Pure Spin Photocurrent in Non-centrosymmetric Crystals: Bulk Spin Photovoltaic Effect

The shift and injection current mechanisms can generate charge currents under light illumination via nonlinear optical (NLO) interaction. We reveal that the similar mechanisms can be applied to spin currents and demonstrate a unified picture of NLO charge and spin currents generation. Symmetry analysis reveals that the NLO spin current can be generated under both linearly and circularly polarized light in any non-centrosymmetric materials regardless of whether time-reversal symmetry (magnetism) exists or not. And a pure spin current can be realized if the system possesses additional mirror symmetry or inversion-mirror symmetry. We apply our theory to several distinct material systems with easy optical accessibility, namely, two-dimensional transition metal dichalcogenides (TMD), anti-ferromagnetic MnBi₂Te₄ (MBT), and the surface states of topological material SnTe. The spin current conductivity is found to be gigantic, often exceeding the charge current conductivity. Such bulk spin photovoltaic effect (BSPE) could find wide applications in energy efficient and ultrafast spintronic devices.