The resistive transition of a Sm 1.85Ce 0.15CuO 4−y single crystal

Abstract

Current-voltage V( I) characteristics and the magnetoresistivity ϱ( T, H) have been measured for a Sm 1.85 Ce 0.15 CuO 4− y single crystal in magnetic fields parallel to the c-axis. We have found that a well defined line H ∗( T) exists in the H-T plane which separates ohmic from non-ohmic behavior. Below H ∗( T), the V( I) curves are described by V/ I≁exp(- U( T, H, I)/ k B T) where U( T, H, I)≁ U( T, H)ln( I 0/ I). From the analysis of the V( I) characteristics, the effective flux creep energy barriers at any temperature and magnetic field are extracted. These data are used to calculate the magnetoresistivity curves ϱ( T, H) which compare very favorably with the experimental data over ⩽80%ϱ n of the resistive transition. The U( I)≁-ln I dependence, indicating the elastic nature of the flux creep process, is predicted by some recent theories: collective creep and vortex glass. We have not found evidence of the phase transition predicted in the latter case. The upper part of the resistive transition can be described in terms of the field enhanced 2D fluctuations. However, the significance of this analysis is questioned.