Role of commensurate and incommensurate low-energy excitations in the paramagnetic to hidden-order transition of URu$_2$Si$_2$

TL;DR

This study uses inelastic neutron scattering to investigate low-energy excitations in URu$_2$Si$_2$ during the transition from paramagnetic to hidden order, revealing wavevector-dependent behavior and nearly critical fluctuations of competing orders.

Contribution

It provides new insights into the wavevector dependence and critical behavior of excitations across the phase transition, suggesting a super-vector order parameter for hidden order and antiferromagnetism.

Findings

Excitations at $oldsymbol{Q}_1$ show suppression below $T_0$.

Excitations at $oldsymbol{Q}_0$ peak at $T_0$, indicating critical LMAF fluctuations.

LMAF fluctuations become nearly critical at the transition temperature.

Abstract

We report low-energy inelastic neutron scattering data of the paramagnetic (PM) to hidden-order (HO) phase transition at $T_0=17.5\,{\rm K}$ in URu$_2$Si$_2$. While confirming previous results for the HO and PM phases, our data reveal a pronounced wavevector 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 $\vec{Q}_1=(1.44,0,0)$ show an abrupt step-like suppression of $1/\Gamma$ below $T_0$, whereas excitations at $\vec{Q}_0=(1,0,0)$, 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 super-vector order parameter with nearly degenerate components for the HO and LMAF leading to nearly isotropic fluctuations in the combined order-parameter space.