Doping dependence of normal- and superconducting-state transport properties ofNd2−xCexCuO4±ythin films

Abstract

The electric and thermomagnetic transport properties of an underdoped, an optimally doped, and an overdoped c-axis oriented, epitaxial ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4\ifmmode\pm\else\textpm\fi{}y}$ thin film have been investigated in the temperature range from 4.2 K to 300 K and in magnetic fields up to 11 T oriented perpendicular to the ${\mathrm{CuO}}_{2}$ planes. In the normal state, the resistivity $\ensuremath{\rho},$ Hall coefficient ${R}_{H},$ and magnetoresistivity $\ensuremath{\Delta}\ensuremath{\rho}/\ensuremath{\rho}(B)$ can be described quantitatively within a simple two-carrier model if the existence of an electronlike and a holelike band is assumed, where each of the two groups of charge carriers is characterized by a temperature-independent Hall coefficient. A corresponding analysis of the thermoelectric effects appears to be more difficult since they depend on more subtle details of the band structure and the scattering mechanisms. In the superconducting regime, the critical field ${B}_{\ensuremath{\rho}}^{*}(T)$ determined from the shift of the resistive transition in an external magnetic field exhibits a positive curvature. In contrast, an analysis of the fluctuation conductivity and the transport entropy of magnetic flux line ${S}_{\ensuremath{\varphi}}$ consistently gives higher values for the upper critical field ${B}_{c2}(T).$ The fluctuation conductivity clearly exhibits two-dimensional scaling behavior, indicating that the quasi-two-dimensional nature of the single-layer compound ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4\ifmmode\pm\else\textpm\fi{}y}$ might be responsible for the difference between ${B}_{\ensuremath{\rho}}^{*}(T)$ and ${B}_{c2}(T).$ The order of magnitude of ${S}_{\ensuremath{\varphi}}$ is consistent with the predictions of the Ginzburg-Landau theory. We derived material parameters as ${B}_{c2}(0),$ the in-plane coherence length ${\ensuremath{\xi}}_{\mathrm{ab}}(0),$ the Ginzburg-Landau parameter $\ensuremath{\kappa},$ and the London penetration depth $\ensuremath{\lambda}(0).$ We also present experimental data on the Hall effect in the mixed state.