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.
Categories: cond-mat.supr-con cond-mat.str-el