrons practically coincides with the Gaussian function [3]. [1]. K.N. Abazajian, M.A. Acero, S.K. Agarwalla et al. (Collaboration), Light Sterile Neutrinos: A White Paper, arXiv:1204.5379v1 (2012). [2]. I. E. Alexeev, S.V. Bakhlanov, N.V. Bazlov, E. A. Chmel, A. V. Derbin, I. S. Drachnev, I.M. Kotina, V.N. Muratova, N.V. Pilipenko, D.A. Semyonov, E.V. Unzhakov, V.K. Yeremin, Nuclear Inst. And Methods in Physics Research A 890, 647 (2018). [3]. A.V. Derbin, A. I. Egorov, I.A. Mitropolskii, V. N. Muratova, S.V. Bakhlanov, and L.M. Tukhkonen, JETP Lett. 65, 605 (1997).
A.V. Derbin, I.S. Drachnev, I.S. Lomskaya, V.N. Muratova. N.V. Pilipenko, D.A. Semenov, L.M. Tykhkonen, E.V. Unzhakov, A.Kh. Khusainov JETP Letters 108, issue 8 (2018)
Nonstationary spinpolarized currents tuning in correlated quantum dot
The possibility to create, manipulate and detect spinpolarized currents is at the very heart of semiconductor spintronics [1]. Stationary spin polarized currents were successfully generated in various semiconductor heterostructures and lowdimensional mesoscopic samples [2]. However, controllable manipulation of charge and spin states, applicable for ultra small size electronic devices design requires analysis of nonstationary effects and transient properties [35]. Consequently, the problem of nonstationary evolution of initially prepared spin and charge state in correlated nanostructures (quantum dots, impurity atoms, etc.) is really vital. In the present paper we analyze nonstationary spinpolarized currents flowing through the correlated singlelevel quantum dot localized between nonmagnetic leads in the presence of applied bias voltage and external magnetic field. We reveal, that spin polarization and direction of the nonstationary currents can be simultaneously inverted by sudden changing of applied bias voltage. We also analyze time evolution of the spin polarization degree and demonstrate the possibility of its sign changing following the applied bias polarity. This effect opens the possibility for the spinpolarization train pulses generation with the opposite degree of polarization. Application of external magnetic field allows to consider correlated singlelevel quantum dot as an effective nonstationary spin filter. [1] I. Zutic, J. Fabian, S. Das Sarma, Rev. Mod. Phys., 76, 323 (2004) [2] M.E. Torio, K. Hallberg, S. Flach, A.E. Miroshnichenko, M. Titov, Eur. Phys. J. B37, 399 (2004) [3] N.S. Maslova, I. V. Rozhansky, V.N. Mantsevich, P.I. Arseyev, N.S. Averkiev, E. Lahderanta, Phys. Rev. B 97, 195445 (2018) [4] V.N. Mantsevich, N.S. Maslova, P.I. Arseyev, Physica E, 93,224 (2017) [5] N.S. Maslova, P.I. Arseyev, V.N. Mantsevich, Solid State Comm. 248, 21 (2016)
Mantsevich V.N., Maslova N.S., Arseyev P.I. JETP Letters 108, №7 (2017)
Sphaleron rate in lattice gluodynamics
It is well known that YangMills theory possesses a nontrivial topological structure: it has an innite series of energetically degenerate but topologically distinct classical vacua. At nite temperature thermal uctuations of elds can lead to (sphaleron) transitions between various vacuums. Due to the chiral anomaly the rate of these transitions describes the evolution of the chiral charge in Quantum Chromodynamics or baryon charge in electroweak theory. [1] G. D. Moore and M. Tassler, JHEP 1102, 105 (2011) doi:10.1007/JHEP02(2011)105 [arXiv:1011.1167 [hepph]].
A.Yu.Kotov JETP Letters 108, issue 6 (2018)
Zitterbewegung of Spin Split Electrons
At the birth of quantum mechanics, E. Schrödinger realized that a free relativistic electron, described by the Dirac Hamiltonian, exhibits oscillations in space resulting from the interference of the positive and the negativeenergy solutions of the Dirac equation [1]. Recently, it was suggested that Zitterbewegung is not limited to free electrons but is a common feature of systems with a gapped or levelsplit spectrum exhibiting a formal similarity to the Dirac Hamiltonian [2]. Here, we study the motion of electrons in a semiconductor system with spinorbit coupling and the Zeeman gap opened by an external magnetic field. It is shown that, in addition to the wellknown Brownian motion, electrons experience an inherent trembling motion of quantummechanical nature. The effect originates from the fact that the electron velocity is not a conserved quantity and contains an oscillating contribution. The Zitterbewegung occurs for all the electrons, also for electrons in thermal equilibrium. Experimental study of the electron Zitterbewegung in such conditions requires the use of noise spectroscopy. We show that the Zitterbewegung of individual electrons can be phasesynchronized by initializing the electrons in the same spin state. In this case, the coherent precession of the individual electron spins drives their backandforth motion in real space giving rise to a macroscopic highfrequency electric current. Such a coherent Zitterbewegung is maintained as long as the coherent spin precession of the electrons is not destroyed by the processes of spin dephasing. We develop a theory of the coherent Zitterwebegung for the cases of ballistic and diffusive electron transport, predict its enhancement at the plasmon resonance conditions, and discuss its relation to the spingalvanic effect [3,4]. [1] E. Schrödinger, Über die kräftefreie Bewegung in der relativistischen Quantenmechanik, Sitz. Press. Akad. Wiss.Phys.Math. 24, 418 (1930). [2] W. Zawadzki and T. M. Rusin, Zitterbewegung (trembling motion) of electrons in semiconductors: a review, J. Phys.: Condens. Matter 23, 143201 (2011). [3] E.L. Ivchenko, Yu.B. LyandaGeller, and G.E. Pikus, Current of thermalized spinoriented photocarriers, Sov. Phys. JETP 71, 550 (1990). [4] S.D. Ganichev, E.L. Ivchenko, V.V. Bel’kov, S.A. Tarasenko, M. Sollinger, D. Weiss, W. Wegscheider, and W. Prettl, Spingalvanic effect, Nature 417, 153 (2002).
