Magnon-scattering spectra of PrBa2Cu3O6, PrBa2Cu3O7, NdBa2Cu3O6, and Nd1.6Ba1.4Cu3O7.3.

Abstract

Raman-scattering spectra from ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$, ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, ${\mathrm{NdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$, and ${\mathrm{Nd}}_{1.6}$${\mathrm{Ba}}_{1.4}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7.3}$ single crystals have been measured at room temperature. Magnon-pair-scattering peaks with ${\mathit{B}}_{1\mathit{g}}$ symmetry have been observed in ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$, ${\mathrm{NdBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$, and ${\mathrm{Nd}}_{1.6}$${\mathrm{Ba}}_{1.4}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7.3}$, and the exchange integrals have been determined as 920, 930, and 950 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. The exchange integral of the compounds with the Cu-${\mathrm{O}}_{5}$ pyramid structure decreases as the Cu-Cu distance increases. On the other hand, it is found that the magnon-pair scattering is suppressed in nonsuperconducting ${\mathrm{PrBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$, suggesting that holes are not localized on the Pr site, but have a probability to populate the Cu-O plane just as in the superconducting Y-Ba-Cu-O--type compounds.