Superconducting system with current in the ground state
Institute of Theoretical Physics, ETH, Zurich, Switzerland
Theoretical Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA
About the paper SUPERCONDUCTING SYSTEM WITH WEAK COUPLING TO CURRENT IN GROUND-STATE by BULAEVSKII L.N., KUZII V.V., SOBYANIN AA
In the paper Pis'ma Zh.Eksp. Teor. Fiz. 25, 314, 1977, Bulaevskii, Sobyanin and Kuzii (BSK) introduced conception of SIS $\pi$ junction consisting two singlet s wave pairing superconductors S separated by the dielectric I with magnetic impurities. This advance was based on previous finding by Kulik (Zh. Eksper. Teor. Fiz. 49, 1211, 1965) that magnetic impurities inside dielectric I in SIS junctions diminish the superconducting critical current. The physical reason for that is that tunneling of Cooper pair with opposite spins of electrons gives negative contribution to the critical current (and thus to junction superconducting energy) as compared with contribution of tunneling via nonmagnetic atoms. BSK argued that if tunneling via magnetic atoms becomes dominant over nonmagnetic ones, critical current will be negative resulting in the junction ground state with the phase difference $\pi$ instead of 0 as in ordinary junctions. In both cases, $\pi$ or 0 junctions, supercurrent in the ground state vanishes. However, BSK showed that if one connects (shorts) the superconducting electrodes with the inductance L (e.g. superconducting wire), one may expect the spontaneous supercurrent circulating in the loop, passing through the junction and through inductance clockwise or counterclockwise. This supercurrent is spontaneous and direction of its circulation is chosen at random. Such a supercurrent will induce a magnetic field which can be detected experimentally. The magnetic flux passing through the loop will have the value in the range from 0 to a half of magnetic flux quanta, i.e. from 0 to $\Phi_0/2$, depending on the value of inductance $L$.
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