Large-shift Raman scattering features in superconducti

We apply the large-shift Raman-scattering technique to explore the metal-insulator transition in YBa₂Cu₃O_6+x. In a doping-dependence study for x∼0 to 1, we find that the previously observed A_2g scattering features near 1.5 eV split in energy as doping is increased and have increasingly mixed A_1g+A_2g symmetry. At the same time, the overall A_2g Raman intensity decreases. We use an annealing/quenching technique on a x∼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 A_2g 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 A_2g 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.