Anomalies of conductivity behavior near the paramagnetic–antiferromagnetic transition in single-crystals

Abstract

The temperature dependences of resistance, R(T ), of two single-crystals La 2 CuO 4+δ 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 δ 0.01, was fairly homogeneous in oxygen distribution (not phase-separated) with Néel temperature T N ≈ 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 δ ≈ 0.05, was inhomogeneous (phase-separated) showing both PM-AFM (T N ≈ 205 K) and superconducting (T c ≈ 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 ≈ 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.

Lattice Effect on the Superexchange Interaction in Antiferromagnetic Bi2.1Sr1.9CaCu2O8+δ

“…There is an antiferromagnetic–paramagnetic transition at the Néel temperature ( T N ) of 90 K, indicated by the kink in the temperature dependence of the magnetic moment in a magnetic field directed along the c axis. No evidence for supporting superconductivity is provided from the magnetic susceptibility data down to 2 K. The kink for the antiferromagnetic transition in studied antiferromagnetic Bi 2.1 Sr 1.9 CaCu 2 O 8+δ is similar to those found in other antiferromagnetic cuprates such as Bi 2 Sr 2 Ca 1– x Y x Cu 2 O 8+δ , La 2 CuO 4+δ , and Bi 2 SrLaCuO y . No magnetic phase transition has ever been reported for Bi-2212 .…”