Bulk Photovoltaic Effect in Two-Dimensional Perovskite Oxides

Perovskite oxides ABO${}_3$ host a rich interplay of charge, spin, lattice, and orbital degrees of freedom, giving rise to diverse quantum phenomena. In low-dimensional ABO${}_3$, reduced symmetry can induce exotic quantum effects such as the two-dimensional electron gas and unconventional superconductivity. Using first-principles density functional theory, tight-binding modeling, and symmetry analysis, we show that ultrathin two-dimensional (2D) ABO${}_3$ films – exemplified by SrTiO${}_3$ – naturally break inversion symmetry, producing a spontaneous out-of-plane bulk photovoltaic (BPV) effect. This differs from previous studies on in-plane BPV current signals and is more applicable and experimentally detectable. Such an effect is highly tunable via thickness, strain, surface termination, crystallographic orientation, and Moiré twisting. These findings are broadly applicable to a wide range of 2D perovskite and other layer-resolved oxides.