LARGE-SHIFT RAMAN SCATTERING FEATURES IN SUPERCONDUCTING YBA2CU3O6+X

We apply the large-shift Raman-scattering technique to explore the metal-insulator transition in ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6+\mathit{x}}$. In a doping-dependence study for x\ensuremath{\sim}0 to 1, we find that the previously observed ${\mathit{A}}_{2\mathit{g}}$ scattering features near 1.5 eV split in energy as doping is increased and have increasingly mixed ${\mathit{A}}_{1\mathit{g}}$+${\mathit{A}}_{2\mathit{g}}$ symmetry. At the same time, the overall ${\mathit{A}}_{2\mathit{g}}$ Raman intensity decreases. We use an annealing/quenching technique on a x\ensuremath{\sim}0.4 sample to explore the transition region further, and find that the Raman intensity increases just after quenching but the peaks do not shift in energy. Room-temperature annealing of the quenched sample shows an initial drop in ${\mathit{A}}_{2\mathit{g}}$ peak intensity over 100 min, a plateau level from 100 to 500 min, and a final slow drop to the original intensity level from 500 to 1100 min. We see the suppressions of ${\mathit{A}}_{2\mathit{g}}$ Raman intensity from doping and annealing as resulting from increases in the carrier concentration. Peak splitting and mixed-symmetry effects seem to be more closely related to variations in the lattice parameters, probably as a result of changes to the in-plane copper d orbitals. \textcopyright{} 1996 The American Physical Society.