Anisotropic optical spectrum of untwinned ${\mathrm{PrBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_7$: Persistence of the charge-transfer insulating state of the ${\mathrm{CuO}}_2$ plane against hole doping

Polarized optical reflectivity spectra of untwinned ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ are reported. The spectra show a large ab-plane anisotropy in the infrared region with higher reflectivity along the chain direction. For the a polarization, the spectrum is similar to that of deoxygenated tetragonal ${\mathrm{PrBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathit{y}}$, with an optical energy gap of about 1.5 eV, indicating that it is difficult to dope holes into the ${\mathrm{CuO}}_2$ planes by the addition of oxygen. The holes are doped into the CuO chains and probably also into Pr atoms. The difficulty of hole doping into the planes may be related to the relatively small charge-transfer energy gap.