Magnetoelastic coupling in URu2Si2: Probing multipolar correlations in the hidden order state

Abstract

Time-reversal symmetry and magnetoelastic correlations are probed by means of high-resolution volume dilatometry in URu 2 Si 2 at cryogenic temperatures, and magnetic fields sufficient to suppress the hidden order state at H HO ( T = 0.66 K ) ≃ 35 T. We report a significant magnetoelastic volume expansion at and above H HO ( T ) , and even above T 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 Δ L / L ∝ 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 ( Δ L / L ∝ 1 − H / H 0 ) for 15 T < H < H HO ( 0.66 K ) ∼ 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.