Effect of annealing on a pseudogap state in untwinned YBa2Cu3O7-δ single crystals

The effect of annealing both in the oxygen atmosphere and at room temperatures on physical properties such as the pseudogap (Delta*(T)) and excess conductivity (sigma'(T)) of untwined YBa2Cu3O7-delta (YBCO) single crystal with a small deviation from oxygen stoichiometry is studied. It was revealed that as the charge carrier density, n(f), increases, T-c also slightly increases, whereas the temperature of the pseudogap opening, T*, decreases noticeably, which is consistent with the phase diagram (PD) of cuprates. The excess conductivity in the vicinity of T-c is represented by the Aslamazov-Larkin and Hikami-Larkin fluctuation theories, illustrating the three-dimensional to two-dimensional (i.e. 3D-2D) crossover with an increase in temperature. The crossover temperature T-0 determines the coherence length along the c axis is xi(c)(0) = 0.86 angstrom, that is 2.6 times larger than for optimally doped YBCO single crystals with defects. Taking into account the short coherence length in high-temperature superconductors, in the model of free charge carriers the phase relaxation time of fluctuating Cooper pairs is determined, tau(phi) (100 K) = (4.55 +/- 0.4) . 10(-13) s, which is slightly (1.2 times) larger than in well-structured YBCO films, and as in films, does not depend on n(f). It is shown that Delta*(T) at different annealing stages practically does not change its shape. As in the well-structured YBCO films, Delta*(T) demonstrates maximum at T-pair similar to 124 K which depends weakly on nf. However, the maximum value of Delta*(T-pair) increases with increasing nf, as it follows from the PD of cuprates. Comparing the experimental data with the Peters-Bauer theory we estimated the density of local pairs < n(up arrow) n(down arrow)> approximate to 0.3 near T-c that is a common value for high-temperature superconductors.