Far-Infrared Properties of ${\mathrm{URu}}_2$${\mathrm{Si}}_2$

Measurements of the far-infrared reflectance of ${\mathrm{URu}}_2$${\mathrm{Si}}_2$ between 10 and 720 ${\mathrm{cm}}^{-1\mathrm{}}$ have been made at temperatures from 2 to 90 K. Above the coherence temperature the optical conductivity increases monotonically with increasing frequency and at lower temperatures the development of the narrow mode responsible for the high dc conductivity is clearly observed. In the antiferromagnetic state an energy gap with a size between 46 and 65 ${\mathrm{cm}}^{-1\mathrm{}}$ is observed whose shape is reminiscent of the energy gap observed in the spin-density-wave state of Cr.