Emergent Polar Metal Phase in a Van der Waals Mott Magnet

Abstract

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.

Figures (7)

• Figure 1: Tuning a Centrosymmetric Mott Insulator to a Correlated Polar Metal This illustration demonstrates our key findings: tuning of a 2D material from a magnetic insulator (left) to a non-magnetic polar metal (right) via pressure. The latter is a special state that not only exhibits ordered displacements of polar ions, but also free carriers which rearrange so as to screen long-range electrical polarisation.
• Figure 2: Powder and Single-Crystal Synchrotron Diffraction Results and Lattice Parameters Evolution under Pressure (a) Integrated diffraction intensities as a function of $2\theta$ for FePSe$_3$ powder sample in a DAC loaded with helium gas as a pressure-transmitting medium (PTM). For each pressure, the intensity was normalized to its maximum, and then an offset proportional to pressure values was added. Blue and orange represent data assigned to the LP and HP phases, respectively. (b) Slices of ($h\overline{h}l$) planes at the lowest pressure (1.4 GPa, LP), before (6.7 GPa, LP) and after (7.8 GPa, LP) the phase transition pressure. The sub-pictures are all scaled in Å$^{-1}$. The red and blue lines are given to guide the eye to show the separation distance at the 1.4 GPa, LP phase. The yellow color blocks highlight the difference in the separation of $c^*$. A zoom-in view is provided at 7.8 GPa to better illustrate the abrupt changes in $c^*$. (c) Pressure-induced relative changes of the lattice parameters in reference to the ambient-pressure structure. Orange and blue represent powder and single crystal cases, respectively. The grey-shaded region indicates the interplanar lattice collapse transition pressure.
• Figure 3: Crystalline structures of the ambient-/low-pressure (LP) and high-pressure (HP) phases of FePSe$_3$ Fe: brown; P: grey; Se: green. (a, b) are the structural isometric view with a minimal stacking unit of $ABC$ triple layers; (b, e) display the view normal to the vdW planes, with the red solid lines indicating the Fe-honeycomb network; (c, f) present the FeSe$_6$ and P$_2$Se$_6$ octahedra along the axis parallel to c. The cartoon aside depicts Se planes in both phases. The light green and dark green with + and - signs represent the Se atoms in the plane above or below the plane defined by Fe or P$_2$ dimer within each vdW layer. The collective displacement of Se atoms in the HP phase is demonstrated with different colors and arrows. (g) shows the normal view of the three individual stacking layers in $ABC$ ordering. The collective rotation of each Se plane is indicated with arrows in different colors.
• Figure 4: Powder Neutron Diffraction Patterns and Magnetic Structure under Pressure (a) Diffraction data taken at temperatures well above (red) and below (blue) the magnetic transition temperature at pressures of 1.2, 4.1 and 8.3 GPa after background subtraction. Nuclear, magnetic peaks for FePSe$_3$, and peaks originating from the diamond anvils are indicated with black, cyan and green, respectively. (b) Resulting diffraction intensity obtained by subtracting the intensity at high temperature (red) from that at low temperature (blue) for each pressure point. Magnetic peaks consistent with the ambient-pressure $(\frac{1}{2},0,\pm\frac{1}{2})$ magnetic propagation vectors are marked with orange arrows. Solid black lines represent refinements of the data. (c) Magnetic ordering of Fe atoms in the LP phase with the nuclear unit cell, in solid black lines as a reference. The black and white arrows on $\rm Fe^{2+}$ atoms show opposing spin moments, parallel to the $c$-axis. Not all magnetic atoms are shown in the magnetic cell, which are four times the nuclear cell.
• Figure 5: Single-Crystal Resistivity Measurements under Pressure (a) The resistivity $\rho$ of a FePSe$_3$ single crystal, loaded in a Cubic Anvil Cell with glycerin as the pressure-transmitting medium. (b) Normalized and offset resistivity as a function of $T^2$, a Fermi-liquid temperature power law, below $\sim$40 K for FePSe$_3$. The fitted curves are plotted in solid black lines with the best-fitting $T^2$ region marked with arrows for each pressure.