Anomalous charge transfer in La2CuO4 near the Néel temperature: Evidence of the transition from spin liquid to two-sublattice antiferromagnetic state

The temperature dependence of the electrical resistance and thermopower of La_{2− x} Sr_xCuO_4+δ single crystals with x ≅ 0.003 and δ<0.05 has been studied in the temperature range from 100 to 400 K. All crystals exhibiting two-dimensional hopping conductivity via neighboring acceptor sites in the CuO₂ plane show a significant difference in the charge transfer below and above the Néel temperature T _N. This difference indicates that the loss of a two-sublattice antiferromagnetic order strongly affects the charge transport in the CuO₂ plane. The obtained data lead to a conclusion that the crystal above T _N occurs in a resonance valence bond state of the Bose type.