The influence of hydrostatic pressure up to P=1.05 GPa on resistivity, excess conductivity σ ′ (T ) and pseudogap ∆ * (T ) is investigated in slightly doped single crystals of Y Ba2Cu3O 7−δ (Tc(P = 0) ≈49.2 K and δ ≈ 0.5). The critical temperature Tc is found to increase with increasing pressure at a rate dTc/dP = +5.1 KGP a −1 , while ρ(300) K decreases at a rate dlnρ/dP = (−19±0.2)% GP a −1 . Near Tc, independently on pressure, the σ ′ (T ) is well described by the Aslamasov-Larkin and HikamiLarkin fluctuation theories, demonstrating a 3D-2D crossover with increase of temperature. The crossover temperature T0 determines the coherence length along the c-axis ξc(0) ≃ (3.43 ± 0.01)Å at P=0, which is found to decrease with increasing P. At the same time, ∆ * and the BCS ratio 2∆ * /kB Tc both increase with increasing hydrostatic pressure at a rate dln∆ * /dP ≈ 0.36 GP a −1 , implying an increase of the coupling strength with increasing P. At low temperatures below Tpair, the shape of the ∆ * (T ) curve is found to be almost independent on pressure. At high temperatures, the shape of the ∆ * (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.
The effect of low-temperature uniaxial deformation on the self-trapping-limited mean free path of excitons in a KI–Tl crystal was revealed from x-ray luminescence spectra. The analysis of the dependence of the intensity ratio of the Tl-center emission (2.85 eV) and the luminescence of self-trapped excitons (π-component; 3.3 eV) on the extent of low-temperature deformation showed that in the KI–Tl crystal (3 × 10−3 mol. %) the self-trapping-limited mean free path of excitons is comparable with the distance between Tl atoms (20–27)a under a deformation ε = 2%. As the compression increases to ε ≥ 2%–5%, the mean free path drops to (27-5.35)a. The results of modeling based on the continuum approximation showed that with increasing temperature and the degree of low-temperature deformation the height of the potential barrier for the exciton self-trapping drops, which is consistent with the reduction of the mean free path of excitons in the KI–Tl crystal.
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.
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