Large-shift Raman scattering features in superconducting YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}}

We apply the large-shift Raman-scattering technique to explore the metal-insulator transition in YBa{sub 2}Cu{sub 3}O{sub 6+{ital x}}. In a doping-dependence study for {ital x}{approximately}0 to 1, we find that the previously observed {ital A}{sub 2{ital g}} scattering features near 1.5 eV split in energy as doping is increased and have increasingly mixed {ital A}{sub 1{ital g}}+{ital A}{sub 2{ital g}} symmetry. At the same time, the overall {ital A}{sub 2{ital g}} Raman intensity decreases. We use an annealing/quenching technique on a {ital x}{approximately}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 {ital A}{sub 2{ital 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 {ital A}{sub 2{ital 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 coppermore » {ital d} orbitals. {copyright} {ital 1996 The American Physical Society.}« less