Field-free Josephson diode effect in altermagnet/normal metal/altermagnet junctions

Abstract

The field-free and highly efficient diodes with the nonreciprocity of supercurrent are believed to be the core block of superconducting computing devices without dissipation. In this paper, we propose a Josephson diode based upon altermagnets with vanishing net macroscopic magnetization. The nonreciprocity of supercurrent can be realized without applying any external magnetic field or ferromagnetic exchange field, which can avoid the magnetic cross-talk between the basic elements of the devices. A high efficiency exceeding $40%$ can be obtained, and the efficiency shows high stability when the structure parameters are changed. The diode efficiency is antisymmetric about the relative orientation angle of the superconducting leads, so that its sign can easily be inverted by adjusting the relative orientation angle. The symmetries satisfied by the current-phase difference relations and the diode efficiency are analyzed by considering the transformations of the junctions under the time reversal, the spin rotation, and the mirror reflection operations. The high efficiency and the high stability of the Josephson diode effect in our junctions make it possible to design field-free dissipationless diode devices.