Altermagnetic superconducting diode effect

Abstract

Nonreciprocal superconductivity, also known as the superconducting diode effect, has been extensively studied in the presence of a magnetic field or some form of ferromagnetic order breaking time-reversal symmetry. We here show that another class of magnetic order known as altermagnetism, which also breaks time-reversal symmetry but does not exhibit a finite net magnetic moment, can also give rise to a superconducting diode effect. Whether this is the case depends on the combination of the system's point group and altermagnetic order parameter, which we explore systematically for two-dimensional crystalline systems. If the superconducting electrons are in a centrosymmetric crystalline environment, an electric field Ez (or other sources of inversion symmetry breaking) can be used to turn on and tune the nonreciprocity, yielding an electric-field-tunable diode effect; there are also noncentrosymmetric point groups, which are not reached by applying Ez≠0 in a centrosymmetric crystal, but still allow for an altermagnetic order parameter with nonreciprocal superconductivity. Depending on the residual magnetic point group, the zeros of the critical current asymmetry, Jc(n̂)−Jc(−n̂), are pinned along high-symmetry crystalline directions n̂=êj or are free to rotate in the plane of the system. In some cases, the zeros can be rotated by tuning the electric field Ez. We discuss all of these phenomena both on the general level using exact symmetry arguments and more explicitly by constructing and solving minimal lattice models. We provide experimental setups to realize the altermagnetic superconducting diode effect.

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