  A dramatic increase in the total thermal conductivity (k) is observed in the
Hidden Order (HO) state of single crystal URu2Si2. Through measurements of the
thermal Hall conductivity, we explicitly show that the electronic contribution
to k is extremely small, so that this large increase in k is dominated by
phonon conduction. An itinerant BCS/mean-field model describes this behavior
well: the increase in kappa is associated with the opening of a large energy
gap at the Fermi Surface, thereby decreasing electron-phonon scattering. Our
analysis implies that the Hidden Order parameter is strongly coupled to the
lattice, suggestive of a broken symmetry involving charge degrees of freedom.
