Lattice Instability of UTe$_2$ Studied by Ultrasonic Measurements

TL;DR

This study investigates the elastic properties of the unconventional superconductor UTe$_2$ using ultrasonic measurements, revealing a specific lattice instability associated with certain elastic modes at low temperatures.

Contribution

First ultrasonic investigation of UTe$_2$ revealing mode-dependent elastic softening indicating lattice instability.

Findings

Elastic constant $C_{55}$ softens at low temperatures

No softening observed in $C_{44}$ and $C_{33}$ modes

Evidence of lattice instability in UTe$_2$

Abstract

The elastic constants of an unconventional superconductor, UTe$_2$, were investigated using ultrasound. In this paper, we report the elastic response of the normal state at temperatures down to 2 K and up to 14 T for $H \parallel b$ at ambient pressure. The transverse ultrasonic mode $C_{55}$, which corresponds to the strain susceptibility of $\varepsilon_{zx}$, shows softening with decreasing temperature, whereas the $C_{44}$ and $C_{33}$ modes do not show such softening. This clear mode dependence strongly suggests that UTe$_2$ has a lattice instability for this specific symmetry.