Dramatic elastic response at the critical end point in UTe$_2$

TL;DR

This study reveals a dramatic elastic response at the critical end point in UTe$_2$, showing lattice instability driven by magnetic fluctuations, with potential universal behavior among itinerant electron metamagnets.

Contribution

The paper uncovers a universal elastic anomaly at the critical end point in UTe$_2$ and UCoAl, linking magnetic fluctuations to lattice instability in itinerant electron metamagnets.

Findings

V-shaped anomaly in elastic constants at critical field

Anomaly maximized at the critical-end-point temperature

Similar elastic anomalies observed in UTe$_2$ and UCoAl

Abstract

The first-order transition line in the \textit{H-T} phase diagram of itinerant electron metamagnets terminates at the critical end point-analogous to the critical point on the gas-liquid condensation line in the \textit{p-T} phase diagram. To unravel the impact of critical magnetic fluctuations on the crystal lattice of a metamagnet at the critical end point, we performed an ultrasonic study of the itinerant electron metamagnet UTe$_2$ across varying temperatures and magnetic fields. At temperatures exceeding 9 K, a distinct V-shaped anomaly emerges, precisely centered at the critical field of the metamagnetic transition in the isothermal field dependence of elastic constants. This anomaly arises from lattice instability, triggered by critical magnetic fluctuations via strong magnetoelastic interactions. Remarkably, this effect is maximized precisely at the critical-end-point temperature. Comparative measurements of another itinerant metamagnet, UCoAl, reveal intriguing commonalities. Despite significant differences in the paramagnetic ground state, lattice symmetry, and the expected metamagnetic transition process between UTe$_2$ and UCoAl, both exhibit similar anomalies in elastic properties near the critical end point. These shared aspects may hold universality for other itinerant electron metamagnets.