far-infrared optical properties of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$ Ca${\mathrm{Cu}}_2$${\mathrm{O}}_8$

The reflectance of crystals of ${\mathrm{Bi}}_2$${\mathrm{Sr}}_2$ Ca${\mathrm{Cu}}_2$${\mathrm{O}}_8$ has been measured in the far-infrared region at 100 K in the normal state and at 1.5 K in the superconducting state. The normal-state properties are dominated at low frequency by a narrow Drude-like peak in the real part of the optical conductivity. At higher frequency there is substantial absorption above this Drude absorption that can be parametrized with two broad oscillators centered at roughly 400 and 1000 ${\mathrm{cm}}^{-1\mathrm{}}$ in the optical conductivity. In the superconducting state, there is a sharp reflectance edge at 300 ${\mathrm{cm}}^{-1\mathrm{}}$ associated with a threshold in the real part of the optical conductivity, but it is not certain that this is a conventional energy gap.