S. A. Tarasenko, A. V. Poshakinskiy, E. L. Ivchenko, I. Stepanov, M. Ersfeld, M. Lepsa, and B. Beschoten JETP Letters 108, issue 5 (2018)
Terahertz cyclotron photoconductivity in strongly unbalanced 2D electronhole system
Cyclotron resonance photoconductivity (CRP) is one of the power tools for study of the interaction of twodimensional particles with electromagnetic radiation especially after the discovery of microwave induced magnetoresistance oscillations [1] that have created a lot of questions in the area, where, after the issue of the wellknown review [2], it seemed that everything was clear. In this work, we report on the observation of CRP of twodimensional (2D) electrons under very unusual conditions – in 2D semimetal in that their number (109 – 1010) cm2 is much (from one to three orders) less than number of holes. So for the first time the cyclotron resonance have been observed from the electrons moving through the hole liquid, which strongly screens an impurity scattering potential and an electronelectron interaction. At first glance, it is impossible to observe CRP in this situation because of a very small absorption rate; however it has been detected in our experiments. Moreover, at 432 µm wavelength no decreasing of the CRP amplitude was observed when electron density decreased from 1010 cm2 to 109 cm2 . The experiments demonstrate that interaction of 2D electrons in semiconductor structures with the high frequency electromagnetic field is not so simple problem. It is likely there is a strong field enhancement in 2D system due to many particle effects in the spirit of a recent theory work [3]. Anyway, the further study of this phenomenon is of undoubted interest. [1] I. A. Dmitriev, A. D. Mirlin, D. G. Polyakov, and M. A. Zudov, Rev. Mod. Phys. 84, 1709 (2012). [2] T. Ando, A. B. Fowler, and F. Stern, Rev. Mod. Phys. 54, 673 (1982). [3] A. D. Chepelianskii, D. L. Shepelyansky, Phys. Rev. B 97, 125415 (2018). Z.D. Kvon JETP Letters 108, issue 4 (2018) Negative differential resistance and other features of spindependent electron transport in doublebarrier hybrid superconductorferromagnetic metalnormal metal structures
Investigation of hybrid structures containing superconductors and magnetic materials attracts great interest due to different interesting phenomena such as spintriplet superconducting pairing, anomalous superconducting and magnetic proximity effects and other ones that were reviewed in several articles [15]. In this work, the spindependent electron transport phenomena have been studied theoretically for doublebarrier structures SIF_{1}FIF_{2}N, where S is a superconductor, F is a ferromagnetic metal, N is a normal metal, IF is a spinactive barrier. It was predicted that under certain conditions the negative differential resistance may be realized in the structures SIF_{1}FIF_{2}N, if the polarization at least one of the barriers is not small: R_{b↑}  R_{b↓} is of the order of ( R_{b↑} + R_{b↓} ), where R_{b↑} , R_{b↓} are the contributions to the (normal state) resistance of the barrier related with spinup and spindown electrons, respectively. It was shown that the negative differential resistance is realized if the superconducting proximity effect is strong, the thickness of the F layer is short enough, the exchange field in this layer is not small with respect to the superconducting energy gap Δ, and the spinorbit relaxation time due to impurity scattering in the F layer is significantly greater than ħ/Δ. Another investigated features of the differential resistance of the SIF_{1}FIF_{2}N structures are its voltage asymmetric dependences and its strong dependence on the mutual orientations of the exchange fields in the barriers and in the F layer, that is the reason of the giant magnetoresistance effect.
Zaitsev A.V. JETP Letters 108, issue 3 (2018) Zener Tunneling between Landau Levels in twodimensional system with onedimensional periodic modulation
Nonlinear magnetotransport in twodimensional (2D) electron systems reveals fascinating novel physical phenomena such as quantal Joule heating [1], zero differential resistance [2] or conductance [3] states, and Zener tunneling between Landau levels [4]. The later effect is related to a backscattering of 2D electrons colliding with a short range, sharp impurity potential. The effect is considered to be absent for a smooth, long range disorder. Surprisingly, this paper shows that a longrange, smooth periodic modulation of the electrostatic potential affects significantly the electron backscattering leading to an unexpected interference of the Zener and commensurability oscillations of the magnetoresistance [5]. The electrostatic modulation is obtained via a fabrication of a periodic array of nanoscaled metallic strips with a period a = 200nm located on top of the studied samples. The interference leads to a dramatic modification of the commensurability oscillations of the magnetoresistance reminiscent of a beating pattern. Due to the long range periodic electrostatic modulation the proposed model relates the observed interference to a modification of the electron spectrum, in particular, the electron lifetime. The model is in a good agreement with the experiment, indicating the relevance of the proposed explanation. The obtained results indicate that the quantization of the electron spectrum is of a paramount importance for nonlinear electron transport in low dimensional systems. 1. Jing Qiao Zhang, Sergey Vitkalov and A. A. Bykov, Phys. Rev. B 80, 045310 (2009). 2. A. A. Bykov, J.Q. Zhang, S. A. Vitkalov, A. K. Kalagin, and A. K. Bakarov, Phys. Rev. Lett. 99, 116801 (2007). 3. A. A. Bykov, Sean Byrnes, Scott Dietrich, and Sergey Vitkalov, Phys. Rev. B 87, 081409(R) (2013). 4. C. L. Yang, J. Zhang, R. R. Du, J. A. Simmons, J. L. Reno, Phys. Rev. Lett. 89, 076801 (2002). 5. D. Weiss, K. von Klitzing, K. Ploog, and G. Weimann, Europhys. Lett. 8, 179 (1989).
A. A. Bykov, I. S. Strygin, E. E. Rodyakina, S. A. Vitkalov JETP Letters 108, issue 2 (2018) Investigation of novel twodimensional CoC phase
Recent progress on novel twodimensional metalbased compounds [1,2] have encouraged us to pay attention to this underinvestigated and highly promising class of materials. Here we would like to present the prediction of a new CoC phase which is very intriguing by uncommon symmetry as well as electronic and mechanical properties. In particular, both the ab initio bending analysis and phonon calculations have shown that 2D CoC demonstrates stability of orthorhombic lattice structure in contrast to probably more expected hexagonal or square types. Moreover, from electronic structure analysis, it was obtained that the cobalt net and carbons dimers are connected through a combination of covalent, ionic and metallic bonding. The estimated mechanical elastic modulus for 2D CoC are comparable to those for hBN and only 30% lower than for the “worldrecord” graphene, whereas Poisson’s ratios and flexural rigidity are higher (or equal) than for the wellknown 2D structures. The predicted metallic states of 2D CoC and promising mechanical properties might be of practical importance for future CoCbased heterostructure synthesis, whereas thorough description of potentially interesting magnetic and optical properties have to motivate further studies. [1] Kano, E.; Kvashnin, D. G.; Sakai, S.; Chernozatonskii, L. A.; Sorokin, P. B.; Hashimoto, A.; Takeguchi, M. OneAtomThick 2D Copper Oxide Clusters on Graphene. Nanoscale 2017, 9 (11), 3980–3985. [2] Zhao, J.; Deng, Q.; Bachmatiuk, A.; Sandeep, G.; Popov, A.; Eckert, J.; Rümmeli, M. H. FreeStanding SingleAtomThick Iron Membranes Suspended in Graphene Pores. Science 2014, 343 (6176), 1228–1232.
Larionov K.V., Popov Z.I., Vysotin M.A., Kvashnin D.G., Sorokin P.B. JETP Letters, 108, issue 1, 2018 On the thermal stability of pentagraphene
Successful exfoliation of oneatomthick graphene layer from the graphite crystal in 2004 [1] stimulated the search for new twodimensional carbon nanostructures. In graphene each carbon atom is bonded to its three nearest neighbors, so that CC bonds form a pattern of hexagons, while pentagons are considered as topological defects. Recently, a new carbon allotrope, pentagraphene, composed entirely of pentagons, has been proposed [2]. Later, however, it was argued that pentagraphene cannot be made experimentally because, first, it is thermodynamically unstable and rapidly restructures toward graphene [3] and, second, intrinsic mechanical stress created by two mutually orthogonal sublattices of carbon dimers results in the growth of strongly curved rather than planar pentagraphene layers [4]. We draw attention to another weak point of pentagrafene, its thermal stability. Tightbinding molecular dynamics simulation showed that after the formation of a single defect of the StoneWales type, the disordered region does not remain localized, but rapidly spreads over the entire sample. The lifetime of the pentagrafene sample until complete disordering of its structure decreases exponentially with increasing temperature and is inversely proportional to the sample area. At room temperature, mesoscopic samples of pentagrafene may have rather high thermal stability. 1. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, and A.A. Firsov, Science 306, 666 (2004). 2. S. Zhang, J. Zhou, Q. Wang, X. Chen, Y. Kawazoe, and P. Jena, Proc. Nat. Acad. Sci. 112, 2372 (2015). 3. C.P. Ewels, X. Rocquefelte, H,W. Kroto, M.J. Rayson, P.R. Briddon, and M.I. Heggie, Proc Nat. Acad Sci. U S A. 112, 15609 (2015). 4. P. Avramov, V. Demin, M. Luo, C.H. Choi, P.B. Sorokin, B. Yakobson, and L. Chernozatonskii, J. Phys. Chem. Lett. 6, 4525 (2015).
