Observation of multiple-gap structure in hidden order state of URu2Si2 from optical conductivity

We have measured the far-infrared reflectance of the heavy-fermion compound URu2Si2 through the phase transition at THO=17.5 K dubbed “hidden order” with light polarized along both the a and c axes of the tetragonal structure. The optical conductivity allows the formation of the hidden order gap to be investigated in detail. We find that both the conductivity and the gap structure are anisotropic, and that the c-axis conductivity shows evidence for a double gap structure, with Δ1,c=2.7 meV and Δ2,c=1.8 meV, respectively, at 4 K, while the gap seen in the a-axis conductivity has a value of Δa=3.2 meV at 4 K. The opening of the gaps does not follow the behavior expected from mean-field theory in the vicinity of the transition.