Comparison of the pressure dependencies ofTcin the 90-K superconductorsRBa2Cu3Ox(R=Gd,Er,andYb) andYBa2Cu3Ox

We have determined the effect of pressure $P$ on the resistance and superconducting transition temperature $T_c$ of $R{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$ ($R=\mathrm{G}\mathrm{d},\mathrm{E}\mathrm{r},\mathrm{a}\mathrm{n}\mathrm{d}\mathrm{Y}\mathrm{b}$) and compare our results to those obtained on high-quality $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_x$. The transition temperature, which is 90 K or greater at ambient pressure for all four compounds, is enhanced by pressure in each at rates between 0.09 and 0.19 K/kbar. Interestingly, the presence of a localized moment in the rare-earth compounds appears to have no influence on the pressure response. From the positive $\frac{d{T}_c}{\mathrm{dP}}$ we infer a positive change in the thermal expansion upon entering the superconducting state.