Openov L.A., Podlivaev A.I. JETP Letters 107, issue 11 (2018) NONWIENER DYNAMICS OF THE GENERALIZED DIKE MODEL AS A BROADBAND ONEPHOTON PACKET DETECTOR
Until recently, the electromagnetic field has been considered as being quantum one with few photons and classical one with quite a few of them. Then a macroscopic quantum state of a field with many photons  a squeezed field  was discovered. In addition, the reverse case was also made possible: a onephoton wave packet may not prove to be a quantum one. An effect that is very sensitive to the state of the "onequantum" object, allowing us to distinguish between the classical and quantum states of a onephoton field was found in the present work. The effect is due to the possibility of complete suppression of collective decay of an ensemble of identical excited atoms localized within the area far smaller than that of the characteristic wavelength [1]. The wellknown Dicke model is generalized for accounting the interaction with a vacuum electromagnetic field of zero photon density up to the second  order algebraic perturbation theory [1,2]. Then the effects of quantum interference of various radiation processes are correctly described, and the dynamics of the atomic ensemble is characterized as nonWiener dynamics [1]. In this work, the joint effect of a broadband onephoton wave packet and a vacuum electromagnetic field on the atomic ensemble is investigated. The master equations of nonWiener dynamics are obtained in [3]. The state of onephoton field can both be prepared in two different ways and presented in different states. If such a field interacts with a localized excited atomic ensemble under suppression of collective decay, then a strong effect is observed. The case of semiexcited atomic ensemble is calculated analytically, which shows diametrically opposite difference in the type of radiation. The quantum onephoton source produces a pulse of superradiation (collective decay), whose intensity is proportional to the square of the number of atoms of the ensemble. On the other hand, in the case of a classical onephoton source an incoherent radiation is generated, similar to that of the one generated by the emission of independent atoms. 1. A.M. Basharov, Phys. Rev. A 84, 013801 (2011). 2. A.I. Maimistov, A.M. Basharov, Nonlinear optical waves, Dordrecht: Kluwer Academic, 1999. 3. A.I. Trubilko, A.M. Basharov. JETP, 2018 (in press)
A.I. Trubilko, A.M. Basharov JETP Letters 107 , issue 9 (2018) Magnetic skyrmions in films with modulated thickness
Experimental observation of the magnetic topological states  magnetic skyrmions in chiral magnets [1] caused the rising interest to them. Such attention is motivated both by the hopes to use their unique properties (such as high mobility in electric current) in novel spintronic devices and by their topologically caused attributes interesting to the fundamental condensed matter physics, topological Hall effect for example [2]. In the chiral magnets the magnetic skyrmions are naturally stabilized by weak relativistic Dzyaloshinskii–Moriya interaction and thus, the skyrmions can exist only within a narrow temperaturefield region which hinders their application. So the search of the possibilities of the skyrmion stabilization in the common magnetic materials at room temperature is the actual problem. The idea of our work is spatially modulate the energy of the domain wall surrounding skyrmion core by nanostructurisation of the film and so artificially create the potential well (or the lattice of such wells) for the skyrmionic state. This well will prevent skyrmion transformation to the labyrinth domain structure. The first possible way to the goal is to spatially modulate the material parameters of the magnetic film [3]. In this presented work we experimentally studied the alternative way of the nanostructurisation, namely the spatial modulation of the thickness of the CoPt multilayered film with the perpendicular anisotropy. The structure is the regular lattice (period is 300 nm) of the stubs (diameters is 150 nm) etched on the surface of the film. The magnetic force microscopy allows to observe skyrmion formation in the system during the magnetizing in the uniform perpendicular field. The skyrmons stay stable even after reducing the field to zero. The magnetization curve of the system is studied both by Hall magnetometry and by magnetooptical methods. The experimentally observed topological magnetic configurations and hysteresis loops are verified by micromagnetic simulations. [1] U. K. Rossler, N. Bogdanov, and C. Pleiderer, Spontaneous skyrmion ground states in magnetic metals, Nature (London) 442, 797 (2006). [2] N. Nagaosa and Y. Tokura, Topological properties and dynamics of magnetic skyrmions, Nat. Nanotech. 8, 899 (2013). [3] M.V. Sapozhnikov, S.N. Vdovichev, O.L. Ermolaeva, N.S. Gusev, A.A. Fraerman, S.A. Gusev, Yu.V. Petrov, Artificial dense lattice of magnetic bubbles, Appl. Phys. Lett. 109, 042406 (2016).
M. V. Sapozhnikov, O. L. Ermolaeva, E.V. Skorohodov, M.N. Drozdov JETP Letters 106, issue 6 (2017) Longlived quantum vortex knots
In the bulk of a superfluid, besides wellknown and experimentally observed quantum vortex rings, theoretically there can exist (developing in time) also solitary topologically nontrivial excitations as vortex knots [13]. The simplest of them are torus knots ${\cal T}_{p,q}$, where $p$ and $q$ are coprime integers, while parameters of torus are the toroidal (large) radius $R_0$ and the poloidal (small) radius $r_0$, both sizes being large in comparison with a width of quantum vortex core $\xi$. It was believed on the basis of previously obtained numerical results that such knots are unstable and they reconnect during just a few typical times, traveling a distance of several $R_0$ (the lifetime is somewhat longer for smaller ratios $B_0=r_0/R_0$). The mentioned results were obtained for not too large ratios $R_0/\xi\lesssim 20$, and with a very coarse step (about 0.1) on parameter $B_0$. The spin kinetics of liquid 3He in contact with the microsized DyF3 powder at ferromagnetic ordering of Dy^{3+}
For the first time the magnetic phase transition in DyF_{3} at low temperatures was observed by ^{3}He NMR. The spin kinetics of liquid ^{3}He in contact with a mixture of microsized powders LaF_{3} (99.67%) and DyF_{3} (0.33%) at temperatures 1.53 K was studied by pulse NMR technique. The DyF_{3} is a dipole dielectric ferromagnet with a phase transition temperature T_{c} = 2.55 K, while as the diamagnetic fluoride LaF_{3} used as a diluent for optimal conditions for observation of ^{3}He NMR. The phase transition in DyF_{3} is accompanied by a significant changes in the magnetic fluctuation spectrum of the dysprosium ions. The spin kinetics of ^{3}He in contact with the substrate is sensitive to this fluctuations. An significant change in the rates of the longitudinal and transverse nuclear magnetization of ^{3}He in the region of magnetic ordering of solid matrix was observed. A technique is proposed for studying the static and fluctuating magnetic fields of a solid matrix at the low temperatures using liquid ^{3}He as a probe. å.í. álakshin, å.I. Kondratyeva, V.V. Kuzmin, ë.R. Safiullin, á.á. Stanislavovas, á.V. Savinkov, á.V. Klochkov, í.S. Tagirov JETP Letters 107 issue 2, 2018 Microparticles at the surface of liquid helium. Quantum version of Archimedes' principle
Microspheres at the surface of liquid are widely used now for visualization of wave and vortex motion [1, 2]. The experiments of this kind had been performed recently to study of turbulence at the surface of liquid helium [3]. That’s why it is of interest to consider the corrections to a classic Archimedes' principle, because while the size of a particle floating at the surface decreases, the forces of surface tension and molecular interaction start to play a significant role. We study the deviations from Archimedes' principle for spherical particles made of molecule hydrogen near the surface of liquid He^{4}. Classic Archimedes' principle takes place if particle radius $R_0$ is greater than capillary length of helium $L_{k} \approx $ 500 µm and the height $h_+$ of the part of the particle above He is proportional to $R_0$ . Over the range of $30 <R_0 <500$ µm Archimedes' force is suppressed by the force of surface tension and $h_{+} \sim R^{3}_{0} / L^{2}_{k}$. When $R_0<30$µm, the particle is situated under the surface of liquid helium. In this case Archimedes' force competes with Casimir force which repels the particle from the surface to the depth of liquid. The distance from the particle to the surface $h_{} \sim R^{5/3}_c / R^{2/3}_0$ if $R_0>R_c...R_c$ can be expressed as $R_c \approx (\frac {\hbar c}{\rho g}) \approx $ 1µm, $\hbar $ is Planck's constant, c is speed of light, $\rho $ is helium density. For the very small particles ( $R_0<R_c)$ $h_{}$does not depend on their size: $h_{}$=$R_c$. 1. S. V. Filatov, S. A. Aliev, A. A. Levchenko, and D. A. Khramov, JETP Letters, , 104(10), 702 (2016). 2. S. V. Filatov, D. A. Khramov, A. A. Levchenko, JETP Letters, 106(5), 330 (2017). 3. A. A. Levchenko, L. P. MezhovDeglin, A. A. Pel’menev, JETP Letters, 106(4), 252 (2017). 4. E. V. Lebedeva, A. M. Dyugaev , and P. D. Grigoriev, JETP, 110(4), 693 (2010). 5. A. M. Dyugaev, P. D. Grigoriev, and E. V. Lebedeva, JETP Letters, 89(3), 145 (2009).
