Press, M. R. ; Jha, Sudhanshu S.
(1991)
*Effects of gap anisotropy on the electromagnetic response of high-T _{c} superconductors*
Physical Review B, 44
(1).
pp. 300-307.
ISSN 0163-1829

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Official URL: http://link.aps.org/doi/10.1103/PhysRevB.44.300

Related URL: http://dx.doi.org/10.1103/PhysRevB.44.300

## 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 σ_{μν}(q,ω) has been calculated for a wide range of the normal-state collision frequency ω_{c} and the parameter Q=max{||q.V_{F}||}, where q is the incident wave vector of the electromagnetic wave and V_{F} 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 2Δ_{ab}, assumed to be the maximum energy gap. We find that with a bilevel gap parameter consisting of an in-plane value Δ_{ab}, a c-axis value of about ¼ to 1/3 of Δ_{ab}, 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 YBa_{2}Cu_{3}O_{7}, in which q is along the c axis. With hQ << Δ_{ab} and ω_{c}, the observed data in the region ω<2Δ_{ab}/h 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=2Δ_{ab}/h, ω_{c} must necessarily be large and close to 2.5Δ_{ab}/h. Whether this type of frequency-dependent ω_{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.

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ID Code: | 13871 |

Deposited On: | 12 Nov 2010 14:34 |

Last Modified: | 02 Jun 2011 15:36 |

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