In-Plane Microwave Conductivity and Quasiparticle Scattering Rate of Superconducting High-<i><i>T</i></i>_{<b><i>c</i></b>}Cuprates

Abstract

The surface impedance Z s = R s + i X s at 9.6 and 51 GHz was measured in single crystals of YBa 2 Cu 3 O 7 , Bi 2 Sr 2 CaCu 2 O 8 , and La 1.85 Sr 0.15 CuO 4 . At low temperatures, the surface resistance R s of all the materials changes linearly in temperature, and the surface reactance (penetration depth) also changes linearly in YBa 2 Cu 3 O 7 and Bi 2 Sr 2 CaCu 2 O 8 . From Z s data, the complex conductivity σ=σ 1 - iσ 2 and the quasiparticle scattering rate 1/τ were determined. The broad peak-like structure in the temperature dependence of σ 1 , which comes from strong suppression of 1/τ below T c , is observed for all the superconductors. The low-temperature scattering rate extracted using the generalized two-fluid model is compared with the published data at different frequencies in YBa 2 Cu 3 O 7 . It is found that the inelastic scattering rate 1/τ at low temperatures strongly depends on the measurement frequency f within this model. In the microwave region, the normalized scattering rate τ n /τ is nearly proportional to the normalized frequency f / T c , and this relation seems to be universal for optimum-doped high- T c superconductors. The implications for the nature of the superconducting state are discussed.