A.M. Dyugaev, E.V. Lebedeva, JETP Letters, 106 issue 12, 2017 Superconductorinsulator transition in disordered NbTiN films
One of the frontiers of quantum condensed matter physics seeks to analyze and classify scenarios of the superconductorinsulator quantum phase transition (SIT). Fermionic scenario [1] rules that disorder, when strong enough, breaks down Cooper pairs thus transforming a superconductor into a metal. The further cranking up disorder strength localizes quasiparticles turning the metal into an insulator. According to Bosonic scenario [2,3] disorder localizes Cooper pairs which survive on the insulating side of the SIT and provide an insulating gap. In the Fermionic scenario, the disorderdriven SIT is a twostage transition through the intermediate state that exhibits finite resistance R_{□} and is ordinarily referred to as quantum metal. In Bosonic scenario, the SIT this intermediate state shrinks into a single point in which the resistance assumes the universal quantum resistance per square R_{c} = 6.45 kΩ/□ [3]. The disorderdriven SIT was reported in films of InO_{x} [4, 5], Be [6], TiN [7]. However, the resistance R_{c }that separates superconducting and insulating states in these films is not universal. The access and detailed study of the phases in the critical vicinity of the SIT in different materials remains one of the major challenges. Here we observe the direct disorderdriven superconductorinsulator transition in NbTiN films with R_{c} = 2.7 kΩ/□ at room temperature. We show that the increasing the film's resistance suppresses the superconducting critical temperature T_{c} in accord with the Fermion model. We find that incrementally increasing R_{□} suppresses the BerezinskiiKosterlitzThouless temperature down to zero, while the critical temperature T_{c} remains finite, which complies with the Bosonic model. Upon further increase of R_{□}, the ground state of system becomes insulating. Finally, we demonstrate that the temperature dependence of the resistance of insulating films follows the Arrhenius law. [1] A. M. Finkel'stein, Superconducting transition temperature in amorphous films, JETP Lett. 45, 46 (1987). [2] A. Gold, Dielectric properties of disordered Bose condensate, Phys. Rev. A 33, 652 (1986). [3] M.P. A. Fisher, G. Grinstein, S. Grivin, Presence of quantum diffusion in two dimensions: Universal resistance at the superconductorinsulator transition, Phys. Rev. Lett. 64, 587 (1990). [4] A. F. Hebard, M. A. Paalanen, Magneticfieldtuned superconductorinsulator transition in twodimensional films, Phys. Rev. Lett. 65, 927 (1990). [5] D. Shahar, Z. Ovadyahu, Superconductivity near the mobility edge, Phys. Rev. B 46, 10917 (1992). [6] E. Bielejec, J. Ruan, W. Wu, Anisotropic magnetoconductance in quenchcondensed ultrathin beryllium films, Phys. Rev. B 63, 1005021 (2001). [7] T. I. Baturina et al., Localized superconductivity in the quantumcritical region of the disorderdriven superconductorinsulator transition in TiN thin films, Phys. Rev. Lett. 99, 257003 (2007).
M. V. Burdastyh, S. V. Postolova, T. I. Baturina, T. Proslier, V. M. Vinokur, A.Yu. Mironov JETP Letters 106 (11) (2017)
Detection of spin excitation transfer in a 2D electron system by photoluminescence of multiparticle exciton complexes
We demonstrate that nonequilibrium spin excitations drift to macroscopically large distances in
1. Yu.A. Bychkov, S.V. Iordanskii, and G.M. Eliashberg, Twodimensional electrons in a strong
Gorbunov A.V., Kulik L.V., Kuznetsov V.A., Zhuravlev á.S., JETP Letters 106, issue 10 (2017) Helical edge transport in the presence of a magnetic impurity
Twodimensional topological insulators are have attracted much recent interest since they feature helical edge states inside their band gap [1,2]. In the absence of timereversal symmetry breaking, spinmomentum locking prohibits elastic backscattering of these helical states, i.e., the helical edge is a realization of an ideal transport channel with conductance equal to e^{2}/h. However, this theoretical prediction was not confirmed by experiments on HgTe/CdTe [36] and InAs/GaSb [7,8] quantum wells. The timesymmetric interaction of the helical states with a "quantum magnetic impurity'' (an impurity which has its own quantum dynamics) is a leading candidate for explaining these experiments. In spite of recent theoretical studies of this problem [914], several key questions has not been addressed in details. We study theoretically the modification of the ideal currentvoltage characteristics of the helical edge in a twodimensional topological insulator by weak scattering off a single magnetic impurity. As a physical realization of such a system we have in mind the (001) CdTe/HgTe/CdTe quantum well (QW) with a Mn impurity that possesses spin S=5/2. Contrary to previous works, we allow for a general structure of the matrix describing exchange interaction between the edge states and the magnetic impurity. For S=1/2 we find an analytical expression for the backscattering current at arbitrary voltage. For larger spin, S>1/2, we derive analytical expressions for the backscattering current at low and high voltages. We demonstrate that the differential conductance may exhibit a nonmonotonous dependence on the voltage with several extrema. [1] X.L. Qi, S.C. Zhang, Topological insulators and superconductors, Rev. Mod. Phys. 83, 1057 (2011). [2] M. Z. Hasan, C. L. Kane, Colloquium: Topological insulators, Rev. Mod. Phys. 82, 3045 (2010). [3] M. Konig, S. Wiedmann, C. Brune, A. Roth, H. Buhmann, L. W. Molenkamp, X.L. Qi, S.C. Zhang, Quantum spin Hall insulator state in HgTe quantum wells, Science 318, 766 (2007) [4] K. C. Nowack, E. M. Spanton, M. Baenninger, M. Konig, J. R. Kirtley, B. Kalisky, C. Ames, P. Leubner, C. Brune, H. Buhmann, L. W. Molenkamp, D. GoldhaberGordon, K. A. Moler, Imaging currents in HgTe quantum wells in the quantum spin Hall regime, Nat. Mater. 