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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 URu$_2$Si$_2$ through the phase transition at T$_{HO}$=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 $Δ_{1,c}=2.7$ meV and $Δ_{2,c}=1.8$ meV respectively at 4 K, while the gap seen in the a-axis conductivity has a value of $Δ_a=3.2$ meV at 4 K. The opening of the gaps does not follow the behaviour expected from mean field theory in the vicinity of the transition.