Time-reversal symmetry and magnetoelastic correlations are probed by means of high-resolution volume dilatometry in ${\mathrm{URu}}_2{\mathrm{Si}}_2$ at cryogenic temperatures, and magnetic fields sufficient to suppress the hidden order state at $H_{\mathrm{HO}}(T=0.66\mathrm{K})\simeq 35$ T. We report a significant magnetoelastic volume expansion at and above $H_{\mathrm{HO}}\left(T\right)$, and even above $T_{\mathrm{HO}}$, possibly a consequence of field-induced $f$-electron localization. We investigate in detail the magnetostriction and magnetization as the temperature is reduced across two decades in temperature from 30 K where the system is paramagnetic, to 0.5 K in the realm of the hidden order state. We find a dominant quadratic-in-field dependence $\mathrm{\Delta }L/L\propto H^2$, a result consistent with a state that is symmetric under time reversal. The data shows, however, an incipient yet unmistakable asymptotic approach to linear ($\mathrm{\Delta }L/L\propto 1-H/H_0$) for $15\text{T}<H<H_{\mathrm{HO}}\left(0.66\text{K}\right)\sim 40$ T at the lowest temperatures. We discuss these results in the framework of a Ginzburg-Landau formalism that proposes a complex order parameter for the HO phase to model the $(H,T,p)$ phase diagram.

• Received 31 August 2018
• Revised 21 March 2019

DOI:https://doi.org/10.1103/PhysRevB.99.235101