Magnetoelastic Coupling In Uru2si2: Probing Multipolar Correlations In The Hidden Order State

Time-reversal symmetry and magnetoelastic correlations are probed by means of high-resolution volume dilatometry in URu2Si2 at cryogenic temperatures, and magnetic fields sufficient to suppress the hidden order state at H-HO (T = 0.66 K)similar or equal to 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 Delta L/L proportional to 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 (Delta L/L proportional to 1 - H/H-0) for 15 T < H < H-HO (0.66 K) similar to 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.