  The pursuit for "ferroelectric metal" which combines seemingly incompatible
spontaneous electric polarization and metallicity, has been assiduously ongoing
but remains elusive. Unlike traditional ferroelectrics with a wide band gap,
ferroelectric (FE) metals can naturally incorporate nontrivial band topology
near the Fermi level, endowing them with additional exotic properties. Here, we
show first-principles evidence that the metallic PtBi2 monolayer is an
intrinsic two-dimensional (2D) topological FE metal, characterized by
out-of-plane polarization and a moderate switching barrier. Moreover, it
exhibits a topologically nontrivial electronic structure with Z2 invariant
equal to 1, leading to a significant FE bulk photovoltaic effect. A slight
strain can further enhance this effect to a remarkable level, which far surpass
that of previously reported 2D/3D FE materials. Our work provides an important
step towards realizing intrinsic monolayer topological FE metals and paves a
promising way for future nonlinear optical devices.
