Anomalies of conductivity behavior near the paramagnetic-antiferromagnetic transition in single-crystals La_{2}CuO_{4+\delta}

The temperature dependences of resistance, R(T), of two single-crystals La_{2}CuO_{4+\delta} samples have been studied with the aim to detect a possible change in the R(T) behavior induced by paramagnetic-antiferromagnetic (PM-AFM) transition. One of the samples with \delta \lesssim 0.01, was fairly homogeneous in oxygen distribution (not phase-separated) with Neel temperature T_{N}\approx 266 K. Conductivity of this sample has been determined by Mott's variable-range hopping below T_N. The other, far less resistive, sample with \delta \approx 0.05, was inhomogeneous (phase-separated) showing both PM-AFM (T_N\approx 205 K) and superconducting (T_c\approx 25 K) transitions. It is found that for the homogeneous sample the resistivity decreases above T_N far faster with temperature than below it (for both directions of measuring current, parallel and perpendicular to basal CuO_2 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) curve near T_N\approx 205 K can be seen. Furthermore, a transition to metallic (dR/dT>0) behavior occurs far enough above T_N. The observed behavior of the samples studied is related to increased delocalization of charge carriers above T_N. This is in accordance with decrease in the AFM correlation length and corresponding enhancement of the hole mobility above T_N known for low-doped lanthanum cuprates.
Comment: 11 pages, 6 figures, submitted to Physica B