Robust finite gap in overdoped Y_0.9Ca_0.1Ba ₂Cu₃O_7-δ thin films

A flattening of the penetration depth variation, Δλ(T), was observed in overdoped Y_0.9Ca_0.1Ba₂Cu ₃O_7-δ thin films at low temperature. That observation was measured also in imperfect thin films, which contain a large density of a-axis grains. Lack of a model combining a crossover from a finite gap into a dx2-y2-wave within the unitary limit for the imperfect films, we have fitted our data using the BCS model. That best fit yields a finite gap of 1.7 meV, which is comparable with the id_xy component obtained for films without a-axis grain where a complex order parameter was used. That flattening is accompanied by a steeper reduction of the quasi-particles scattering rate below 15 K for both quality thin films.