Infrared studies of high temperature superconductors

Abstract

Infrared measurements have been used both to study the superconducting energy gap and the normal state electronic character of the high Tc oxide superconductors. While the question of the existence of a gap is not yet completely resolved, we find strong evidence for a very large energy gap, 2†≌8kTc, in the a-b plane response of crystal Y1Ba2Cu3O7. Simple calculations also support the assertion that the much smaller gap values, 2†≾4kTc, inferred from polycrystalline data are associated with the c-axis response. The relationship between infrared, tunnelling and NQR measurements of the energy gap is not yet clear. The normal state conductivity, which is obtained from a Kramers-Kronig transform of the reflectivity, remains controversial. We find that the normal state conductivity above ≈ 1000cm-1 is primarily Durde-like with modest interband contributions beginning at about 6000cm-1. The area of our Drude curve corresponds to a density of 5×1021 cm-3 assuming a mass of 1 at high frequency. Below 1000cm-1 we suggest that a frequency dependent scattering rate will be required to obtain an appropriate correspondence between the infrared conductivity and the strongly temperature dependent d.c. conductivity. Reliably obtaining this frequency dependence should substantially improve our understanding of transport in the normal state. © 1988, Elsevier Science Publishers B.V.. All rights reserved.