Investigation of high-energy phonon anomalies in HTSC cuprates with inelastic neutron scattering (with local structures of layered triangular oxides)
The Cu-O bond-stretching vibrations in the HTSC cuprates are of a prime interest with its possible relevance to superconductivity. The purpose of this dissertation is to investigate the oxygen phonon anomalies in the cuprates by means of neutron spectroscopy. We have chosen YBa2Cu3O 6+x and La2−xSrxCuO2 as model systems. ^ The bond-stretching LO phonons in the optimally doped phase of YBa 2Cu3O6+x showed distinct in-plane anisotropy, characterized by zone-boundary softening and a split-dispersion along b and a directions, respectively. It is consistent with the idea that stripe-like dynamic charge inhomogeneities exist on the CuO2 plane. At low temperatures, the mode along the b direction also showed a discontinuity, suggesting that the inhomogeneity becomes two-dimensional in the superconducting states. A similar behavior was observed from an underdoped phase, too. On the other hand, the phonon densities of states of cation-substituted La2−xSrxCuO 2 have shown that the dispersions of the bond-stretching modes are independent of the stripe periodicity or TC when the charge concentration is maintained. We argue that the fluctuation of doped charges is responsible for the zone boundary softening, while inhomogeneities are for the discontinuity regardless of the periodicity. ^ The various results in this study consistently suggested the electronic nature of the bond-stretching phonon anomalies. Most importantly, the bond-stretching longitudinal optical phonons seem to interact strongly with dynamic charge fluctuations or charge inhomogeneities on the CuO2 plane. From their temperature-dependence, we suspect that the charge inhomogeneities are closely related to superconducting pair formation. In this light, it is very likely that the phonons play an important role in the mechanism of high-temperature superconductivity. ^ In addition, we have used the neutron diffraction to investigate local structures of triangular layered oxide. It was found that Jahn-Teller orbitals of Ni3+ in LiNiO2 form a local ordering, where three neighboring 3dz2−r2/3 orbitals share an oxygen site to form a trimer. We also found a possible signature of hole-doped water in the recently discovered Na 0.3CoO2-1.4D2O superconductor. These studies successfully demonstrated the usefulness of the neutron pair distribution function analysis in studying local structural features, which may be essential in understanding macroscopic properties. ^
Engineering, Materials Science
"Investigation of high-energy phonon anomalies in HTSC cuprates with inelastic neutron scattering (with local structures of layered triangular oxides)"
(January 1, 2003).
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