Emergence of Elastic Softening Featuring Ultra-Slow Dynamics Around Magnetic Critical Endpoint in UCoAl

TL;DR

This study reveals ultra-slow elastic dynamics and softening near the magnetic critical endpoint in UCoAl, highlighting the role of quadrupole degrees of freedom and unprecedented relaxation times in solids.

Contribution

It uncovers ultra-slow elastic relaxation phenomena near the CEP in UCoAl, suggesting involvement of orbital degrees of freedom beyond simple magnetic fluctuations.

Findings

Significant elastic softening near the CEP

Longest relaxation time observed in solids (3.5×10^{-8} s)

Ultrasonic dispersion linked to quadrupole effects

Abstract

We conducted an investigation on the temperature and magnetic field dependence of the elastic properties of UCoAl. The longitudinal elastic stiffness, $C_{33}$, exhibits significant softening as the system approaches the critical endpoint (CEP). This softening is indicative of an ultrasonic dispersion phenomenon, where the anomaly in the elastic constants diminishes with increasing measurement frequency. Fine structures were observed near the CEP in higher frequencies. The magnetic field dependence of $C_{33}$ can be explained by assuming a specific field dependence of the relaxation time. Remarkably, we recorded a relaxation time of 3.5$\times$10$^{-8}$ s in the vicinity of the CEP, which is the longest observed value among solids. These peculiar ultrasonic properties cannot be explained solely by Ising-like ferromagnetic fluctuations, suggesting the involvement of the quadrupole (orbital) degree of freedom in the formation of the CEP. We discussed the origin of these observed phenomena in relation to the magnetic ground state of the UCoAl system.