Anomalies of conductivity behavior near the paramagnetic–antiferromagnetic transition in single-crystals La2CuO4+δLa2CuO4+δ

The temperature dependences of resistance, R(T)R(T), of two single-crystals La2CuO4+δLa2CuO4+δ samples have been studied with the aim to detect a possible change in the R(T)R(T) behavior induced by paramagnetic–antiferromagnetic (PM–AFM) transition. One of the samples with δ≲0.01δ≲0.01, was fairly homogeneous in oxygen distribution (not phase-separated) with Néel temperature TN≈266K. Conductivity of this sample has been determined by Mott's variable-range hopping below TNTN. The other, far less resistive, sample with δ≈0.05δ≈0.05, was inhomogeneous (phase-separated) showing both PM–AFM (TN≈205K) and superconducting (Tc≈25K) transitions. It is found that for the homogeneous sample the resistivity decreases above TNTN far faster with temperature than below it (for both directions of measuring current, parallel and perpendicular to basal CuO2CuO2 planes). A similar behavior of conductivity near PM–AFM transition is also found for the phase-separated and less resistive sample. In this case a clear kink in R(T)R(T) curve near TN≈205K can be seen. Furthermore, a transition to metallic (dR/dT>0dR/dT>0) behavior occurs far enough above TNTN. The observed behavior of the samples studied is related to increased delocalization of charge carriers above TNTN. This is in accordance with decrease in the AFM correlation length and corresponding enhancement of the hole mobility above TNTN known for low-doped lanthanum cuprates.