Giant Bulk Photovoltaic Effect in Two-Dimensional Topological Ferroelectric Semimetal

Ferroelectric (FE) metals have attracted growing interest because they combine ferroelectricity with metallic behavior and enable polarization-controlled band topology near the Fermi level. However, the number of known ferroelectric metals is still limited. Here, we employ first-principles calculations and symmetry analysis to identify three two-dimensional (2D) ferroelectric metal candidates-XO monolayers (X = Nb, Ta, Rh)-featuring considerable out-of-plane polarization and moderate switching barriers. Notably, the polarization of RhO reaches 5.68 pC/m, significantly exceeding those of most reported 2D FE metals. The XO monolayers are also ferroelectric topological semimetals, hosting symmetry-protected Weyl points along the $S$ - $Y$ path enforced by the mirror symmetry M_y, with nontrivial edge states emerging at the projected Weyl points. We also demonstrate an intrinsically large bulk photovoltaic effect (BPVE), arising from the nontrivial band topology and the broken inversion symmetry in the ferroelectrics. Such BPVE responses can be further enhanced through charge doping and strain engineering. Our work establishes XO monolayers as a versatile 2D platform that integrates switchable ferroelectricity with topological semimetal behavior and a giant BPVE, enabling multifunctional nanoelectronic and optoelectronic applications.