Effect of physical and chemical pressure on the superconductivity of high-temperature oxide superconductors

We have compared the changes in the structure and superconductivity of the ${\mathrm{Gd}}_{1\mathrm{-}\mathit{x}}$${\mathrm{Y}}_{\mathit{x}}$${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ system produced by rare-earth-ion substitution (chemical pressure) and by hydrostatic pressure. Although the application of both chemical and physical pressure result in an overall compression of the unit cell, the effect on ${\mathit{T}}_{\mathit{c}}$ is of opposite sign. A detailed comparison of the evolution of the structure under both kinds of pressure shows that the origin of this qualitatively different behavior in ${\mathit{T}}_{\mathit{c}}$ is correlated with the apical oxygen to ${\mathrm{CuO}}_2$-plane distance which also presents opposite trends. The origin of this difference is found in the extremely inhomogeneous character of the chemical pressure which, in spite of inducing an overall compression of the structure, produces an expansion of certain regions of the unit cell.