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 σ1(ω) 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+ħω, 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 2Δc≃3kTc is suggested by c-axis polarized measurements, while a much larger characteristic energy of 2Δa−b≃8kTc is evident in the (a-b)-plane conductivity. From the area missing from the conductivity up to this very large gap, a reasonable estimate (≃1700 Å) 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, 2Δa−b≃8kTc; for the cubic Ba0.6K0.4BiO3 superconductor a more conventional energy scale, 2Δ≃4kTc 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 (2Δ≃3 to 5kTc).

Source-reported materials — not catalogue approval

Similar papers