Determination of the upper critical field of the electron-doped superconductorSm1.85Ce0.15CuO4−yfrom resistive fluctuations

The upper critical field ${\mathit{H}}_{\mathit{c}2}$(T) for the magnetic field applied parallel to the c axis of a ${\mathrm{Sm}}_{1.85}$${\mathrm{Ce}}_{0.15}$${\mathrm{CuO}}_{4\mathrm{\ensuremath{-}}\mathit{y}}$ single crystal has been determined from an analysis of magnetoresistance data in terms of the Ginzburg-Landau fluctuation theory for layered superconductors. The fluctuations are predominantly two dimensions (2D) away from ${\mathit{T}}_{\mathit{c}}$(H) with a crossover to 3D as ${\mathit{T}}_{\mathit{c}}$(H) is approached. The ${\mathit{H}}_{\mathit{c}2}$(T) curve shows negative curvature and is in excellent agreement with the Werthamer, Helfand, Hohenberg, and Maki theory for conventional superconductors without paramagnetic limiting effects. The values of the initial slope (${\mathit{dH}}_{\mathit{c}2}$/dT${)}_{\mathit{T}\mathit{c}}$=-7.38 kOe/K, coherence length \ensuremath{\xi}(0)=48.4 \AA{}, and orbital critical field ${\mathit{H}}_{\mathit{c}2}$(0)=97.6 kOe, derived from this analysis, are in good agreement with the values we previously obtained from fits of the Hao-Clem variational model to reversible magnetization data below ${\mathit{T}}_{\mathit{c}}$.