Conductivity relaxation in strongly underdoped YBa2Cu3O7−δ single crystals

Temperature dependences of the basal-plane electrical resistance of underdoped YBa2Cu3Ot−δ single crystals are investigated in a broad temperature range down to the superconducting transition, after a fast cooldown from 650° C as well as after a room-temperature annealing. A stepped behavior of the superconducting transition has been observed, pointing to an inhomogeneity of the samples. Annealing has been revealed to lead to a narrowing of the superconducting transition and a reduction of both, the residual resistivity and the phonon scattering coefficient. This is accompanied by a slight increase of the Debye temperature attributed to a decrease of the lattice parameter caused, in turn, by a redistribution of the labile oxygen. The revealed changes of the parameters of the temperature dependence of the electrical resistance point to an equalization of the concentration of the labile oxygen in the layers and its ordering.