Anisotropic and thermally activated resistive behavior in ${\mathrm{Nd}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$

An anisotropic-field-induced resistive transition has been measured for highly c-axis-oriented ${\mathrm{Nd}}_{1.85}$ ${\mathrm{Ce}}_{0.15}$ ${\mathrm{CuO}}_{4\mathrm{-}\mathrm{\delta }}$ thin films as a function of a magnetic field H parallel and perpendicular to the c axis. It is found that the observed resistivity ρ below ${\mathit{T}}_{\mathit{c}}$ scales as ρ(T)=${\mathrm{\rho }}_0$ exp{-U(H)[1-T/${\mathit{T}}_{\mathit{c}}$(H)${]}^{\mathit{n}}$/${\mathit{k}}_{\mathit{B}}$T} with n=2 for H⊥c and n=3 for H∥c. This resistive behavior is explained by thermally activated flux motion based on a depinning model for flux lines in a quasi-two-dimensional superconductor. Numerical fitting leads to an estimation of the upper critical field, which results in a straight H-T phase boundary in quite a wide range below ${\mathit{T}}_{\mathit{c}}$ with upward deviation at lower temperatures.