Superconductivity effects in hopping conductivity of La2CuO4 single crystal with excess oxygen

The hopping conductivity of La2CuO4+δ single crystal with excess oxygen has been studied. It is found that the transition from variable-range hopping (with the temperature dependence of resistance R∝exp(T0/T)1/4) to simple activation conductivity (R∝exp(Δ/kT)) occurs at a temperature lower than 20 K. This transition is accompanied with significant changes in the behavior of magnetoresistance and current–voltage characteristics. It is shown that such a behavior of hopping conductivity below 20 K is due to the presence of superconducting inclusions in insulating sample due to phase separation of La2CuO4+δ. The observed peculiarities in the conducting properties are in good agreement with the known effects of competition between electron localization and superconductivity in homogeneous systems.