Infrared-excited Raman scattering and photoluminescence of deep intragap states in semiconducting YBa2Cu3O6+x.

Abstract

The semiconducting parent compounds of the high-${\mathit{T}}_{\mathit{c}}$ superconducting system ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$, at low oxygen content (0x0.25) and different oxygen isotopes, have been investigated by means of infrared (IR) excited (1.16 eV) Raman scattering and photoluminescence (PL). The IR-excited Raman spectrum of the ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6}$ is dominated by a group of three bands in the spectral range of the apex oxygen O(4) ${\mathit{A}}_{\mathit{g}}$ mode (475 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the $^{16}\mathrm{O}$ compound). The Raman spectrum changes dramatically upon oxygen doping with the appearance of a strong resonant band at 507 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ accompanied by several overtones. By studying site-selective $^{18}\mathrm{substituted}$ samples this band is assigned to ${\mathit{A}}_{\mathit{g}}$ vibrations of apex O(4) atoms adjacent to the O(1) oxygen atoms occupying a chain site upon doping. The energy shift of this phonon and its associated overtones is caused by a strong lattice relaxation around the O(4)-Cu(1)-O(4) complex due to electronic charge-transfer processes involving O(4) sites. This behavior implies a strong coupling between O(4) vibration and charge-transfer excitations. PL, with a maximum at about 1.3 eV, has been observed in samples with very low oxygen concentrations. Peak energy and intensity are temperature and doping dependent. Both Raman and PL data are interpreted assuming the existence of a narrow band, associated with the O(4)-Cu(1)-O(4) complex (in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$) which is located, for small x, within the ${\mathrm{CuO}}_{2}$ plane charge-transfer gap.