Linear temperature variation of the penetration depth in YBa2Cu3O7 − δ thin films

We have measured the penetration depth λ(T) on YBa2Cu3O7 thin films from transmission at 120, 330 and 510 GHz, between 5 and 50 K. Our data yield simultaneously the absolute value and the temperature dependence of λ(T). In high-quality films λ(T) exhibits the same linear temperature dependence as single crystals, showing its intrinsic nature, and λ(0) = 1570 A. In a lower quality one, the more usual T2-dependence is found, and λ(0) = 3400 A. This suggests that the T2 variations is of extrinsic origin. Our results put the d-wave-like interpretation in a much better position.