The Bulk Photovoltaic Effect: Origin of Shift Currents in the Many-Body Picture

The bulk photovoltaic effect (BPVE) has garnered significant attention in recent years with the shift current being the most widely studied phenomenon. Low-dimensional materials have been shown to exhibit large BPVE; however their reduced electronic screening necessitates a many-body description of the BPVE due to strong electron-hole interactions. We investigate the shift current response from a many-body perspective and show that, similar to the case in the independent particle approximation (IPA), it originates from a many-body shift vector describing shifts in charge centers of the many-body excited states. We present two equivalent interpretations of the shift current connected by a sum-rule. The shift vector can be interpreted as a sum of virtual transitions dominated by nearly-degenerate optically-active excitons overlapping in reciprocal space. The many-body sum-rule provides a tractable way to compute the position matrix elements involving cumbersome wave-vector derivatives. We predict that the shift current can be enhanced by transitions between nearly-degenerate, tightly-bound bright excitons. Ab initio results for the many-body shift current and shift vector are presented for CdS and MoS2. Exciton-phonon coupling is included through the exciton-phonon self-energies.

Lai, M., Xuan, F., & Quek, S. Y. arXiv preprint arXiv:2402.02002 (2024).