12, 787 (2013). [5] G. Grabecki, J. Wrobel, M. Czapkiewicz, L. Cywinski, S. Gieratowska, E. Guziewicz, M. Zholudev, V. Gavrilenko, N. N. Mikhailov, S. A. Dvoretski, F. Teppe, W. Knap, T. Dietl, Nonlocal resistance and its fluctuations in microstructures of bandinverted HgTe/(Hg,Cd)Te quantum wells, Phys. Rev. B 88, 165309 (2013). [6] G. M. Gusev, Z. D. Kvon, E. B. Olshanetsky, A. D. Levin, Y. Krupko, J. C. Portal, N. N. Mikhailov, S. A. Dvoretsky, Temperature dependence of the resistance of a twodimensional topological insulator in a HgTe quantum well, Phys. Rev. B 89, 125305 (2014). [7] E. M. Spanton, K. C. Nowack, L. Du, G. Sullivan, R.R. Du, K. A. Moler, Images of edge current in InAs/GaSb quantum wells, Phys. Rev. Lett. 113, 026804 (2014). [8] L. Du, I. Knez, G. Sullivan, R.R. Du, Observation of quantum spin Hall states in InAs/GaSb bilayers under broken timereversal symmetry, Phys. Rev. Lett. 114, 096802 (2015). [9] J. Maciejko, Ch. Liu, Y. Oreg, X.L. Qi, C. Wu, S.C. Zhang, Kondo effect in the helical edge liquid of the quantum spin Hall state, Phys. Rev. Lett. 102, 256803 (2009). [10] Y. Tanaka, A. Furusaki, K. A. Matveev, Conductance of a helical edge liquid coupled to a magnetic impurity, Phys. Rev. Lett. 106, 236402 (2011). [11] J. I. Vayrynen, M. Goldstein, L. I. Glazman, Helical edge resistance introduced by charge puddles, Phys. Rev. Lett. 110, 216402 (2013). [12] J. I. Vayrynen, M. Goldstein, Y. Gefen, L. I. Glazman, Resistance of helical edges formed in a semiconductor heterostructure, Phys. Rev. B 90, 115309 (2014). [13] V. Cheianov, L. I. Glazman, Mesoscopic fluctuations of conductance of a helical edge contaminated by magnetic impurities, Phys. Rev. Lett. 110, 206803 (2013). [14] L. Kimme, B. Rosenow, A. Brataas, Backscattering in helical edge states from a magnetic impurity and Rashba disorder, Phys. Rev. B 93, 081301 (2016).
Kurilovich P.D. , Kurilovich V.D., Burmistrov I.S. , Goldstein M. JETP Letters 106 (9) (2017) Breather chimeras in the system of phase oscillators
Chimera is, according to Greek mythology, a monstrous creature combining the parts of different animals (a lion with a head of a goat and a tail of a snake). Physicists recently adopted this name for complex states in nonlinear dynamical systems, where instead of an expected symmetric synchronous state one observes coexistence of synchronous and asynchronous elements [1]. Since the discovery of chimeras by Kuramoto and Battogtokh in 2002 [2], these states have been reported in numerous theoretical studies and experiments. [1] M. J. Panaggio, D. M. Abrams, Chimera states: coexistence of coherence and incoherence in networks of coupled oscillators, Nonlinearity 28 , R67 (2015). [2] Y. Kuramoto, D. Battogtokh, Coexistence of Coherence and Incoherence in Nonlocally Coupled Phase Oscillators, Nonlinear Phenom. Complex Syst. 5 , 380 (2002). [3] M. Rosenblum, A. Pikovsky, SelfOrganized Quasiperiodicity in Oscillator Ensembles with Global Nonlinear Coupling, Phys. Rev. Lett. 98 , 064101 (2007). [4] L. A. Smirnov, G. V. Osipov, A. Pikovsky, Chimera patterns in the KuramotoBattogtokh model, J. Phys. A: Math. Theor. 50 , 08LT01 (2017).
Bolotov M.I., Smirnov L.A., Osipov G.V., Pikovsky A. JETP Letters 106, issue 6 (2017) Faraday Waves and Vortices on the Surface of Superfluid HeII.
Wellknown Faraday waves can be parametrically generated on a free surface of ordinary (classical) fluids such as water or on superfluid helium HeII surface when a sample cell is vibrated vertically. Standingwave patterns appear on the surface, and their frequencies are onehalf the driving frequency. The acceleration threshold for the parametric excitation of Faraday waves on the surface of water is near an order of magnitude higher than on the surface of HeII at the same frequencies [1]. Generation of vorticity by interacting nonlinear surface waves has been predicted theoretically in a number of papers [2, 3] and generation of vortices by noncollinear gravity waves on a water surface has been observed experimentally [4].Our study has shown that classical 2D vortices can be generated by Faraday waves on the surface of superfluid HeII also, more over one can observe formation of the vortex lattice in addition to the wave lattice on the surface of HeII in a rectangular cell. Combined with predictions [5] that the sharpest features (about nm sizes) in the cell walls can induce nucleation of quantum vortex filaments and coils on the interface and formation a dense turbulent layer of quantum vortices near the solid walls with a nonclassical average velocity profile which continually sheds small vortex rings into the bulk of vibrating HeII, this opens up new prospects for studying the properties of a quantum liquid and turbulent phenomena on the surface and in bulk of supefluid liquids.
[1] Haruka Abe, Tetsuto Ueda, Michihiro Morikawa, Yu Saitoh, Ryuji Nomura, Yuichi Okuda, Faraday instability of superfluid surface, Phys. Rev. E 76, 046305 (2007).
