Crystal symmetry selected pure spin photocurrent in altermagnetic insulators

The generation of time-reversal-odd spin current in metallic altermagnets has attracted considerable interest in spintronics. However, producing pure spin current in insulating materials remains both challenging and desirable, as insulating states are frequently found in antiferromagnets. Nonlinear photogalvanic effects offer a promising method for generating spin current in insulators. We here reveal that spin and charge photocurrents in altermagnets are protected by spin point group symmetry. The shift and injection current mechanisms simultaneously induce spin-momentum locking in a specific direction in altermagnets regardless of spin-orbit coupling (SOC), exhibiting pure spin current. This stands in contrast to parity-Time symmetric antiferromagnets, where SOC induces a charge current alongside the spin current, thereby preventing the generation of a pure spin current. We applied our predictions using first-principles calculations to several distinct materials, including wurtzite MnTe and multiferroic BiFeO3. Additionally, we elucidated the linear-inject-current mechanism in BiFeO3 induced by SOC, which may account for the enhanced bulk photovoltaic effect in multiferroics.