Anomalous behavior of the quasi-one-dimensional quantum material Na$_{2}$OsO$_{4}$ at high pressure

TL;DR

This study investigates the high-pressure behavior of Na$_{2}$OsO$_{4}$, revealing an unexpected increase in insulating properties due to enhanced lattice distortion, with implications for understanding pressure-induced electronic transitions in quantum materials.

Contribution

The paper combines experimental and theoretical approaches to demonstrate that increased lattice distortion under high pressure enhances insulating behavior in Na$_{2}$OsO$_{4}$, contrary to typical expectations.

Findings

An anomaly at 6.3 K indicating a phase transition.

The material becomes more insulating before metallization at 23 GPa.

Enhanced OsO$_{6}$ distortion widens the band gap and reduces electron occupancy.

Abstract

Na$_{2}$OsO$_{4}$ is an unusual quantum material that, in contrast to the common 5${d}^{2}$ oxides with spins = 1, owns a magnetically silent ground state with spin = 0 and a band gap at Fermi level attributed to a distortion in the OsO$_{6}$ octahedral sites. In this semiconductor, our low-temperature electrical transport measurements indicate an anomaly at 6.3 K with a power-law behavior inclining through the semiconductor-to-metal transition observed at 23 GPa. Even more peculiarly, we discover that before this transition, the material becomes more insulating instead of merely turning into a metal according to the conventional wisdom. To investigate the underlying mechanisms, we applied experimental and theoretical methods to examine the electronic and crystal structures comprehensively, and conclude that the enhanced insulating state at high pressure originates from the enlarged distortion of the OsO$_{6}$. It is such a distortion that widens the band gap and decreases the electron occupancy in Os's ${t}_{2g}$ orbital through an interplay of the lattice, charge, and orbital in the material, which is responsible for the changes observed in our experiments.