Emergent Polar Metal Phase in a Van der Waals Mott Magnet

We report the emergence of a two-dimensional (2D) polar metal phase in van der Waals compound FePSe3 under moderate pressures. This layered material is a Mott insulator with antiferromagnetic order under ambient conditions. We show that FePSe3 uniquely allows tuning a 2D correlated insulator into an exotic metal state where a loss of inversion symmetry leads to periodic polar displacements of ions, within a conducting phase - a polar metal. Our combined synchrotron and neutron diffraction data allow us to present a long-sought, unambiguous high-pressure structural model and show the polar displacements of this new phase. We also observe the suppression of mag- netic ordering at the insulator-to-metal transition correspondent with this structural change. Our work outlines a comprehensive temperature-pressure phase diagram of FePSe3, combining detailed structural, magnetic and transport data. The high-pressure phase exhibits activated semiconductor behavior at high temperatures, a T2-dependence in its resistivity at lower temperatures - despite the conditions required for a ‘good metal’ Fermi-Liquid description not being met in this case - and a low-temperature resistivity upturn which is suppressed as the system is tuned away from the concomitant transitions. The realisation of a tunable 2D polar metal state in FePSe3 due to the loss of its inversion symmetry combined with pressure-induced metallicity offers a promising new platform to investigate this exotic phase at accessible pressures.