Crossover in temperature dependence of penetration depth λ(T) in superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films

The ab-plane penetration depth λ(T) in twinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ films exhibits a crossover in its T dependence at 25 K. Below 25 K, δλ≡λ(T)-λ(0)∝${\mathit{T}}^2$. This differs from recent measurements on a crystal which find δλ∝T below 40 K. Above 25 K, the normalized data λ(T/${\mathit{T}}_{\mathit{c}}$)/λ(0) vs T/${\mathit{T}}_{\mathit{c}}$ 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 d-wave superconductors.