Effect of pressure on optical excitations in semiconductingMBa2Cu3O6(M=Y,Sm,Gd)

We report optical reflectance measurements in the 0.5-4-eV region for ceramic samples of $M{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_6$ ($M=\mathrm{Y},\mathrm{}\mathrm{S}\mathrm{m},\mathrm{}\mathrm{Gd}$) as a function of pressure up to 34 GPa. Two distinct optical transitions at 1.75 and 2.6 eV can be identified in the visible region. These transitions show only a weak shift with pressure (pressure coefficients $\frac{\mathrm{dE}}{dP=-2\mathrm{} \mathrm{and} 10}$ meV/GPa, respectively) or rare-earth substitution. Based on the strength of these transitions, their pressure behavior, and the observation of a similar optical response for ${\mathrm{La}}_2$Cu${\mathrm{O}}_4$, we assign the 1.75-eV excitation in $M{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_6$ as due to the $\mathrm{O}\mathrm{}\left(p\right)\mathrm{}-\mathrm{C}\mathrm{u}\mathrm{}\left(d\right)\mathrm{}$ charge-transfer gap in the two-dimensional Cu-O network.