Linear temperature variation of the penetration depth in YBa 2 Cu 3 O 7 − δ thin films

We have measured the penetration depth λ( T ) on YBa 2 Cu 3 O 7 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 Å. In a lower quality one, the more usual T 2 -dependence is found, and λ(0) = 3400 Å. This suggests that the T 2 variations is of extrinsic origin. Our results put the d -wave-like interpretation in a much better position.