Search

We measured the phonon dispersions of YBa2Cu3O6.15 and YBa2Cu3O6.95 by time-of-flight inelastic neutron scattering. The in-plane bond-stretching modes in the metallic phase showed a distinct a-b plane anisotropy beyond what is expected for structural origin. Such anisotropy in the longitudinal optical modes, which is absent in the TO, suggests strong in-plane anisotropy in the underlying electronic structure. Apical oxygen bond-stretching modes showed a large frequency change between the insulating and the metallic phases. This large softening also is beyond structural origin, and suggests the effect of local electronic environment.

In earlier studies of phonons in V3Si, a gradual softening of the (hh0) branch was observed and attributed to the martensitic transition in this compound, which precedes the onset of superconductivity by a few degrees K. In this work, the temperature dependence of the transverse acoustic (TA) branch along the hh0 direction was studied in greater detail and it is shown that while the TA hh0 mode starts to soften at relatively high temperatures (>200 K), an anomalous softening, which is rather localized in q, occurs just below Tc. The intensity of this soft-phonon peak correlates extremely well with the onset of the superconducting phase and appears to vary as the order parameter with temperature and applied field. The similarities in the phonon softening and Fermi-surface anisotropy between this compound and the rare-earth nickel borocarbides suggest the existence of a common mechanism for the superconducting transition.

This paper reports single-crystal inelastic neutron scattering measurements of the phonon-dispersion relations of the aluminium silicate minerals andalusite and sillimanite, Al2SiO5. The measurements were undertaken using the triple-axis spectrometer at the Dhruva reactor, Trombay for andalusite and at the Oak Ridge National Laboratory, USA for sillimanite. The phonon-dispersion relations (up to 50 meV) along various high-symmetry directions have been measured and have been analyzed on the basis of lattice dynamics shell model calculations. The calculated structure factors based on the model calculations were used as guides for planning these single-crystal measurements and have been used to identify regions in reciprocal space with large cross sections. The calculated phonon-dispersion relations are in good agreement with the measured data. The model has been successfully used to derive the high pressure–temperature thermodynamic properties like the specific heat and the equation of state.

We present the results of inelastic neutron-scattering measurements of a twinned crystal of YBa2Cu3O6.95 using the MAPS spectrometer of the ISIS facility of the Rutherford–Appleton Laboratory as well as the HFIR of the Oak Ridge National Laboratory. The dispersion of the Cu–O bond-stretching modes indicates a strong dielectric anisotropy within the plane and associated electronic anisotropy, supporting the dynamic stripe model. The results suggest a possibility that phonons play a major role in the mechanism of high-temperature superconductivity.

Since its beginnings in Oak Ridge and Argonne in the late 1940's, neutron scattering has been established as the premier tool to study matter in its various states. Since the thermal neutron wavelength is of the same order of magnitude as typical atomic spacings and because they have comparable energies to those of atomic excitations in solids, both structure and dynamics of matter can be studied via neutron scattering. The High Flux Isotope Reactor (HFIR) provides an intense source of neutrons with which to carry out these measurements. This paper summarizes the available neutron scattering facilities at the HFIR.