Axial oxygen-centered lattice instabilities inYBa2Cu3O7: An application of the analysis of extended x-ray-absorption fine structure in anharmonic systems (original) (raw)
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Anisotropy in the electronic screening of oxygen lattice modes in YBa (2) Cu (3) O (6.95)
Inelastic neutron scattering data from a twinned single-crystal of YBa 2 Cu 3 O 6.95 are presented that show a distinct a-b plane anisotropy in the oxygen vibrations. The Cu-O bond-stretching type phonons are simultaneously observed along the a and b directions due to a 4 meV splitting arising from the orthorhombicity. The present results show the bond-stretching branch along b (parallel to the chain) has a continuous dispersion, while the branch along a is discontinuous, suggesting a possibility of short-range cell-doubling along a. Furthermore, the LO mode along a is split in energy from its TO partner at non-zero q-vectors, while the b mode is not. These results imply strong anisotropy in the electronic screening and a one-dimensional character in underlying charge fluctuations.
Polaron origin for anharmonicity of the axial oxygen in YBa_{2}Cu_{3}O_{7}
Physical Review Letters, 1992
We present exact diagonalization results of an electron-phonon model Hamiltonian for the 0(4)-Cu(0-O(4) cluster in YBa2Cu307. For large enough electron-infrared-phonon coupling the motion of holes and ions becomes strongly correlated, i.e., polaronic, leading to a double-well structure for the infrared mode, as encountered in x-ray-absorption fine structure. In contrast, the associated Raman mode shows a single-well behavior even for large electron-Raman-phonon coupling. The appearance of polarons accompanies nonadiabatic behavior yielding optical spectroscopy predictions differing from those obtained from harmonic or anharmonic lattice dynamics models.
Oxygen-ordering superstructures in underdopedYBa2Cu3O6+xstudied by hard x-ray diffraction
Physical Review B, 2003
High-energy x-ray diffraction is used to investigate the bulk oxygen-ordering properties of YBa 2 Cu 3 O 6ϩx. Four different superstructures of Cu-O chains aligned along the b axis and ordered with periodicity ma, along the a axis have been observed. For xϽ0.62, the only observed superstructure is ortho-II with mϭ2. At room temperature, we find ortho-III (mϭ3) for 0.72рxр0.82, ortho-V (mϭ5) in a mixed state with ortho-II at xϭ0.62, and ortho-VIII (mϭ8) at xϭ0.67. Ortho-II is a three-dimensional ordered structural phase, the remaining ones are essentially two-dimensional. None of the superstructures develops long-range ordering. The temperature dependence of the observed superstructure ordering is investigated explicitly and a structural phase diagram is presented.
2000
Local lattice distortions in YBa 2 Cu 3 O y single crystals are studied by polarized x-ray absorption spectroscopy (XAS) for E//ab and E//c orientations between 300 K and 20 K. From the Fourier transform analysis of the Cu K-EXAFS oscillations, we find that the out-of-plane Cu-O correlation shows an anomalous temperature dependence of radial distribution below T 1 =240-250 K, indicating an increase of lattice fluctuations, while no anomalies are found for the Cu-Ba correlation. On further lowering temperature, the Fourier transfom shows that the lattice fluctuation are reduced below T 2 ~T * =140-150 K associated with pseudogap opening. Below T * , the radial distribution of oxygen atoms became asymmetric, indicating that the distorted domain is formed. A strong electron-lattice coupling is indicated by the anomalous local lattice distortions at the cross-over temperatures characterizing various electronic transitions in the title system.
Origin of modulated structures inYBa2Cu3O6.63: A first-principles approach
Physical Review B, 2005
Recent diffraction studies have shown the existence of lattice modulations in yttrium barium cuprates ͑YBCO͒. We show that these modulations are caused by the ordering of O u Cuu O chains in the CuO planes according to a scheme of quasi-one-dimensional ordering developed previously. Remarkable agreement is illustrated in the case of underdoped YBCO between experimental diffraction patterns of diffuse intensity and calculated satellite intensity obtained from ab initio electronic structure calculations of atomic displacements in unit cells containing missing oxygen chains.
Structure ofYBa2Cu3O7−δversus temperature by x-ray-absorption spectroscopy
Physical Review B, 1987
We present extended x-ray-absorption fine-structure data on YBa2Cu307taken at four temperatures: 80, 112, 170, and 297 K. Spectra from both the Y edge and Cu edge are analyzed for the mean distances to near neighbors and the mean-square relative displacements o.. The distances show little temperature dependence. The o. 's are unusual because they vary more strongly at low temperature (80-110 K) than at high temperature (170-300 K). This could indicate some softening of the bond strengths near 170 K which may entail a subtle structural change. From a study of the Cu E edge we conclude that the x-ray-absorption near-edge structure (XANES) does not provide evidence for the existence of Cu(III) in YBa2Cu307s. We find that a peak at-9002 eV in the XANES, the origin of which has been the subject of conflicting opinions in La2 "Sr Cu04~, is associated with interference effects in Cu(II) complexes and its presence is not evidence for Cu(III) in YBa2Cu307-$.
Oxygen ordering and the orthorhombic-to-tetragonal phase transition inYBa2Cu3O7−x
Physical Review B, 1987
In situ neutron powder diffraction measurements show that the orthorhombic-to-tetragonal phase transition in YBa2Cu307-, which occurs near 700 C in a pure oxygen atmosphere, is an order-disorder transition in which the disordering of oxygen atoms into a normally vacant site destroys the one-dimensional Cu-0 chains present in the room-temperature orthorhombic structure. For both structures, the oxygen stoichiometry decreases monotonically with increasing temperature. The transition temperature depends on the oxygen partial pressure and occurs when the stoichiometry is near YBa2Cu3065. The tetragonal structure has a partially occupied, nearly octahedral Cu-0 arrangement, in contrast to the orthorhombic structure which has one-dimensional Cu-0 chains. The observed depression of the superconducting transition temperature in tetragonal YBazCu307-", which has been quenched from high temperature, could result either from the disordering of oxygen atoms which destroys the one-dimensional chains or from the absence of Cu + ions.