Role of commensurate and incommensurate low-energy excitations in the paramagnetic to hidden-order transition of URu2Si2.

We report low-energy inelastic neutron scattering data of the paramagnetic (PM) to hidden-order (HO) phase transition at T0 = 17.5 K in URu2Si2. While confirming previous results for the HO and PM phases, our data reveal a pronounced wave-vector dependence of low-energy excitations across the phase transition. To analyze the energy scans we employ a damped harmonic oscillator model containing a fit parameter 1/Τ which is expected to diverge at a second-order phase transition. Counter to expectations the excitations at Q0 ≈ (1.4,0,0) show an abrupt steplike suppression of 1/Τ below T0, whereas excitations at Q0 = (1,0,0), associated with large-moment antiferromagnetism (LMAF) under pressure, show an enhancement and a pronounced peak of 1/Τ at T0. Therefore, at the critical HO temperature T0, LMAF fluctuations become nearly critical as well. This is the behavior expected of a "supervector" order parameter with nearly degenerate components for the HO and LMAF leading to nearly isotropic fluctuations in the combined order-parameter space. [ABSTRACT FROM AUTHOR]