Role of $\mathrm{Nd}/\mathrm{Ba}$ Disorder on the Penetration Depth of ${\mathrm{Nd}}_{1+\mathit{x}}{\mathrm{Ba}}_{2-\mathit{x}}{\mathrm{Cu}}_3{O}_{7-\mathit{\delta }}$ Thin Films

We report on a study on the effect of $\mathrm{Nd}/\mathrm{Ba}$ disorder on the $\mathrm{ab}$-plane penetration depth of epitaxial ${\mathrm{Nd}}_{1+x}{\mathrm{Ba}}_{2-x}{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ thin films. While in stoichiometric samples $\lambda \left(T\right)$ at low temperature is linear, Nd-rich films exhibit a quadratic law. For low Nd excess $(x<\mathrm{}0.04\mathrm{})$, a satisfying fit is obtained using the “dirty” $d$-wave model assuming that Nd ions at Ba sites act as strong scattering centers. At high $x(x>\mathrm{}0.15\mathrm{})$ the data are explained if $\mathrm{Nd}/\mathrm{Ba}$ disorder becomes less effective as a source of scattering. The effect of localization has been discussed to account for the experimental results.