Irreversibility line and thermally activated flux flow in La1.6−xNd0.4SrxCuO4 films

Abstract

The resistive transition broadening of the c-axis oriented La1.6−xNd0.4SrxCuO4 epitaxial films (x=0.1, 0.12, 0.14, and 0.16) has been systematically investigated under magnetic fields up to 14 T for both H⊥c and H‖c configurations. For H‖c, the irreversibility line corresponds to a melting line in high temperature and low field regions, whereas it shows a rapid increase following a decoupling theory in low temperature and high field regions. It is found that the in-plane resistivity ρab below the mean-field transition temperature Tc(H) follows Arrhenius-type thermally activated flux flow model ρ(T)=ρ0exp{−U0(H)[1−T∕Tc(H)]n∕kBT}, where n=3 for H‖c and n=1.5 for H⊥c. For H‖c, the field dependence of activation energy follows a power law, i.e., U0(H)∝H−α(α∼1), which can be interpreted in terms of thermal depinning of vortices in a quasi-two-dimensional superconductor. For H⊥c, a logarithmic dependence of activation energy is observed, which results from the plastic creep of flux lines associated with the intrinsic pinning in layered high-Tc superconductors.