Low temperature penetration depth and the effect of quasi-particle scattering measured by millimeter wave transmission in YBa2Cu3O7-δ thin films

Measurement of the penetration depth λ(T) as a function of temperature using millimeter wave transmission in the range 130-500GHz are reported for three YBa2Cu3O7-δ (YBCO) laser ablated thin films. Two films, deposited on a LaAlO3 substrate (Tc=90.2 K), exhibit a narrow resistive transition (0.3 K). One has been subsequently irradiated with He+ ions in order to increase the scattering rate of the quasi-particles (Tc=87.8 K). The third film, grown on MgO (Tc=8.5 K), exhibits also a fairly narrow transition (0.8 K) and a high crystalline quality. The experiment provides the absolute value λ(T≤30 K) for the penetration depth at low temperature: the derivation from the transmission data and the experimental uncertainty are discussed. We find a zero temperature penetration depth λ0=1990 ± 200 A, 2180 ± 200A and 2180 ± 200 A, for YBCO-500 A/LaAlO3(pristine), YBCO-1300 A/MgO and YBCO-500 A/LaAlO3 (irradiated) respectively. λ(T≤30 K) exhibits a different behavior for the three films. In the pristine sample, λ(T≤30 K) shows a clear temperature and frequency dependence, namely the temperature dependence is consistent with a linear variation, whose slope decreases with frequency: this is considered as an evidence for the scattering rate being of the order of the measuring frequency. A two fluids analysis yields 1/τ(T≤30 K)∝1.7 x 1012s-1. In the two other samples, λ(T≤30 K) does not display any frequency dependence, suggesting a significantly larger scattering rate. (orig.)