Shift current response in elemental two-dimensional ferroelectrics

A bulk material without inversion symmetry can generate a direct current under illumination.This interface-free current generation mechanism,referred to as the bulk photovoltaic effect(BPVE),does not rely on p-n junctions.Here,we explore the shift current generation,a major mechanism responsible for the BPVE,in single-element two-dimensional(2D)ferroelectrics represented by phosphorene-like monolayers of As,Sb,and Bi.The strong covalency,small band gap,and large joint density of states afforded by these elemental 2D materials give rise to large shift currents,outperforming many state-of-the-art materials.We find that the shift current,due to its topological nature,depends sensitively on the details of the Bloch wave functions.It is crucial to consider the electronic exchange-correlation potential beyond the generalized gradient approximation as well as the spin-orbit interaction in density functional theory calculations to obtain reliable frequency-dependent shift current responses.