Pressure dependence of the $T_c$ of Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$ up to 170 kbar

The superconducting onset temperature $T_{\mathrm{co}}$ of single-phased $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{9-\delta }$ (with $\delta \simeq 2$) measured resistively in a diamond anvil cell is found to increase at a rate of $\frac{d{T}_{\mathrm{co}}}{\mathrm{dp}}=0.043\mathrm{}$ K ${\mathrm{kbar}}^{-1\mathrm{}}$ up to 170 kbar. This is much weaker than for La-Ba-Cu-O for which $\frac{d{T}_{\mathrm{co}}}{\mathrm{dp}}=0.64\mathrm{}$ K ${\mathrm{kbar}}^{-1\mathrm{}}$. The pressure dependence of the high-$T_c$ superconductors measured so far cannot be explained within a standard electron-phonon Bardeen-Cooper-Schrieffer theory. Predictions of resonating-valence bonds and bipolaronic theories are discussed.