TEMPERATURE DEPENDENCE OF THE PENETRATION DEPTH IN YBa2Cu3O7 THIN FILMS FROM FAR INFRARED TRANSMISSION

We report transmission measurements in the range 120 GHz-500 GHz on a set of YBa2Cu3O7-delta epitaxial thin films. We deduce the absolute value of the electromagnetic penetration depth X(T) from the frequency variation of the transmission. Comparing two films selected for their different characteristics, we find that the smallest value of lambda(0 K) (1570 +/- 200 angstrom) is associated with a linear temperature dependence of lambda(T), and the largest (3400 +/- 200 angstrom) with a quadratic temperature dependence, as usually found in YBa2Cu3O7 thin films. The linear variation of lambda(T) with temperature maps the one observed in high quality single crystals. This, together with the lambda(0 K) values, leads us to conclude to the intrinsic character of the T dependence and to the extrinsic character of the T-2 behavior, which we assign to weak links. We discuss the implications of such results for the symmetry of the pairing wave function and for the lifetime of the quasi-particles.