The pressure dependence of the magnetic penetration depth λ in polycrystalline samples of YBa2Cu3Ox with different oxygen concentrations x=6.45, 6.6, 6.8, and 6.98 was studied by muon spin rotation (μSR). The pressure dependence of the superfluid density ρs∝1/λ2 as a function of the superconducting transition temperature Tc is found to deviate from the usual Uemura line. The ratio (∂Tc/∂P)/(∂ρs/∂P) is smaller by a factor of ≃2 than that of the Uemura relation. In underdoped samples, the zero-temperature superconducting gap Δ0 and the BCS ratio Δ0/kBTc both increase with increasing external hydrostatic pressure, implying an increase of the coupling strength with pressure. The relation between the pressure effect and the oxygen isotope effect on λ is also discussed. In order to analyze reliably the μSR spectra of samples with strong magnetic moments in a pressure cell, a special model was developed and applied.
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