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The lattice dynamics of the spin-Peierls (SP) system CuGeO 3 have been analyzed by inelastic neutron scattering combined with shell model calculations. The low-lying modes of the symmetry of the structural distortion in the dimerized phase are identified and studied as a function of temperature. Surprisingly, the static distortion in the SP phase does not correspond to one eigenmode of the high symmetry phase. The two modes associated with the structural distortion exhibit frequency shifts and pronounced linewidth broadening well above the transition temperature; however, there is no soft mode behavior for these modes. [S0031-9007(98)05674-9] PACS numbers: 75.40.Gb, 61.12.Ex, 63.20.-e A spin-Peierls (SP) transition results from spin-lattice coupling and, hence, should lead to peculiarities in the magnetic as well as in the lattice properties. Concerning the lattice dynamics in the SP compound CuGeO 3 (CGO) [1], infrared and Raman studies determined almost all optical G-point frequencies [2]; however, any dimerization must be related to modes away from the zone center, which can be investigated only by neutron scattering. Several neutron studies have focused on the acoustic branches [3-5] revealing a particularly low LA branch along the b axis whose relevance for the spin-phonon coupling still needs to be clarified. The search for a soft mode related to the structural distortion at the SP transition has so far remained unsuccessful. Answering whether the associated phonon mode softens at T SP or not, is, however, essential for the understanding of the underlying mechanism.Additional information on the lattice dynamics was obtained by optical techniques in the distorted phase, where the high-symmetry zone boundary (0.5 0 0.5) is folded into a new zone center, and where new optical active phonon modes should appear. Indeed, several groups have reported new Raman scattering peaks below T SP [6-13], at ϳ1, 3.2, 6.8, 11.1, and 24.6 THz. However, there is controversy about the interpretation of these intensities. There seems to be agreement on the facts that the lowest peak has a magnetic origin [6][7][8][9][10][11][12][13][14], and that the two highest frequencies are due to phonons. The 6.8 THz feature is commonly assumed to have a magnetic origin [6][7][8][9][10][11][12][13] and the one at 3.2 THz is interpreted as either magnetic [6,7] or phononic [8,10]. We will show by combining inelastic neutron scattering results with lattice dynamical model calculations that the four intensities correspond to the frequencies of the four phonon modes of the same symmetry as the distortion below T SP . However, none of these modes shows an indication of softening at the SP transition.The inelastic neutron scattering experiments were performed on the triple axis spectrometers 2T and 1T installed at the Orphée reactor. Double focusing crystals were used as monochromator [pyrolytic graphite (PG) (002) and Cu-(111)] and analyzer ]. Two crystals of about 600 mm 3 volume each were coaligned for measurements in the (010) geometry; for o...

Longitudinal optical phonons with oxygen character were measured in La0.7Sr0.3MnO3 by inelastic neutron scattering in the (1 0 0) cubic direction and results were compared with shell model predictions. Measurements were performed in several Brillouin zones, which enabled us to identify the eigenvectors independent of the shell model. All major disagreements between model predictions and experimental results are primarily due to the anomalous downward dispersion of the bond-stretching vibration. The main new result is that the rhombohedral distortion of the cubic lattice makes the bond-stretching vibrations interact strongly with bond-bending modes folded into the cubic Brillouin zone.

We report results of inelastic neutron scattering measurements of phonon dispersions in optimally doped YBa 2 Cu 3 O 6.95 and compare them with model calculations. The focus is on the in-plane oxygen bond-stretching phonon branches. The study of these modes is complicated by anticrossings with c-axis-polarized branches; such effects are interpreted through lattice-dynamical shell-model calculations. The in-plane anisotropy of the bond-stretching phonons was firmly ascertained from measurements on a detwinned sample. Studying not only the in-plane modes involving in-phase motion for the two Cu-O layers within a unit cell but also those with opposite-phase motion was of great help for establishing a clear experimental picture. The measurements confirm that the in-plane oxygen bond-stretching phonon branches disperse steeply downwards from the zone center in both the a and the b directions indicating a strong electron-phonon coupling. For the b-axis-polarized bond-stretching phonons, there is an additional feature of considerable interest: a sharp local frequency minimum was found to develop on cooling from room temperature to Tϭ10 K at wave vector qϷ0.27 r.l.u.

The lattice dynamics in Sr2RuO4 has been studied by inelastic neutron scattering combined with shell-model calculations. The in-plane bond-stretching modes in Sr2RuO4 exhibit a normal dispersion in contrast to all electronically doped perovskites studied so far. Evidence for strong electron phonon coupling is found for c-polarized phonons suggesting a close connection with the anomalous c-axis charge transport in Sr2RuO4.