The effect of annealing both in the oxygen atmosphere and at room temperatures on physical properties such as the pseudogap (Δ*(T)) and excess conductivity (σ′(T)) of untwined YBa 2 Cu 3 O 7−δ (YBCO) single crystal with a small deviation from oxygen stoichiometry is studied. It was revealed that as the charge carrier density, n f , increases, Т с also slightly increases, whereas the temperature of the pseudogap opening, T*, decreases noticeably, which is consistent with the phase diagram (PD) of cuprates. The excess conductivity in the vicinity of T c is represented by the Aslamazov-Larkin and Hikami-Larkin fluctuation theories, illustrating the three-dimensional to two-dimensional (i.e. 3D-2D) crossover with an increase in temperature. The crossover temperature T 0 determines the coherence length along the c axis is ξ c (0) = 0.86 Å, that is 2.6 times larger than for optimally doped YBCO single crystals with defects. Taking into account the short coherence length in high-temperature superconductors, in the model of free charge carriers the phase relaxation time of fluctuating Cooper pairs is determined, τ φ (100 K) = (4.55 ± 0.4) · 10 −13 s, which is slightly (1.2 times) larger than in well-structured YBCO films, and as in films, does not depend on n f . It is shown that Δ*(T) at different annealing stages practically does not change its shape. As in the well-structured YBCO films, Δ*(T) demonstrates maximum at T pair ~124 K which depends weakly on n f . However, the maximum value of Δ*(T pair ) increases with increasing n f , as it follows from the PD of cuprates. Comparing the experimental data with the Peters-Bauer theory we estimated the density of local pairs ≈ 0.3 near T c that is a common value for high-temperature superconductors.

The effect of hydrostatic pressure up to P = 1.7 GPa on the fluctuation conductivity σ′(T) and pseudogap ∆*(T) in Y0.95Pr0.05Ba2Cu3O7−δ single crystal with critical temperature Тс = 85.2 K (at P = 0) was investigated. The application of pressure leads to the increase in Tc with dTc/dP = +1.82 K∙GPa−1 while the resistance decreases as dlnρ(100 K)/dP = −(10.5 ± 0.2) %∙GPa−1. Regardless of the pressure, in the temperature interval from Tc to T0 (~88 K at P = 0) the behaviour of σ′(T) is well described by the Aslamazov – Larkin (AL – 3D) fluctuation theory, and above the T0 by the Lawrence – Doniach theory (LD). The Maki-Thompson (MT – 2D) fluctuation contribution is not observed. This indicates the presence of structural defects in the sample induced by Pr. Here it is determined for the first time that when the pressure is applied to the Y1−xPrxBa2Cu3O7−δ single crystal, the pseudogap increases as dlnΔ*/dP = 0.17 GPa–1.

The temperature dependence of excess conductivity σ′(Т) has been studied in three polycrystalline samples of the FeSe0.94 superconductor, prepared by different technologies. The measured temperature dependences of the Δ*(T) parameter, which is associated with the pseudogap in cuprates, were analyzed using the local pair model. At high temperatures, all three samples exhibit a high narrow maximum along Δ*(T) at Ts1∼250 K, which is typical for magnetic superconductors. Below T ≈ 225 K, the dependences Δ*(T) become different. Over almost the entire temperature range below Ts1, the S2 sample, prepared by solid state reaction without impurities, exhibits a Δ*(T) that is typical for Fe-pnictides. An exception is the interval between the structural change temperature Ts = 85 K and Tc, where this Δ*(T) exhibits an atypical, broad maximum. An analysis of the obtained dependence suggests the discovery of a pseudogap in this FeSe0.94 sample, below Ts. Samples S1, containing 4 wt.%Ag, and S3, having a nominal composition but containing nonsuperconducting hexagonal phase inclusions, both prepared by partial melting, show identical Δ*(T), but different from S2. They have a number of features that correlate with temperatures at which there are also features along M(T), and the Hall coefficient RH(T) changes signs several times with decreasing T, which indicates that there is change in the type of charge carriers in FeSe. The Δ*(T) dependence of the S3 sample below Ts has almost no maximum, since the nonsuperconducting impurities of the hexagonal phase in S3 prevent the formation of paired fermions near Tc. As a result, S3 also has the minimum local pair density <n↑n↓> = 0.26, determined by comparing Δ*(TG)/Δmax near Tc using the Peters–Bauer theory, whereas the dependence Δ*(T) does not follow the theory. S1 has the maximum <n ↑ n ↓> = 0.47, supposedly due to the influence of Ag impurities. In S2, which is pure, <n ↑ n ↓> ≈ 0.3, which is the same as that of YBa2Cu3O7−δ, and both dependences Δ*(Т) for S1 and S2 follow the theory over a wide temperature range.

We report the effect of the ab-plane magnetic field B up to 8 T on the resistivity ρ(T) and fluctuation conductivity σ′'(T) in YBa2Cu3O7-δ thin films. As expected, up to ∼ 2.5 T the magnetic field monotonously increases ρ, the width of the resistive transition, ΔTc, and coherence length along the c axis, ξC(0), but decreases both Tc and the range of superconducting (SC) fluctuations ΔTfl. The fluctuation conductivity exhibits a crossover at characteristic temperature T0 from the 3D Aslamasov-Larkin (AL) theory near Tc to the 2D fluctuation theory of Maki-Thompson (MT). However, at B = 3 T, the MT term is completely suppressed, and above T0 σ′'(T) is unexpectedly described by the fluctuation contribution of 2D AL, suggesting the formation of a 2D vortex lattice in the film under the action of a magnetic field. At the same time, ΔTf sharply increases by a factor of about 7, and ξC(0) demonstrates a very unusual dependence on Tc when B increases above 3 T. Our results demonstrate the possibility of the formation of a vortex state in YBCO and its evolution with increasing B.