Inelastic x-ray scattering measurements of phonon dynamics in URu2Si2

We report high-resolution inelastic x-ray scattering measurements of the acoustic phonons of URu2Si2. At all temperatures, the longitudinal acoustic phonon linewidths are anomalously broad at small wave vectors revealing a previously unknown anharmonicity. The phonon modes do not change significantly upon cooling into the hidden order phase. Our data suggest that the increase in thermal conductivity in the hidden order phase cannot be driven by …