Effect of Structural Relaxation on the In-Plane Electrical Resistance of Oxygen-Underdoped ReBa_2Cu_3O_{7-delta } (Re = Y,Ho) Single Crystals

The effect of jumpwise temperature variation and room-temperature storing on the basal-plane electrical resistivity $rho _{ab}$ of underdoped ReBa $_2$ Cu $_3$ O $_{7-delta }$ (Re = Y, Ho) single crystals is investigated. Reducing the oxygen content has been revealed to lead to the phase segregation accompanied by both, labile component diffusion and structural relaxation in the sample volume. Room-temperature storing of ${text {YBa}}_2{text {Cu}}_3{text {O}}_{7-delta }$ single crystals with different oxygen hypostoichiometries leads to a substantial widening of the rectilinear segment in $rho _{ab}(T)$ in conjunction with a narrowing of the temperature range of existence of the pseudogap state. It is established that the excess conductivity obeys an exponential law in a broad temperature range, while the pseudogap’s temperature dependence is described satisfactory in the framework of the BCS-BEC crossover theory. Substituting yttrium with holmium essentially effects the charge distribution and the effective interaction in CuO planes, thereby stimulating disordering processes in the oxygen subsystem. This is accompanied by a notable shift of the temperature zones corresponding to transitions of the metal-insulator type and to the regime of manifestation of the pseudogap anomaly.