High-frequency surface impedance and penetration depth of YBa/sub 2/Cu/sub 3/O/sub 7/ films: coherent time-domain spectroscopy method

Abstract

The transmission (/spl Jscr/) and phase shift (/spl thetav/) of 0.2-1.5 THz radiation have been measured simultaneously on unpatterned, high-quality YBa/sub 2/Cu/sub 3/O/sub 7/ films (T/sub c/=90 K) in the normal and superconducting states using a coherent, pulsed, time-domain technique. The complex surface impedance has been extracted without contacting or patterning the superconducting films using /spl Jscr/ and /spl thetav/. Above T/sub c/, both the real (R/sub s/) and imaginary (X/sub s/) parts show a metallic /spl radic//spl omega/T dependence with almost equal magnitude as predicted by the normal skin effect. Below T/sub c/, R/sub s/ scales with /spl omega//sup 2/, but only for /spl omega//2/spl pi/ <400GHz; it increases more slowly with frequency above 400 GHz. The reactance X/sub s/ is linear in /spl omega/ for T<<T/sub c/, but follows a weaker power dependence with frequency at higher temperatures, indicating that the penetration depth is also frequency dependent. We attribute these features to a rapid decrease of the quasiparticle relaxation rate, 1//spl tau/, in the superconducting state (where 1//spl tau/ is comparable to the photon energy of the THz beam), and a significant influence of the remaining normal carriers in the THz regime. Differences between the microwave and submillimeter-wave responses will be discussed.<<ETX>>