Far-infrared study of superconducting Tl 2Ba 2CaCu 2O 8

Abstract

We report the ab-plane optical properties of a Tl 2Ba 2CaCu 2O 8 single crystal with T c = 110 K. Measurements were made over 130–30000 cm −1 at room temperature and over 130 and 3000 cm −1 at temperatures between 10 and 300 K. The real part of the optical conductivity, determined by Kramers-Kronig analysis, has a metallic behavior above T c in the far infrared, but non-Drude behavior at higher frequencies. Below T c, the far-infrared spectral weight is substantially reduced, although there is no superconducting gap feature in the spectrum. A generalized Drude analysis of the conductivity yields a frequency-dependent scattering rate which develops a threshold around 1000 cm −1 in the superconducting state. No strong indication of this threshold can be seen above T c. An alternative two-component analysis also fits the optical data. We found that about 20% of the total doping-induced spectral weight and about 90% of the free-carrier spectral weight contributes to the superfluid condensate. The London penetration depth is about 1700 Å. We conclude that our sample is in the clean limit and close to optimal doping.