The problem of how high pressure impacts various electric transport mechanisms in HTSC compounds REBa₂Cu₃O_7-δ (RE = Y, or another rare-earth ion) is considered. The features of the crystal structure and the effects that structural defects of different morphologies have on the electrical conductivity of these compounds in the normal, pseudogap, and superconducting states, are discussed. A review of the experimental data obtained in studies on the effect of high hydrostatic pressure on the various electric transport mechanisms of REBa₂Cu₃O_7-δ compounds, with varying composition and technical prehistory, is conducted. Different theoretical models devoted to the subject of how high pressure impacts the electrical conductivity of 1-2-3 HTSC system compounds are discussed.

The pseudogap (PG) derived from the analysis of the excess conductivity ${\sigma }^{\prime }\left(T\right)$ in superlattices and double-layer films of ${\text{YBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }{\text{-PrBa}}_2{\text{Cu}}_3{\text{O}}_{7-\delta }$ (YBCO-PrBCO), prepared by pulsed laser deposition, is studied for the first time. The ${\sigma }^{\prime }\left(T\right)$ analysis has been performed within the local-pair (LP) model based on the assumption of the paired fermion (LPs) formation in the cuprate high-$T_c$ superconductors (cuprates) below the representative temperature $T^{*}\gg T_c$ resulting in the PG opening. Within the model, the temperature dependencies of the PG, ${\mathrm{\Delta }}^{*}\left(T\right)$, for the samples with different number of the PrBCO layers ($N_{\mathrm{Pr}}$) were analyzed in the whole temperature range from $T^{*}$ down to $T_c$. Near $T_c,{\sigma }^{\prime }\left(T\right)$ was found to be perfectly described by the Aslamazov–Larkin (AL) and Hikami–Larkin (HL) [Maki–Thompson (MT) term] fluctuation theories, suggesting the presence of superconducting fluctuations in a relatively large (up to 15 K) temperature range above $T_c$. All sample parameters were found to change with increase of $N_{\mathrm{Pr}}$, finally resulting in the appearance of the pronounced maximum of ${\mathrm{\Delta }}^{*}\left(T\right)$ at high temperatures. The result is most likely due to increasing influence of the intrinsic magnetism of PrBCO (${\mu }_{\mathrm{Pr}}\approx 4{\mu }_B$) and suggests the possibility to search in that way the change of interplay between the superconductivity and magnetism in cuprates.

A new approach is proposed for analysis of the excess conductivity ${\sigma }^{\prime }\left(T\right)$ arising below a characteristic temperature $T^{*}⪢T_c$ in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-y}$ (YBCO) films with different oxygen concentrations. The approach is based on the idea that ${\sigma }^{\prime }\left(T\right)$ is formed as a result of the formation at $T⩽T^{*}$ of local pairs (tightly bound bosons) obeying Bose–Einstein statistics in the temperature interval $T_m<T<T^{*}$ . At temperatures $T_c^{mf}<T<T_m$ the pairs obey BCS theory ($T_c^{mf}$ is the critical temperature separating the phase transition region from the region of critical fluctuations). Thus in Y123 systems a transition from Bose–Einstein condensation to condensation of the BCS type occurs with decreasing temperature. An equation in which the dynamics of formation of the tightly bound bosons is taken into account is proposed which gives a good description of the temperature dependence ${\sigma }^{\prime }\left(T\right)$ and in which the parameter ${\Delta }^{*}$ , identified with the pseudogap in high-$T_c$ superconductors, is contained in explicit form. The temperature dependence ${\Delta }^{*}\left(T\right)$ is obtained for all the films studied.

The fluctuation-induced conductivity measured on ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ -${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ superlattices (SL) is analyzed using various theoretical models describing weak fluctuations in layered superconductors and considering both Aslamazov-Larkin (AL) and Maki-Thompson (MT) terms. From the analysis ${\xi }_{\mathrm{c}}$(0)=1.6±0.1nÅ and ${\mathrm{\tau }}_{\mathrm{\phi }}$(100nK)≃3×${10}^{\mathrm{-}13}$ s in clean limit are derived for the interlayer coherence length and temperature-dependent phase-relaxation time, respectively. Large ${\mathrm{\tau }}_{\mathrm{\phi }}$ values and AL-MT crossover revealed for all samples studied are attributed to the two-dimensional electronic state nature of SL above the crossover temperature. Possible mechanisms of charge scattering in high-${\mathrm{T}}_{\mathrm{c}}$ superconductors are also discussed.

The fluctuation conductivity in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{\text{7−y}}$ (YBCO) films with $\text{T≈80}$ K is investigated for the first time. Unlike the optimally doped samples, these films exhibit a transition from a Maki–Thompson (MT) mechanism for the scattering of fluctuational pairs to a Lawrence–Doniach (LD) mechanism and, as the temperature approaches $T_c,$ to an Aslamazov–Larkin (AL) mechanism. It is shown that the coherence length ${\xi }_c\text{(0)}$ along the $c$ axis and the phase relaxation time ${\tau }_{\phi }\text{(100\hspace{0.17em}}\mathrm{K})$ of the fluctuational pairs are determined by the temperature of this second transition. The features observed on both the fluctuation conductivity and resistive behavior are sharply enhanced as $T_c$ approaches 80 K, probably because of a significant increase in the intensity of the magnetic interaction in high-$T_c$ superconductors at these temperatures. In spite of this, for two samples a value ${\tau }_{\phi }\text{(100\hspace{0.17em}}\mathrm{K}{\text{)=(3.35±0.01)×10}}^{\text{−13}}\hspace{0.17em}\mathrm{s}$ is measured, i.e., the same value as for optimally doped YBCO films. It is shown that the dependence of ${\xi }_c\text{(0)}$ on $T_c$ obeys the standard theory of superconductivity. The mechanisms for the scattering of charge carriers and the superconducting pairing in YBCO are analyzed.