Relationship between pressure-induced charge transfer and the superconducting transition temperature in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ superconductors

A model is proposed for the pressure dependence of ${\mathit{T}}_{\mathit{c}}$ based on the inverted parabolic relationship between ${\mathit{T}}_{\mathit{c}}$ and the hole carrier density n that takes into account the fact that ${\mathit{T}}_{\mathit{c}}$ drops to zero for a minimum value ${\mathit{n}}^{\min }$ or a maximum value ${\mathit{n}}^{\max }$ of n. The models proposed previously by Almasan et al. and Neumeier and Zimmermann are recovered, and are shown to be quite similar. Detailed calculations for ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ superconductors show that one can obtain large enhancements in ${\mathit{T}}_{\mathit{c}}$ under pressure in materials that are highly underdoped, or have relatively modest rates of pressure-induced charge transfers. On the other hand, ${\mathit{T}}_{\mathit{c}}$ enhancements in compounds which are near optimum doping or which have large rates of pressure-induced charge transfers are found to be quite modest.