Levchenko A.A., MezhovDeglin L. P., Pel’menev A.A. JETP Letters 106, issue 4 (2017)
Superradiance Properties of a Suspension of Composite
Nanoscale integration of organic and metallic particles is expected to open up new opportunities for the design highperformance nanoscale devices. Optimization of heterostructures requires experimental and theoretical analysis of their specific physical properties. Nanosystem consisting in gold It was found that observed effects can be explained by means of the feedback caused by the nonlinear interaction of polarizations with their total reflection in the metallic core. At low temperatures Bloch vectors related with twolevel systems form an analog of a ferromagnetic state. With increasing fluctuations, antiferromagnetic states are formed along with the desynchronization of ferromagnetic one. These properties allows us to explain the observed changes in the intensity of the and line form of laser generation with temperature. Experimental and numerical results of the work demonstrate that the synchronization of the polarization of dye molecules caused by inverse nonlinear coupling yields an analog of plasmonpolariton superradiance. 1. D.J. Bergman and M.I. Stockman, Phys.Rev.Lett. 90, 027401 (2003). 2. M. Haridas et al, J. Appl. Phys.114, 064305 (2013). 3. M. Praveena et al, Phys. Rev. B 92, 235403 (2015). A. S. Kuchyanov, A.A. Zabolotskii, Plekhanov A.I. JETP Letters 106 (2) (2017) Energy Spectrum of the Spin States in $Sr_2FeSi_2O_7$ and Origin of the Magneto Electric Coupling
Recently Sr_{2}FeSi_{2}O_{7} comes into focus as a possible compound with unusual magnetoelectric coupling or, in other words, as a novel potential multiferroic [1,2]. Results of terahertz spectroscopy in the paramagnetic state show that the multiplet Fe^{+2}(S=2) of the ground state splits due to the spinorbit coupling. However the energy intervals between the lowlying singlet state and excited states are quite small so that all spin states are populated at the temperature of about 100 K. The Fe^{+2} ion occupies the center of a tetragonally distorted tetrahedron. In the present communication the origin of the magnetoelectric coupling is described as follows. The odd crystal field from the tetrahedral environment induces the coupling of the orbital momentum of the Fe^{+2}( ^{5}D) state with the external electric field. On the other hand, the orbital momentum is coupled with spin via the spin –orbit interaction. Both angular momenta are coupled with the external magnetic field, which is enhanced due to the presence of the superexchange interaction between neighboring Fe^{+2 }ions. Combining all these couplings, the author derived the affective spin Hamiltonian for the magnetoelectric coupling, which made it possible to calculate relative intensities of the electric dipole transitions between spin states and estimate the magnetization caused by the external electric field as well as the electric polarization induced by the magnetic field.
M.V. Eremin JETP Letters 105 (11) (2017) Electrontopological transition in copperoxide highTC superconductors before superconducting transition
It is well known the conductivity of hightemperature superconductors (HTSCs) with T_{C} ~100 K (YBaCuO, BiSrCaCuO, etc.) is provided at T~300 K by hole (h) fermions [1]. It is also known the superconducting transition in such cuprates is accomplished by means of the Cooper pairing, while the fluctuating Cooper pairs with charge 2e exist even at T=T_{C}+(~30 K) [2]. Hence it inevitably follows in the interval T_{C}<T<300 K the hole Fermi surface (FS) of these HTSCs transforms into an electron one as a result of a topological transformation (the Lifshitz transition (LT) [3]. There is one of the central questions in the problem of the pseudogap state [1] of copperoxide highT_{C} superconductors: how and at what temperatures this transformation occurs. To evidence the charge carrier conversion the Hall effect is used usually. As for the BiSrCaCuO and YBaCuO, their Hall coefficients (R_{H}) have several features in the temperature range T_{C}…300 K [4,5]. The most significant of them is observed before the T_{C} in the region of fluctuation conductivity and can be interpreted as a manifestation of a scale holeelectron (he) conversion in a system of charge carriers, i.e. as the LT. However, this point of view is not universally accepted. As for the data on the transformation of the FS obtained by the ARPES (Angle Resolved Photoemission Spectroscopy) method [7], they, like [4,5], support several rearrangements of the FS, including those occurring near T_{C}. Meanwhile, it is the possibility to evidence the he conversion in a hole HTSC (the last condition is sure), which does not require either electric or magnetic fields to create the Hall potential difference. The technique developed by us [7,8] is based on the phenomenon of rearrangement of the spectrum of charge carriers in the nearsurface layer of a hole HTSC being in contact with a normal metal (Me). This phenomenon is a consequence of the annihilation of "aboriginal" hole fermions in the HTSC/Me interface with electrons penetrated from Me. The essence of this technique is the registration of changes in the resistance of the HTSC/Me interface r_{ó}, which is characterized by a small number of hole carriers. The appearance of the temperature singularities of r_{C} and the sign of r_{C } variation (dr_{ó}) make it possible to obtain an idea of the character of the changes in the system of charge carriers of the HTSC array. The dependences r_{C}(T) of the Bi(Pb)SrCaCuO/Pb and YBaCuO/In interfaces have been studied and anomalies near the temperature of the pseudogap opening and before the superconducting transition have been observed. We are shown that in Bi(Pb)SrCaCuO and YBaCuO, when the temperature T=T_{C}+(~10 K) is reached, that do not concerns to fluctuating Cooper pairs condensation. So, there is due to changing the topology of the FS. As a result, significant piece of FS becomes electronic. The most probable reason for the topological transition is the achievement of the temperature of the 2D3D crossover (the temperature of the threedimensionality of HTSC), which is a consequence of a modification in the electronic subsystem that leads to a change in the interaction mechanisms of the fluctuation Cooper pairs [9, 10]. 1. The Physics of Superconductors, Vol.1. Conventional and HighT_{C} Superconductors. Ed. by K.H. Bennemann and J.B. Katterson, Berlin, Springer, (2003). 2. K. Kawabata, S. Tsukui, Y. Shono, O. Michikami, H. Sasakura, K. Yoshiara, Y. Kakehi, T. Yotsuya, Phys. Rev. B58, 2458 (1998). 3. I.M. Lifshits, JETP 38, 1569 (1960) (in Russian). 4. Q. Zhang, J. Xia, M. Fang, Z. He, S. Wang, Z. Chen, Physica C 162164, 999 (1989). 5. A.L. Solovjov, FNT 24, 215 (1998) (in Russian). 6. T. Kondo, A.D. Palczewski, Y. Hamaya, T. Takeuchi, J.S. Wen, Z.J. Xu, G. Gu, A. Kaminski, arXive: 1208.3448v1 (2012). 7. V.I. Sokolenko, V.A. Frolov, FNô 39, 134 (2013) (in Russian). 8. V.A. Frolov, VANô, Ser.: Vacuum, pure materials, superconductors, 1, 176 (2016) (in Russian). 9. Y.B. Xie, Phys. Rev., B46, 13997 (1992). 10. A.L. Solovjov, V.M. Dmitriev, FNT 35, 227 (2009) (in Russian).
Sokolenko V.I., Frolov V.A. JETP Letters 105, issue 10 (2017)
QUANTUM GENERALIZATION OF THE THOMAS  FERMI APPROACH : EXACTLY SOLVABLE EXAMPLE
Correct allowing for the interparticle interaction in manybody systems faces considerable mathematical difficulties. The most frequently used approximation in such problems is the mean field approximation (MFA) which neglects fluctuations and the particles are considered as a continuous medium of inhomogeneous density. If , moreover, the system is described by the classical distribution function ( the statistics can be a quantum one) we obtain the well known Thomas  Fermi approach .However there are situations when at least some of the degrees of freedom of the system have to be treated in accord with quantum mechanics. Such examples are electrons in quantum wells or dipolar excitons in an electrostatic trap. In such cases the density of particles appearing in MFA is to be expressed via wave functions of a particle in the effective potential. The latter, in its turn, depends on the wave functions and occupation numbers, so one has to solve a selfconsistent problem. In case of a shortrange interparticle pair potential (2D gas of dipolar excitons) a nonlinear wave equation arises while for the longrange ( Coulomb) pair interaction the corresponding equation becomes integrodifferential (nonlocal effects). Two different systems are considered: bose  gas of dipolar excitons in a ring shape trap and fermigas of electrons in a quantum well of a MOSstructure. The trapped excitons are described by the GrossPitaevskyi nonlinear equation and for the very simple case of the rectangular potential of the “empty” trap the exact analytical solution is found. The most interesting result of this problem is criterion for existence of bound state in the effective potential ( in the one particle problem a 1D symmetric potential well always contains at least one bound state) . Methodologically instructive is the way of obtaining the eigenvalue of the GrossPitaevskyi equation: the ground state energy is found from the normalization condition. In case of electrons in a quantum well one deals with nonlinear integrodifferential equation for which the exact solution is unknown. The direct variational method was used to find the frequency of the intersubband transition. This frequency turned out to be scaled with the electron concentration N as $N^{2/3}$.
