Altermagnetic superconducting diode effect from non-collinear compensated magnetism in Mn$_3$Pt

Altermagnets have recently emerged as a distinct class of magnetic systems that exhibit spin splitting of electronic bands while retaining zero net magnetization. This unique combination makes them a promising platform for time-reversal symmetry-breaking superconducting phenomena, although identifying concrete material platforms remains an important open challenge. Here, we develop a theory for the superconducting diode effect observed experimentally in a Mn$_3$Pt-superconductor heterostructure. Using both a symmetry analysis and model calculations on the breathing kagome lattice, we show how the altermagnetic spin textures in Mn$_3$Pt generate a spin splitting of the electronic bands that remains magnetization-free even in the presence of spin-orbit coupling and, upon taking into account the proximity coupling across the interface, produces a superconducting diode effect. We also demonstrate that the angular dependence of the critical current provides a probe of the magnetic order. We hope that our work will contribute to the understanding and further discovery of candidate materials for novel altermagnet-superconductor hybrid devices.