Magnetic flux locking in two weakly coupled superconducting rings

Abstract

We have analyzed the quantum interference effects in the macroscopic "superconducting molecule". The composite system consists of two massive superconducting rings, each interrupted by a Josephson junction, which are at the same time weakly coupled with one another. The special case of coupling via the Josephson four-terminal junction is considered. The structure of the macroscopic quantum states inan applied magnetic field is calculated. It is shown that, depending on the values of the magnetic fluxes through each ring, the system displays two groups of states, the "orthostates" with both induced currents going in the same directions, and the "parastates" which the opposite currents and with the total induced flux locked to zero value. The transition to the flux locked state with changing of the total applied flux is sudden and is preserved in a certain interval which is determained by the difference of the fluxes applied through each rings. It makes the system sensitive to small gradients of the external magnetic field.