# Highly responsive ground state of PbTaSe$_2$: structural phase transition and evolution of superconductivity under pressure

## TL;DR

This study reveals a pressure-induced structural phase transition in PbTaSe$_2$ that significantly affects its superconducting properties, including the existence of two distinct superconducting phases and a sharp change in transition temperature.

## Contribution

It uncovers a first-order structural phase transition in PbTaSe$_2$ under pressure and links it to changes in superconductivity, supported by experimental and first-principles calculations.

## Key findings

- Identification of a first-order structural phase transition near 0.12 GPa.
- Observation of two superconducting phases separated by the transition.
- A ~1 K decrease in $T_c$ across the transition.

## Abstract

Transport and magnetic studies of PbTaSe$_2$ under pressure suggest existence of two superconducting phases with the low temperature phase boundary at $\sim 0.25$ GPa that is defined by a very sharp, first order, phase transition. The first order phase transition line can be followed via pressure dependent resistivity measurements, and is found to be near 0.12 GPa near room temperature. Transmission electron microscopy and x-ray diffraction at elevated temperatures confirm that this first order phase transition is structural and occurs at ambient pressure near $\sim 425$ K. The new, high temperature / high pressure phase has a similar crystal structure and slightly lower unit cell volume relative to the ambient pressure, room temperature structure. Based on first-principles calculations this structure is suggested to be obtained by shifting the Pb atoms from the $1a$ to $1e$ Wyckoff position without changing the positions of Ta and Se atoms. PbTaSe$_2$ has an exceptionally pressure sensitive, structural phase transition with $\Delta T_s/\Delta P \approx - 1700$ K/GPa near 4 K, this first order transition causes an $\sim 1$ K ($\sim 25 \%$) step - like decrease in $T_c$ as pressure is increased through 0.25 GPa.