Symmetry-Programmed Pure Spin Photocurrents in Sliding Ferroelectric Altermagnets.

The bulk spin photovoltaic (BSPV) effect can generate pure spin currents under illumination, but a general symmetry rule connecting pure spin photocurrent tensors across multiple sliding ferroelectric states remains lacking. Here, we establish a symmetry-programming principle for BSPV responses in sliding ferroelectric altermagnets. By combining first-principles calculations with spin space symmetry analysis, we show that interlayer sliding maps the allowed nonlinear spin photocurrent tensors between symmetry-related ferroelectric configurations. Bilayer MnPS3 realizes a six-state triangular-lattice example with direct reversal and C3-related reorientation, whereas bilayer Fe2MoS4 provides a four-configuration square-lattice prototype governed by a C2-type connection rule. In the target transport channels, the magnetic-sublattice-connecting symmetry allows the spin photocurrent while forcing the corresponding charge photocurrent to vanish, thereby producing a pure spin photocurrent. The SOC-included calculations preserve this symmetry-enforced spin-charge decoupling and the sliding-state switching relations. These results establish a symmetry-based framework for ferroelectric tuning of optospintronic responses.