Dc and Ac conductivity of La2CuO4+delta

The complex conductivity of La2CuO4+delta has been investigated for frequencies 20 Hz less than or equal to nu less than or equal to 4 GHz and temperatures 1.5 K less than or equal to T less than or equal to 450 K. Two single crystals with delta approximate to 0 and delta approximate to 0.02 were investigated, using dc (four-probe), reflectometric and contact-free techniques. At high temperatures the de conductivity is thermally activated with low values of the activation energy. For low temperatures Mott's variable range hopping dominates. The real and imaginary parts of the ac conductivity follow a power-law dependence sigma - nu(s), typical for charge transport by hopping processes. A careful analysis of the temperature dependence of the ac conductivity and of the frequency exponent s has been performed. It is not possible to explain all aspects of the ac conductivity in La2CuO4+delta by standart hopping models. However, the observed minimum in the temperature dependence of the frequency exponent s strongly suggests tunneling of large polarons as dominant transport process.