Effects Of Gap Anisotropy On The Electromagnetic Response Of High-Tc Superconductors

We study the nature of the electromagnetic absorption in a superconductor with an anisotropic energy gap and a nonspherical Fermi surface that is either completely closed or open along the direction of the c axis of the crystal. The real part of the electromagnetic conductivity sigma-mu-v(q,omega) has been calculated for a wide range of the normal-state collision frequency omega-c and the parameter Q = max {(q.V(F))}, where q is the incident wave vector of the electromagnetic wave and VF is quasi-particle velocity at the Fermi surface. For simplicity, the model gap parameter DELTA(k) is assumed to vary only with the angle theta between the direction k of the quasiparticle wave vector and the c axis of the crystal (chosen to be the z direction). We employ a formulation for calculating the linear conductivity in which the collision frequency is directly related to the imaginary part of the single-particle self-energy resulting from various elastic and inelastic collisions. In the presence of gap anisotropy, there is finite absorption below the in-plane gap 2-DELTA-ab, assumed to be the maximum energy gap. We find that with a bilevel gap parameter consisting of an in-plane value DELTA-ab, a c-axis value of about 1/4 to 1/3 of DELTA-ab, and a sharp transition between them at (cos-theta) approximately 0.5, we are able to fit quite well the experimental infrared absorption data for the single crystal YBa2CU307, in which q is along the c axis. With hBARQ << DELTA-ab and omega-c, the observed data in the region omega < 2-DELTA-ab/hBAR can be fitted even with a low normal state collision frequency derived from the normal-state dc conductivity. However, we find that to get a good fit in the region beyond omega = 2-DELTA-ab/hBAR, omega-c Must necessarily be large and close to 2.5-DELTA-ab/hBAR. Whether this type of frequency-dependent omega-c implies electron-electron collision effects in the normal state beyond the normal Fermi-liquid picture or wether this is merely due to the existence of another inelastic scattering channel in the system with a low threshold, cannot be resolved unambiguously.