Pressure dependence of ${\mathit{T}}_{\mathit{c}}$ and charge transfer in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ (6.35≤x≤7) single crystals

The superconducting critical temperature ${\mathit{T}}_{\mathit{c}}$ of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathit{x}}$ single crystals (6.35≲x≲7) has been measured resistively as a function of pressure P (0≤P≲20 kbar). The initial rate ${\mathit{dT}}_{\mathit{c}}$/dP exhibits three distinct regimes, a narrow plateau near x≊7 with ${\mathit{dT}}_{\mathit{c}}$/dP≊0.04 K/kbar, a plateau in the range 6.4≲x≲6.8, where ${\mathit{dT}}_{\mathit{c}}$/dP≊0.43 K/kbar, and a maximum value ∼0.8 K/kbar at x≊6.35. An analysis of the ${\mathit{T}}_{\mathit{c}}$(x,P) data using a phenomenological model yields a pronounced peak near x=6.8 in dΔ${\mathit{n}}_{\mathit{h}}$(x)/dP, where Δ${\mathit{n}}_{\mathit{h}}$(x) is the change in the hole density in the ${\mathrm{CuO}}_2$ planes relative to the value corresponding to the fully oxygenated sample.