Altermagnetism-induced non-collinear superconducting diode effect and unidirectional superconducting transport

TL;DR

This paper proposes a novel superconducting diode effect induced by altermagnetism, enabling non-reciprocal transport with preserved superconducting gap and potential for energy-efficient electronic devices.

Contribution

It introduces the use of altermagnets coupled with s-wave superconductors to achieve a resilient, symmetry-protected superconducting diode effect with controllable anisotropy.

Findings

Altermagnet coupling induces a non-collinear SC-diode effect with fourfold symmetry.

Transition to FF state results in unidirectional ($C_1$) anisotropy.

Superconducting gap remains sizable without significant suppression.

Abstract

Current studies of non-reciprocal superconducting (SC) transport have centered on the forward-backward asymmetry of the critical current measured along a single axis. In most realizations, this diode effect is achieved via introducing ferromagnetism or applying an external magnetic field, which drives system into an effective Fulde-Ferrell (FF) state but often at the cost of severely suppressing the SC gap and thus compromising device robustness. Here we propose and theoretically demonstrate that coupling a conventional $s$-wave SC thin film to a $d$-wave altermagnet offers a more resilient alternative. The momentum-dependent spin splitting inherent to altermagnets induces a non-collinear SC-diode effect in the BCS state, with the critical-current anisotropy exhibiting a fourfold ($C_4$) symmetry. Upon entering the FF state at large splitting, this anisotropy gradually evolves into a unidirectional ($C_1$) pattern. Crucially, the FF pairing momentum locks to the discrete crystal axes, eliminating the rotational Goldstone mode and preserving a sizable SC gap without any abrupt or significant suppression. These combined features make the altermagnetic proximity an appealing platform to engineer symmetry-protected, energy-efficient and programmable SC diodes for next-generation electronic devices.