Effects of annealing on the fluctuation conductivity and pseudogap in slightly doped HoBa₂Cu₃O_7- single crystals

The effect of annealing at room temperature on the fluctuation conductivity (FLC) sigma' (T) and pseudogap (PG) delta*(7) in the basal ab plane of ReBa2Cu3O7-delta (Re = Ho) single crystals with a lack of oxygen has been studied. It is shown that at all stages of annealing, the FLC near T-c can be described by the Aslamazov-Larkin and Maki-Thompson fluctuation theories, demonstrating a 3D-2D crossover with increasing temperature. The crossover temperature T-0 was used to determine the coherence length along the c axis, xi(c)(0) = (2.82 +/- 0.2) angstrom. At the intermediate stage of annealing, an anomalous increase in 2D FLC was revealed, which is associated with the influence of uncompensated magnetic moments in HoBa2Cu3O7-5 (HoBCO): mu(eff), (Ho) = 9.7 mu(B). For the quenched sample S1, the temperature dependence of the PG has a shape typical of single crystals with a large number of defects. However, delta*(T) has two small additional maxima at high temperature, which is a feature of HoBCO single crystals with pronounced twins and indicates the two-phase nature of the sample. Upon annealing, the shape of delta*(T) noticeably changes, very likely due to an increase in the magnetic interaction (sample S2). More important is the change in the slope of the data at high temperatures, which has become about 3.5 times steeper. The ordering of the oxygen distribution due to the diffusion process during annealing somewhat compensates for the influence of magnetic interaction. But the slope does not change (sample S3). Interestingly, the slope turns out to be the same as for FeAs-based superconductors, suggesting the possibility of the existence of spin density waves in HoBCO in the PG state. The comparison of the pseudogap parameter delta * ( T ) /delta(max) * near T-c with the Peters-Bauer theory revealed a slight increase in the density of local pairs < n up arrow n down arrow >, which should explain the observed increase in T-c by 9 K during annealing.