Isotopic and isomorphic substitution of YBa2Cu3O6 and YBa2Cu3O7: Far-infrared spectroscopy and assignment of phonons.

Abstract

We report results of far-infrared spectroscopic measurements on a series of isotopically ${(}^{18}$O) exchanged and isomorphically substituted materials in the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ and ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ families. Based upon these measurements, we propose assignments for phonons in both the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ semiconducting phase and the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$ superconducting phase. In ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}^{18}$${\mathrm{O}}_{7}$, the lowest-frequency phonon at 155 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ is seen to shift \ensuremath{\sim}2% to lower frequency when compared to the $^{16}\mathrm{O}$ material, demonstrating the role of oxygen in this vibration (which we have previously assigned as a c-axis-directed Cu-O chain bending mode of ${B}_{1u}$ symmetry). This transverse-optical branch may be important for phonon-mediated superconductivity in this material, as indicated in a recent theoretical treatment by Weber and Matthiess [Phys. Rev. B 37, 599 (1988)].