Oxygen Breathing Mode Research Articles

Dynamics of atoms in the high-temperature superconductor YBa2Cu3O7??

In order to obtain insight into the role played by the phonons in the superconducting phase transition, we have performed infrared reflectivity and inelastic neutron scattering experiments on single phase YBa2Cu3O7−δ(δ≊0.1,T c =94 K, ΔT 10–90=1 K). These experimental results, together with published Raman scattering and specific heat data are used to determine the parameters of a rigid-ion model; the model is able to reproduce qualitatively the experimental data. On the basis of our model we derive and discuss the phonon density of states and the pattern of atomic displacements of some important vibrational modes. In particular, the model reveals the existence of high-frequency oxygen breathing modes and of low-frequency copper dimerization modes in both the CuO3-chains and the CuO2-planes. Furthermore, Jahn-Teller like oxygen modes exist in the chains. The possible role of these modes with respect to the pairing mechanism is discussed. In addition there exist low-frequency “chain-degrading” modes which favour jumps of oxygen atoms out of the CuO3-chains into neighbouring vacant sites in the basal plane.

Ionic contributions to lattice instabilities and phonon dispersion in La2CuO

We have applied the potential-induced breathing model, a nonempirical ionic model, to La/sub 2/CuO/sub 4/. The ionic model predicts the observed tetragonal-to-orthorhombic distortion, predicts a lower-symmetry ground state, and predicts a stable oxygen breathing mode. We find unstable phonon branches that we relate to phase transitions. Harmonically unstable double-well modes will couple to the charge density in the copper-oxide planes and possibly contribute to high-T/sub c/ superconductivity. This ionic coupling is not included in recent calculations.

THE EFFECT OF LOCAL MODE ON Tc OF THE OXIDE SUPERCONDUCTORS

Although there may exist a large intrasite Coulomb repulsion, the local mode can still induce a attractive interaction between electrons through oxygen breathing mode. This interaction plays an imporatnt role in raising Tc. The result approaches that of experiment by a combination of the local mode and the Van Hove singularity.

Phonon density-of-states for the high-T c superconductor La1.85Sr0.15CuO4 and its non-superconducting reference La2CuO4

From inelastic neutron-scattering experiments, we determined the generalized phonon density-of-states for the high-Tc superconductor La1.85Sr0.15CuO4 and its semiconducting reference La2CuO4 atT=300K andT=6K. The spectra differ only little from each other. In particular, we do not observe any pronounced soft-mode behaviour either on cooling or when comparing the superconducting sample to the non-superconducting reference. We find, however, modest but distinct indications of coupling between electrons and oxygen breathing-modes at high frequencies.

Lattice Dynamics and Ionicity in the High-Temperature Superconductors

Using a non-empirical ionic model, we find harmonically unstable, double well potential surfaces for both La2CuO4 and YBa2Cu3O7 which may be related to the high temperature superconductivity. We have applied the Potential Induced Breathing (PIB) Model to La2CuO4 and YBa2Cu3O7 and find that the ionic description gives semiquantitative agreement with structural and vibrational observations. Furthermore, comparison with self-consistent linearized augmented plane wave [1] (LAPW) calculations shows that ionic contributions to the bonding and lattice dynamics of oxide superconductors are large. For La2CuO4 we find that an ionic description predicts the observed tetragonal to orthorhombic distortion, predicts a lower symmetry ground state, and predicts a stable oxygen breathing mode. We find harmonically unstable phonon branches that we relate to phase transitions. Unstable (anharmonic) modes will couple via ionic forces to the charge density in the metallic planes. This ionic coupling has not been included in previous lattice dynamics calculations. For YBa2Cu3O7 we find similar unstable phonon branches that involve oxygen motions perpendicular to the Cu-O bonds. We find the crystalline charge density is best approximated by overlapping ions if Cu and O ions of charge +1.6 and -1.8, respectively, are used.

Defects in Gel-Derived Glasses

ABSTRACTDefects in gel-derived glasses were reviewed for the three principal stages of the sol-gel process: solution, porous gel, and consolidated glass. The most prominent defects in gel-derived SiO2 are Raman active defects characterized by sharp bands at 490 cm−1(D1) and 608 cm−1(D2) 18O enrichment, experimental estimates of heats of formation, and structural studies of cyclic model compounds were used to identify the structural origins of D1 and D2 in gels. We conclude that D1 and D2 in gel-derived SiO2 are the oxygen breathing modes of 4- and 3-fold siloxane rings, respectively, as originally proposed by Galeener to explain these Raman-active defects in conventional vitreous silica (v-SiO2). During heat treatments near Tg (defined by n = 1013.5 poises), the gel-derived SiO2 structure rapidly approaches that of conventional v-SiO2.