Crossover in temperature dependence of penetration depth [lambda]([ital T]) in superconducting YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]] films

The [ital ab]-plane penetration depth [lambda]([ital T]) in twinned YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]] films exhibits a crossover in its [ital T] dependence at 25 K. Below 25 K, [delta][lambda][equivalent to][lambda]([ital T])[minus][lambda](0)[proportional to][ital T][sup 2]. This differs from recent measurements on a crystal which find [delta][lambda][proportional to][ital T] below 40 K. Above 25 K, the normalized data [lambda]([ital T]/[ital T][sub [ital c]])/[lambda](0) vs [ital T]/[ital T][sub [ital c]] for our films agree extremely well with the crystal data, including quasilinear behavior between 25 and 40 K. Differences and similarities of films and crystals agree well with a theory of slightly disordered [ital d]-wave superconductors.