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 YBa₂Cu₃O_7–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 T_c = 90 K (sample S1), σ′(T) near T_c 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, ξ_с(0). With a decrease in nf (samples S2 with T_c = 84 K and S3 with T_c = 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 T_pair ~ 114 K and a linear section descending to T₀₁ ~ 94 K, which limits the region of superconducting fluctuations above T_c. As n_f decreases, the shape of Δ*(T) noticeably changes and becomes typical for YBCO films with a symmetric maximum at T_pair, which is the BEC–BCS transition temperature in high-T_c superconductors. As n_f 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 n_f, since S/T ~ 1/n_f