Pure momentum-shift bulk photovoltaic effect in ferroelectric flat-band Mott insulators

The shift current photovoltaic effect is conventionally understood as the real-space displacement of a wave packet induced by photoexcitation. However, this interpretation becomes insufficient in flat-band systems, where quasiparticles are too massive to accelerate in real space under the optical electric field. Here, we developed a physically consistent method to decompose the shift current into real-space and momentum-space components. A surprising pure momentum-space shift current is found theoretically in flat-band Mott insulator Nb$_3$X$_8$ (X = Cl, Br, I) monolayers. This work underscores that significant shift current responses can emerge even in systems with minimal interband polarization differences, highlighting the potential for exploring novel bulk photovoltaic effects in flat-band Mott insulators.