OPTICAL CONDUCTIVITY OF A SUPERCONDUCTOR WITH DX2-Y2 SYMMETRY

Using a simplified Eliashberg formulation in which we keep only that part of the susceptibility which is separable and of d(x2-y2) symmetry in the pairing channel and keep the isotropic part in the renormalization channel, we have calculated the ac conductivity in the superconducting state. At zero temperature, in the clean limit, we find a finite amount of absorption at any frequency, however small, although it is substantially reduced over its normal-state value. Besides this boson-assisted absorption, when impurities are added a new channel is opened up in which momentum is given up to the impurity system and absorption proceeds directly through the creation of a hole-particle pair. Born and unitary impurity scattering are considered.