Temperature Dependence of the Penetration Depth of YBa ₂ Cu ₃ O 7-δ Films near T c

The temperature dependence of the penetration depth, λ, of films of YBa 2 Cu 3 O 7-δ measured using radio frequency techniques, has been found to have a mean-field–like exponent well within some estimates of the critical region. This result is found only if finite-size boundary conditions at the film edge are included in the analysis. This observation agrees with a recent renormalization group calculation in which the critical exponent for λ is mean-field–like, while the others are XY -like. It could also imply that the critical regime has not been reached, but then recent heat capacity measurements with XY -like behavior would have to be flawed.