Lattice Instability of UTe${}_2$ Studied by Ultrasonic Measurements

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 ${\epsilon }_{zx}$, 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.