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 T-0 = 17.5K 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/Gamma which is expected to diverge at a second-order phase transition. Counter to expectations the excitations at (Q) over right arrow (1) approximate to (1.4,0,0) show an abrupt steplike suppression of 1/Gamma below T-0, whereas excitations at (Q) over right arrow (0) = (1,0,0), associated with large-moment antiferromagnetism (LMAF) under pressure, show an enhancement and a pronounced peak of 1/Gamma at T-0. Therefore, at the critical HO temperature T-0, 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.