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The effect of external hydrostatic pressure on the electrical resistance of optimally doped and underdoped YBa 2 Cu 3 O 7 - δ single crystals is investigated in a broad temperature range, which includes the normal state and the region of superconducting fluctuations. The temperature dependences of the resistivity in the normal state are determined by scattering of charge carriers by phonons and defects. The application of pressure leads to significant changes in the electronic structure of the sample and reduces the degree of their defectiveness. This is accompanied by changeû in the lattice characteristics as the sample volume decreases. The fluctuation conductivity only exists within the range of ∼ 0.1 T c . With increasing δ the superconducting characteristics are close to the values typical for conventional low-temperature superconductors. [ABSTRACT FROM AUTHOR]

Abstract: We have investigated the conductivity in the basis plane of YBaCuO single crystals slightly doped by Al and Pr with a pre-specified topology of twin boundaries. The excess conductivity for the analyzed samples shows dependence in a wide temperature range , where is the mean field temperature of superconducting transition. The temperature dependence of the pseudogap can be satisfactorily described in terms of the BCS–BEC crossover theoretical model. [Copyright &y& Elsevier]

Evolution of the excess conductivity in YBCO single crystals is investigated after their step-by-step irradiation with fast electrons. The dependence of the coherence length ξ c (0) along the c -axis on the irradiation dose dependence is obtained. It is revealed that an increase of the irradiation dose from 1.4 to 8.8 × 1018 cm − 2 leads to a considerable extension of the temperature range of the excess conductivity. This is accompanied by a factor of about 1.5 increase of ξ c (0) and a shift ot the 2D-3D crossover point in temperature. [ABSTRACT FROM AUTHOR]

Abstract: We investigate the conductivity in the basis plane of the non-stoichiometric ReBa2Cu3O7−δ (Re=Y, Ho) single crystals. The excess conductivity (Δσ) is described by in the wide temperature range T f < T < T*, where T* is the mean field temperature of the superconducting transition. We determined that the temperature dependence of the pseudogap (PG) is described in terms of the BCS–BEC crossover theoretical model. It is observed that a reduction in the oxygen content leads to the widening of the temperature interval, in which the PG region is realized, and to the narrowing of the area of the ρ(T) linear dependence in the ab-plane. [Copyright &y& Elsevier]

The influence of the hydrostatic pressure P up to 0.95 GPa on the excess conductivity σ ′( T ) and the pseudogap Δ ∗ ( T ) in optimally doped YBa 2 Cu 3 O 7− δ single crystals ( T c ≃ 91.1 K at ambient pressure) is investigated by electrical resistivity measurements. A pronounced enhancement of the pseudogap under pressure of d lnΔ ∗ / dP ≈ 0.32, which is only a factor of 1.12 smaller than in slightly doped single crystals, is revealed for the first time. This implies a somewhat more moderate increase of the coupling strength in optimally doped cuprates with increasing pressure. Simultaneously, the ratio 2Δ ∗ ( T c )/ k B T c ≈ 5 at P = 0 GPa, which is typical for high-temperature superconductors with strong coupling, increases by 16% with increasing pressure. At the same time, the pressure effect on T c is minor: dT c / dP ≈ +0.73 K GPa −1 , whereas d ln ρ / dP ≈ (−17 ± 0.2)% GPa −1 is comparable with that in lightly doped YBCO single crystals. This suggests that the mechanisms of the pressure effect on ρ ( T ) and T c are noticeably different. Independently of pressure, near T c , σ ′( T ) is well described by the Aslamazov-Larkin (3D-AL) and 2D Hikami-Larkin fluctuation theories, exhibiting a 3D–2D crossover with increasing temperature. However, the temperature interval T c < T < T 01 , in which σ ′( T ) obeys the classical fluctuation theories, is exceptionally narrow (≈1.16 K). Nevertheless, a peculiarity at the temperature T 01 , up to which the wave function phase stiffness in the superconductor is maintained, is clearly observed in the dependence Δ ∗ ( T ). Below T 01 a fast growth of Δ ∗ ( T ) is revealed for the first time. It can be associated with a sudden increase of the superfluid density, n s , that is the density of fluctuating Cooper pairs (short-range phase correlations) forming in the sample when T approaches T c . [ABSTRACT FROM AUTHOR]

The influence of hydrostatic pressure P up to 1.05 GPa on resistivity ρ , excess conductivity σ ′ ( T ) and pseudogap Δ ⁎ ( T ) is investigated in slightly doped single crystals of YBa 2 Cu 3 O 7− δ ( T c ( P = 0 ) ≈ 49.2 K and δ ≈ 0.5 ). The critical temperature T c is found to increase with increasing pressure at a rate dT c / dP =+ 5.1 KGPa − 1 , while ρ ( 300 K ) decreases at a rate d ln ρ / dP = ( − 19 ± 0.2 ) % GPa − 1 . Near T c , independently on pressure, σ ′ ( T ) is well described by the Aslamasov–Larkin and Hikami–Larkin fluctuation theories, demonstrating a 3D–2D crossover with increase of temperature. The crossover temperature T 0 determines the coherence length along the c -axis ξ c ( 0 ) ≃ ( 3.43 ± 0.01 ) A ˚ at P =0, which is found to decrease with increasing P . At the same time, Δ ⁎ and the BCS ratio 2 Δ ⁎ / k B T c both increase with increasing hydrostatic pressure at a rate d ln Δ ⁎ / dP ≈ 0.36 GPa − 1 , implying an increase of the coupling strength with increasing P . At low temperatures below T pair , the shape of the Δ ⁎ ( T ) curve is found to be almost independent of pressure. At high temperatures, the shape of the Δ ⁎ ( T ) curve changes noticeably with increasing P , suggesting a strong influence of pressure on the lattice dynamics. This unusual behavior is observed for the first time. [ABSTRACT FROM AUTHOR]

Highlights: [•] The substitution of Y by Ho significantly affects the charge distribution. [•] The disordering in the oxygen subsystem which is affected by pressure. [•] Twin boundaries are effective scattering centers of normal carriers. [•] Near the conductivity is described by the Aslamasov–Larkin theory. [•] demonstrates two representative maxima which disappear under pressure. [ABSTRACT FROM AUTHOR]