Chaplik A.V. JETP Letters 105 (9) (2017) Toward a selfconsistent theory of Fermi systems with flat bands
A model of fermion condensation, advanced more than 25 years ago, still remains the subject of hot debates, due to the fact that within its frameworks, nonFermiliquid (NFL) behavior, ubiquitously exhibited by strongly correlated Fermi systems, including electron systems of solids, is properly elucidated. The model is derived with the aid of the same Landau postulate that the ground state energy $E$ is a functional of its quasiparticle momentum distribution $n$, giving rise to the conventional Landau state. However, the model discussed deals with completely different solutions, emergent beyond a critical point, at which the topological stability of the Landau state breaks down, and therefore relevant solutions of the problem are found from the wellknown variational condition of mathematical physics $\delta E(n)/\delta n({\bf p})=\mu$ where $\mu$ is the chemical potential. Since the left side of this condition is nothing but the quasiparticle energy $\epsilon({\bf p})$, the variational condition does imply formation of the flat band or, in different words, a fermion condensate (FC). In fact, variational condition furnishes an opportunity to find solely the FC quasiparticle momentum distribution $n_*({\bf p}\in \Omega)$. New allotropes of carbon based in the C60 and C20 fullerenes with outstanding mechanical properties
Materials harder than diamond are always attract great attention from the scientists all over the world. Many attempts were made towards the synthesis especially of carbon material harder than diamond, which is the hardest possible material nowadays. A special interest belongs to materials called as fullerites. There are several experimental and theoretical works, where the synthesis and investigation of superhard fullerite were carried out. [1]–[4] Such materials reveal outstanding mechanical properties with the bulk modulus of several times higher than that of diamond. In this case the computational approaches and methods allow the theoretical investigations and prediction of a new materials with desired properties without using very expensive experimental equipment. Here we used the stateoftheart theoretical methods of computational predictions to predict new carbon phases based on the fullerene molecules of different sizes (C_{60} and C_{20}). Using the evolutionary algorithm, implemented in USPEX package, [5] we considered more than 3000 possible crystal structures to find the most stable ones. The important point, that predicted phases are based on the polymerized fullerites, displaying the superior mechanical properties. We defined the crystal structure of predicted 4 stable allotropes by simulating the XRD patterns. All predicted structures are highly symmetric. The mechanical properties were studied in details in terms of elastic tensor, bulk and shear moduli and velocities of acoustic waves. All predicted structures display elastic constants and bulk modulus very close to diamond, which allows to say that we indeed predict new superhard phases. The possible way of synthesis of such phases was proposed consisting in the cold compression of a mixture of graphite and C_{60} fullerenes. The important feature of predicted phases (besides the mechanical properties) is that they have relatively small band gap ~2.5 eV, while the cI24 phase has the direct gap of 0.53 eV. All obtained data allows the conclusion that predicted superhard semiconducting phases based on the polymerized fullerenes reveal necessary properties for applications in the electronic as basic elements.
[1] V.D. Blank, S.G. Buga, G.A. Dubitsky, N. R Serebryanaya, M.Y. Popov, and B. Sundqvist, Carbon 36, 319 (1998). [2] M. Popov, V. Mordkovich, S. Perfilov, A. Kirichenko, B. Kulnitskiy, I. Perezhogin, and V. Blank, Carbon 76, 250 (2014). [3] Y.A. Kvashnina, A.G. Kvashnin, M.Y. Popov, B.A. Kulnitskiy, I.A. Perezhogin, E.V. Tyukalova, L.A. Chernozatonskii, P.B. Sorokin, and V.D. Blank, J. Phys. Chem. Lett. 6, 2147 (2015). [4] Y.A. Kvashnina, A.G. Kvashnin, L.A. Chernozatonskii, and P.B. Sorokin, Carbon 115, 546 (2017). [5] C.W. Glass, A.R. Oganov, and N. Hansen, Comput. Phys. Commun. 175, 713 (2006).
Kvashnina Yu.A., Kvashnin D.G., Kvashnin A.G., Sorokin P.B. JETP Letters 105 ( 7) (2017)
Stochastic clustering of materials by plasma  surface interaction
Recently stochastic clustering with statistical selfsimilarity (fractality) has been found on material surface exposed under extreme plasma thermal loads in fusion devices (see [1]). In such devices, multiple processes of erosion and redeposition of the eroded material, surface melting and motion of the surface layers lead to a stochastic surface growth on the scales from tens of nanometers to hundreds of micrometers. The moving of eroded material species during redeposition from plasma and agglomeration on the surface is governed by stochastic electric fields generated by the hightemperature plasma. The specific property of the nearwall plasma in fusion device is the nonGaussian statistics of electric field fluctuations with longrange correlations [2]. It leads to the stochastic agglomerate growth with a selfsimilar structure (hierarchical granularity  fractality) of nonGaussian statistics contrary to a trivial roughness observed in ordinary processes of stochastic agglomeration. The dominant factor in such process in fusion device is the collective effect during stochastic clustering rather than the chemical element composition and physical characteristics of the solid material. In support of this view it is reported in this Letter, that such similar stochastic fractal structure with hierarchical granularity and selfsimilarity is formed on various materials, such as tungsten, carbon materials and stainless steel exposed to hightemperature plasma in fusion devices. In the literature it is discussed hypotheses of universal scalings of stochastic objects and processes with multiscale invariance property (statistical selfsimilarity), see e.g. [3]. The kinetic models propose the describing of the stochastic clustering with a selfsimilar structure and considering the power law solutions for the number N of agglomerating clusters with mass m (see e.g. [4]), N(m)=Cm^{(3+}^{h}^{)/2}, where h is a selfsimilarity exponent of the agglomeration kinetic model, C is a constant factor. It is surprisingly found in this Letter that such the power laws (with power exponents from 2.4 to 2.8) describing the roughness of the test specimens from fusion devices are strictly deviated from that of the reference samples formed in a trivial agglomeration process forming Brownianlike rough surface (such as samples exposed to lowtemperature glow discharge plasma and rough steel casting with the power law exponent in the range of 1.97 to 2.2). Statistics of stochastic clustering samples from fusion devices is typically nonGaussian and has a "heavy" tails of probability distribution functions (PDF) of stochastic surface heights (of the Hurst exponent from 0.68 to 0.86). It is contrary to the Gaussian PDF of the reference samples with trivial stochastic surface. Stochastic clustering of materials from fusion devices is characterised by multifractal statistics. Quantitative characteristics of statistical inhomogeneity of such material structure, including multifractal spectrum with broadening of 0.5 ¾ 1.2, are in the range observed for typical multifractal objects and processes in nature. This may indicate a universal mechanism of stochastic clustering of materials under the influence of hightemperature plasma.
1. V.P. Budaev et al., JETP Letters vol. 95, 2, 78 (2012). 2. V.P. Budaev, S.P. Savin, L.M. Zelenyi, PhysicsUspekhi 54 (9), 875 (2011) 3. A. L. Barabasi and H. E. Stanley, Fractal Concepts in Surface Growth (Cambridge Univ. Press, Cambridge, 1995). 4. C. Connaughton, R. Rajesh, O. Zaboronski, PRL 94 (19), 194503 (2005).
