Normal state dynamical conductivity of layered superconductors

We have performed a model calculation of normal state macroscopic and microscopic dynamical conductivity for layered superconductors, which consists of one and two conducting layers per unit cell in the long-wavelength limit. Our calculation incorporates: (i) weak tunnelling of current between the layers; (ii) strong electron-electron interactions, which result in frequency- and temperature-dependent transport relaxation time; and (iii) optical phonons, which contribute to dynamical conductivity in the infrared frequency regime. Both the a-b plane and c-axis dynamical conductivity are calculated for longitudinal as well as transverse component of the field. It is found that both intralayer and interlayer interactions contribute to dynamical conductivity of a layered superconductor. Our computed macroscopic conductivity as a function of frequency and temperature shows good agreement with experimental results on YBa-2Cu3O7 (YBCO). In agreement with prior reported detailed numerical calculations, our model calculation of the c-axis conductivity also shows a broad peak (which is attributed from tunnelling between layers) in infrared frequency regime. We find that there exist one and two plasma modes, respectively in normal state of layered superconductors consisting of one and two conducting layers per unit cell, both in the a-b plane and along c-axis. On the other hand, several transverse electric (TE) modes are found to exist in a layered superconductor. One of the two plasma modes in a layered superconductor having two conducting layers per unit cell is found to exist for wave vector values larger than the critical value determined by intrinsic parameters of the superconductor. The complex frequency, which describes a plasma mode or a TE mode, consists of large imaginary part as compared to its real part. The frequency- and temperature-dependent transport relaxation time, which is needed to obtain a good agreement between theory and experiments, leads to larger imaginary part of complex frequency and the broad peaks in microscopic dynamical conductivity. (C) 2000 Published by Elsevier Science B.V. All rights reserved.