Comparative analysis of the temperature dependences of the resistivity, pseudogap, and thermoelectric power in polycrystals YBa2Cu3O7–δ with a decrease in the density of charge carriers

Abstract

Comparative analysis of the temperature dependences of resistivity ρ(T), excess conductivity σ′(T), pseudogap (PG) Δ*(T), and thermoelectric power S(T), measured on textured YBa2Cu3O7–δ (YBCO) polycrystals with different charge carrier density nf, depending on the level of doping with oxygen, modified by annealing, has been carried out. It is shown that for an optimally doped (OD) sample with Tc = 90 K (sample S1), σ′(T) near Tc is well described by the Aslamazov–Larkin (AL−3D) and Maki–Thompson (MT-2D) fluctuation theories, demonstrating 3D–2D crossover with increasing temperature. The crossover temperature T0 was used to determine the coherence length along the с axis, ξс(0). With a decrease in nf (samples S2 with Tc = 84 K and S3 with Tc = 80 K), the MT contribution is suppressed, and the σ′(T) dependence obeys the Lawrence–Doniach model, which is typical for samples with defects. The dependence Δ*(T) obtained for S1 has a form typical for OD single crystals of YBCO with a maximum at Tpair ∼114 K and a linear section descending to T01 ∼94 K, which limits the region of superconducting fluctuations above Tc. As nf decreases, the shape of Δ*(T) noticeably changes and becomes typical for YBCO films with a symmetric maximum at Tpair, which is the BEC–BCS transition temperature in high-Tc superconductors. As nf decreases, the slope S(T) changes from positive to negative, demonstrating a feature at the PG opening temperature T*. Accordingly, the dependence of S(T)/T on log T changes from linear to nonlinear, which indicates a change in the nature of interactions in the YBCO electronic subsystem with decreasing nf, since S/T ∼1/nf.