Evidence for a bulk complex order parameter in Y_0.9Ca _0.1Ba₂Cu₃O_7-δ thin films

We have measured the penetration depth of overdoped Y_0.9Ca _0.1Ba₂Cu₃O_7-δ (Ca-YBCO) thin films using two different methods. The change of the penetration depth as a function of temperature has been measured using the parallel-plate resonator (PPR), while its absolute value was obtained from a quasi-optical transmission measurement. Both sets of measurements are compatible with an order parameter of the form ΔΔ_dx2-y2 + iδ_dxy, with Δ = 14.5 ± 1.5 meV and δ= 1.8 meV, indicating a finite gap at low temperature. Below 15 K, the drop of the scattering rate of uncondensed carriers becomes steeper in contrast to a flattening observed for optimally doped YBCO films. This decrease supports our results on the penetration depth temperature dependence. The findings are in agreement with tunneling measurements on similar Ca-YBCO thin films.