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We have investigated the evolution of the temperature dependence of the in-plane and off-plane conductivity of YBa2Cu3O7−δ single crystals due to the decrease of the oxygen content in the samples caused by a heat treatment. The concentration dependence of the in-plane and off-plane energy gaps has been determined. It has been shown that the enhancement of the localization effects results in a suppression of the pseudogap state. At the same time, the decrease of the oxygen content results in a relative narrowing of the temperature interval in which the regime of fluctuation conductivity is realized.

The effect of low-temperature annealing on the superconducting transition temperature and electrical resistance of single crystals of YBa2Cu3O7−δ with an oxygen deficiency δ=0.5–0.6 quenched from temperatures of 620–650 °C is considered. The transition temperature increases during annealing, while the resistance decreases. The isothermal relaxation of resistance is measured and used for estimating the activation energy of the relaxation process, which coincides with the activation energy of oxygen diffusion. The obtained results are attributed to oxygen ordering in Cu-O planes without changing its concentration. The observed step form of resistive transitions to the superconducting state is interpreted as the formation of clusters characterized by different oxygen concentrations and ordering. The cluster size is estimated.

We report on the second peak line H sp and the possible phases of vortex matter near T c for a detwinned underdoped YBa 2 Cu 3 O 7Ϫ␦ single crystal. Our local ac and dc permeability measurements revealed that the second peak line H sp does not terminate either at the critical temperature or on the irreversibility line H irr . Pronounced hysteretic behavior is observed in the regime below the second peak line H sp . Below ͑above͒ a characteristic field the end point of the second peak differs from ͑coincides with͒ the irreversibility point, H end point H irr (H end point ϵH irr ). A possible reentrance of a dynamically ordered solid phase in the hightemperature regime near the irreversibility line is discussed.