Effects of gap anisotropy on the electromagnetic response of high-Tc superconductors

Abstract

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 σμν(q,ω) has been calculated for a wide range of the normal-state collision frequency ωc and the parameter Q=max{‖q⋅VF‖}, where q is the incident wave vector of the electromagnetic wave and VF is quasiparticle velocity at the Fermi surface. For simplicity, the model gap parameter Δ(k) is assumed to vary only with the angle θ 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 2Δab, assumed to be the maximum energy gap. We find that with a bilevel gap parameter consisting of an in-plane value Δab, a c-axis value of about 1/4 to 1/3 of Δab, and a sharp transition between them at ‖cosθ‖∼0.5, we are able to fit quite well the experimental infrared absorption data for the single crsytal YBa2Cu3O7, in which q is along the c axis. With ħQ≪Δab and ωc, the observed data in the region ω<2Δab/ħ 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 ω=2Δab/ħ, ωc must necessarily be large and close to 2.5Δab/ħ. Whether this type of frequency-dependent ωc 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.

Source-reported materials — not catalogue approval

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