Peculiarities in the pseudogap behavior in optimally doped YBa2Cu3O7−δ single crystals under pressure up to 1 GPa

• Investigated is the effect of high pressure on the excess conductivity and the pseudogap in optimally doped YBCO single crystals.
• A pronounced enhancement of the pseudogap under pressure is revealed.
• The temperature interval, in which the excess conductivity obeys the classical theories, is exceptionally narrow.
• The model of local pairs allows one to get self-consistent results for the temperature dependence of the pseudogap.

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 YBa2Cu3O7−δ single crystals (Tc ≃ 91.1 K at ambient pressure) is investigated by electrical resistivity measurements. A pronounced enhancement of the pseudogap under pressure of dlnΔ∗/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∗(Tc)/kBTc ≈ 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 Tc is minor: dTc/dP ≈ +0.73 K GPa−1, whereas dlnρ/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 Tc are noticeably different. Independently of pressure, near Tc, 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 Tc < T < T01, in which σ′(T) obeys the classical fluctuation theories, is exceptionally narrow (≈1.16 K). Nevertheless, a peculiarity at the temperature T01, up to which the wave function phase stiffness in the superconductor is maintained, is clearly observed in the dependence Δ∗(T). Below T01 a fast growth of Δ∗(T) is revealed for the first time. It can be associated with a sudden increase of the superfluid density, ns, that is the density of fluctuating Cooper pairs (short-range phase correlations) forming in the sample when T approaches Tc.