Large Resistivity Reduction in Mixed-Valent CsAuBr$_3$ Under Pressure

TL;DR

This study investigates how applying high pressure drastically reduces resistivity in CsAuBr₃, revealing a transition from insulator to metal and then to semiconducting behavior, driven by band structure changes related to valence state transitions.

Contribution

It provides the first detailed high-pressure resistivity measurements on CsAuBr₃, identifying a large resistivity drop and elucidating the electronic band structure evolution responsible for this behavior.

Findings

Resistivity decreases by over 6 orders of magnitude below 10 GPa.

Transition from insulator to metal occurs below 10 GPa.

Resistivity stabilizes at high pressures above 14 GPa.

Abstract

We report on high-pressure $p \leq 45$ GPa resistivity measurements on the perovskite-related mixed-valent compound CsAuBr$_3$. The compounds high-pressure resistivity can be classified into three regions: For low pressures ($p < 10$ GPa) an insulator to metal transition is observed; between $p= 10$ GPa and 14 GPa the room temperature resistivity goes through a minimum and increases again; above $p = 14$ GPa a semiconducting state is observed. From this pressure up to the highest pressure of $p = 45$ GPa reached in this experiment, the room-temperature resistivity remains nearly constant. We find an extremely large resistivity reduction between ambient pressure and 10 GPa by more than 6 orders of magnitude. This decrease is among the largest reported changes in the resistivity for this narrow pressure regime. We show - by an analysis of the electronic band structure evolution of this material - that the large change in resistivity under pressure in not caused by a crossing of the bands at the Fermi level. We find that it instead stems from two bands that are pinned at the Fermi level and that are moving towards one another as a consequence of the mixed-valent to single-valent transition. This mechanism appears to be especially effective for the rapid buildup of the density of states at the Fermi level.