LARGE-SHIFT RAMAN-SCATTERING FEATURES IN SUPERCONDUCTING YBA2CU3O6+X

We apply the large-shift Raman-scattering technique to explore the met al-insulator transition in YBa2Cu3O6+x. In a doping-dependence study f or x similar to 0 to 1, we find that the previously observed A(2g) sca ttering features near 1.5 eV split in energy as doping is increased an d have increasingly mixed A(1g) + A(2g) symmetry. At the same time, th e overall A(2g) Raman intensity decreases. We use an annealing/quenchi ng technique on a x similar to 0.4 sample to explore the transition re gion further, and find that the Raman intensity increases just after q uenching but the peaks do not shift in energy. Room-temperature anneal ing of the quenched sample shows an initial drop in A(2g) peak intensi ty 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 re sulting from increases in the carrier concentration. Peak splitting an d 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.