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 FePSe$_3$ under moderate pressures. This layered material is a Mott insulator with antiferromagnetic order under ambient conditions. We show that FePSe$_3$ 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 magnetic ordering at the insulator-to-metal transition correspondent with this structural change. Our work outlines a comprehensive temperature-pressure phase diagram of FePSe$_3$, combining detailed structural, magnetic and transport data. The high-pressure phase exhibits activated semiconductor behavior at high temperatures, a $T^2$-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 FePSe$_3$ 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.

💡 Research Summary

The authors investigate the pressure‑induced evolution of the van‑der‑Waals Mott insulator FePSe₃, a layered honeycomb antiferromagnet at ambient conditions. Using a combination of synchrotron X‑ray diffraction (both powder and single‑crystal), neutron diffraction, and electrical transport measurements under hydrostatic pressure, they construct a detailed temperature–pressure phase diagram and identify a novel high‑pressure phase that simultaneously exhibits metallic conductivity and a non‑centrosymmetric (polar) crystal structure—a “polar metal”.

Key structural findings: Between 7.6 and 8.1 GPa the (00l) diffraction peaks shift abruptly, indicating a ~9 % collapse of the interlayer spacing (c‑axis). The a‑b lattice parameters change only smoothly, confirming that the transition is dominated by a c‑axis compression rather than a reconstruction of the honeycomb network. Single‑crystal data show that the (hhl) reflections separate dramatically along