Infrared studies of the superconducting energy gap and normal-state dynamics of the high-Tc superconductor YBa2Cu3O7

Abstract

A detailed study of infrared properties (reflectivity, conductivity, and dielectric response), emphasizing reproducible results from fully oxygenated YBa2Cu3O7 crystals (Tc 93 K) and films, is presented. The extrapolated values of 1() at low frequency are roughly consistent with the measured temperature-dependent dc resistivity. Although not well understood, this infrared conductivity can be interpreted in terms of a frequency-dependent scattering rate of kT+Latin small letter h with stroke, with a low-frequency mass enhancement of roughly 2 to 4 associated with a carrier-spin related interaction. Infrared measurements polarized along the c axis suggest a conductivity anisotropy of roughly 40:1 near Tc in the normal state. In the superconducting state an energy scale of 2c 3kTc is suggested by c-axis polarized measurements, while a much larger characteristic energy of 2a-bf8kTc is evident in the (a-b)-plane conductivity. From the area missing from the conductivity up to this very large gap, a reasonable estimate (f1700) for the (a-b)-plane penetration depth is obtained. Evidence for non-BCS temperature dependence, strong pair breaking scattering, and possible fluctuation effects is discussed. A comparison to infrared data from Bi2Sr2CaCu2O8-y shows a similarly large energy scale, 2a-bf8kTc; for the cubic Ba0.6K0.4BiO3 superconductor a more conventional energy scale, 24kTc is observed. The unusually large energy scale obtained from the (a-b)-plane measurements of the layered cuprates lies far beyond the range of previously studied superconducting energy gaps (23 to 5kTc). © 1990 The American Physical Society.