Josephson effect in bipolar magnetic semiconductors

Abstract

We theoretically investigate equilibrium currents in a one-dimensional Josephson junction incorporating a bipolar magnetic semiconductor (BMS). We show that the intrinsic exchange splitting of the spin-resolved bands enables purely electrical control of the $0$--$π$ transition through gate-tunable modulation of the BMS chemical potential, eliminating the need for an external magnetic field. This provides a viable route toward electrically tunable $π$-junction behavior and highlights the potential of BMS-based Josephson devices for phase-controllable superconducting electronics. Furthermore, in the presence of Rashba spin--orbit coupling, we find an anomalous Josephson effect characterized by a finite equilibrium supercurrent at zero phase difference. This behavior originates from the intrinsic breaking of time-reversal symmetry associated with the spin-polarized electronic structure of the BMS. Interestingly, despite the simultaneous breaking of time-reversal and inversion symmetries---conditions often associated with nonreciprocal superconducting transport---we do not observe a Josephson diode effect. Our results therefore highlight an important distinction between anomalous Josephson transport and superconducting nonreciprocity: the former does not necessarily imply a finite critical-current asymmetry between opposite current directions.