V.P. Budaev, JETP Letters vol. 105, issue 5 (2017) Fluctuational shift of nematic  isotropic phase transition temperature
Modern physics of liquid crystals is much younger than its traditional condensed matter material counterparts. Therefore the field is not yet completely elaborated and exhausted, and one may still expect discoveries of new mesogen materials exhibiting of new types of liquidcrystalline ordering. A few years ago such a discovery of socalled bentcore or dimer mesogens which can form short pitch heliconical nematic state (also known as twistbend nematics, $N_{TB}$) [1, 2], attracted a lot of interest to this new state of matter with nanoscale orientational modulation. First, to understand the nature of the phase, basically different from conventional uniform nematics and from modulated in mass density smectics (see e.g., Landau theory approach, [3,4]). Second, to exploit potentially very perspective applications of the $N_{TB}$ liquid crystals. Along this way, very recently S.M.Saliti, M.G.Tamba, S.N. Sprunt, C.Welch, G.H.Mehl, A.Jakli, J.T.Gleeson [5] observed of the unprecedentedly large magnetic field induced shift $\Delta T_c(H)$ of the nematic  isotropic transition temperature. What is even more surprising $\Delta T_c(H)$ does not follow the thermodynamics textbook wisdom prediction $H^2$ scaling. Our interpretation of such a behavior is based on singular longitudinal fluctuations of the nematic order parameter. Since these fluctuations are governed by the Goldstone director fluctuations they exist only in the nematic state. External magnetic field suppresses the singular longitudinal fluctuations of the order parameter. The reduction of the fluctuations changes the equilibrium value of the modulus of the order parameter in the nematic state, and leads to additional (with respect to the mean field contribution) fluctuational shift of the nematic  isotropic transition temperature. The mechanism works for any nematic liquid crystals, however the magnitude of the fluctuational shift increases with decrease of the Frank elastic moduli. Since some of these moduli supposed to be anomalously small for the bentcore or dimer mesogen formed nematic liquid crystals, just these liquid crystals are promising candidates for the observation of the predicted fluctuational shift of the phase transition temperature. Paradox of photons disconnected trajectories being located by means of "weak measurements" in the nested MaxZehnder interferometer
In a recent letter A. Danan et al. [A. Danan, D. Farfurnik, S. BarAd et al., Phys. Rev. Lett. 111, 240402 (2013)] have experimentally demonstrated an intriguing behavior of photons in an interferometer. Simplified layout of the experimental setup represents a nested MachZehnder interferometer (MZI) and is shown below. The surprising result is obtained when the inner MZI is tuned to destructive interference of the light propagating toward mirror F. In that case the power spectrum shows not only peak at the frequency of mirror C but two more peaks at the frequencies of mirrors A and B, and no peaks at the frequencies of mirrors E and F. From these results authors conclude that the path of the photons is not represented by connected trajectories, because the photons are registered inside the inner MZI and not registered outside it. These unusual results have raised an active discussion. Nevertheless, until now there was no comprehensive and clear analysis of the experiment within the framework of the classical electromagnetic waves approach. In this letter, we calculate the signal power spectrum at the output of the nested MZI, based on traditional concept of the classical electromagnetic waves (or quantum mechanics). This concept imply the continuity of the wave (photon) trajectories. We give intuitive clear and comprehensive explanation of paradoxical results. So, there is no necessity for a new concept of disconnected trajectories.
Simplified experimental setup with two nested MachZehnder interferometers. A, B, C, E, and F stands for mirrors; BS1 and BS2, and PBS1 and PBS2 stands for ordinary and polarized beam splitters respectively. The elements BS1, A, B, and BS2 form an inner MZI whereas the elements PBS1, C, E, F and PBS2 form an outer MZI. Various mirrors inside the MZI vibrate with different frequencies. The rotation of a mirror causes a vertical shift of the light beam reflected off that mirror. The shift is measured by a quadcell photodetector QCD. When the vibration frequency of a certain mirror appears in the power spectrum, authors conclude that photons have been near that particular mirror
G.N.Nikolaev JETP Letters 105 (3) (2017) Dark matter from dark energy in qtheory
The dynamics of the quantum vacuum is one of the major unsolved problems of relativistic quantum field theory and cosmology. The reason is that relativistic quantum field theory and general relativity describe processes well below the Planck energy scale, while the deep ultraviolet quantum vacuum at or above the Planck energy scale remains unknown. Following the condensed matter experience we develop a special macroscopic approach called qtheory, which incorporates the ultraviolet degrees of freedom of the quantum vacuum into an effective theory and allows us to study the dynamics of the quantum vacuum and its influence on the evolution of the Universe. The vacuum in our approach is considered as the Lorentzinvariant analog of a condensedmatter system (liquid or solid) which is stable in free space. The variable q is the Lorentzinvariant analog of the particle number density, whose conservation regulates the thermodynamics and dynamics of manybody systems. This approach is universal in the sense that the same results are obtained using different formulations of the qfield. In the paper, we choose the qfield in terms of a 4form field strength, which has, in particular, been used by Hawking for discussion of the main cosmological constant problem  why is the observed value of the cosmological constant many orders of magnitude smaller than follows from naive estimates of the vacuum energy as the energy of zeropoint motion. In qtheory, the huge zeropoint energy is naturally cancelled by the microscopic (transPlanckian) degrees of freedom, as follows from the GibbsDuhem identity, which is applicable to any equilibrium ground state including the one of the physical vacuum. In the paper, we consider a further extension of qtheory. We demonstrate that, in an expanding Universe, the variable effectively splits into two components. The smooth part of the relaxing vacuum field is responsible for dark energy, while the rapidly oscillating component behaves as cold dark matter. In this way, qtheory provides a combined solution to the missingmass problem and the cosmological constant problem. If this scenario is correct, the implication would be that direct searches for darkmatter particles remain unsuccessful in the foreseeable future. F.R. Klinkhamer and G.E. Volovik, JETP Letters 105, issue 2 (2017) NEW METHOD OF INVESTIGATIONS
The ability to detect nonequilibrium spin accumulation (imbalance) by all electrical means is one of the key ingredients in spintronics . Transport detection typically relies on a nonlocal measurement of a contact potential difference induced by the spin imbalance by means of ferromagnetic contacts or spin resolving detectors . A drawback of these approaches lies in a difficulty to extract the absolute value of the spin imbalance without an independent calibration. An alternative concept of a spintocharge conversion via nonequilibrium shot noise was introduced and investigated in experiment recently . Here, the basic idea is that a nonequilibrium spin imbalance generates spontaneous current fluctuations, even in the absence of a net electric current. Being a primary approach , the shot noise based detection is potentially suitable for the absolute measurement of the spin imbalance. In addition, the noise measurement can be used for a local noninvasive sensing. In this letter, we calculate the impact of a spin relaxation on the spin imbalance generated shot noise in the absence of inelastic processes. We find that the spin relaxation increases the noise up to a factor of two, depending on the ratio of the conductor length and the spin relaxation length. The design of the system. A diffusive normal wire of the length L is attached to normal islands on both ends. Nonequilibrium energy distribution on the left hand side of the wire generates the shot noise at a zero net current. The spin imbalance on the lefthand side of the wire is due to the electric current flowing from one ferromagnetic lead (red) to another one with opposite magnetization (blue).
V.S. Khrapai and K.E. Nagaev JETP Letters 105, №1 (2017)
