Altermagnetism-induced noncollinear superconducting diode effect and unidirectional superconducting transport

Current studies of nonreciprocal 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 the 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 noncollinear 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.