Composition dependence of the in-plane copper-oxygen bond-stretching LO phonon mode in yttrium barium copper oxide

Composition dependence of the in-plane Cu-O bond-stretching LO mode in YBa2Cu3O6+x was investigated in order to gain insight into the microscopic mechanism of the metal-insulator transition in cuprates. We report experimental results suggesting the presence of spatial electronic inhomogeneities in superconducting YBa2Cu 3O6+x. These results were obtained by time-of-flight inelastic neutron scattering measurements on YBa2Cu3O6+x single crystals with different oxygen concentrations (x = 0.15, 0.35, 0.6, 0.7 and 0.95). In addition to the well-known zone boundary softening of the in-plane Cu-O bond-stretching longitudinal optical phonon mode, our results showed an insulator-like unsoftened branch coexisting with the softened branch at intermediate doping. We also determined that the extent of softening was the same for all doping levels and was equal to 14 meV. It has been proposed that this phonon softening is the result of strong electron-phonon coupling. In addition, we found that the spectral weight of the softened mode at the zone boundary increased while the intensity of the unsoftened branch decreased with increasing doping. This suggests a nano-scale electronic phase separation in superconducting YBa2Cu3O6+x. Lattice dynamical calculations have been carried out to study the evolution of phonon modes upon doping. The lattice dynamical models assuming a homogeneous, one-component system were not able to reproduce the experimentally observed phonon behavior. To explain out experimental findings, we propose a two-phase scenario for underdoped cuprates. The effect of doping---phonon softening---is confined to a certain volume fraction of the material, and this volume fraction is increasing with doping. Furthermore, triple-axis and time-of-flight inelastic neutron scattering measurements on the underdoped YBa2Cu3 O6.6 performed at different temperatures revealed that a new mode appeared at 65 meV at higher temperatures (200 K and above). This mode is dispersionless and might indicate the presence of polarons in YBa 2Cu3O6.6 at higher temperatures, possibly shedding light on nature of transition from the insulating parent compound to metallic cuprates