Phonon Dispersion of Transverse Basal Plane Modes in Alkali-Graphite Compounds

1982 ◽  
Vol 20 ◽  
Author(s):  
W. A. Kamitakahara ◽  
H. Zabel ◽  
R. M. Nicklow
Keyword(s):  
Basal Plane ◽  
Phonon Dispersion ◽  
Low Frequency ◽  
Layered Materials ◽  
Brillouin Zone Boundary ◽  
Neutron Inelastic Scattering ◽  
Acoustic Branch ◽  
Transverse Modes ◽  
Pure Graphite ◽  
Stage 1

ABSTRACTDispersion curves for low-frequency transverse modes propagating in the basal plane have been measured in the compounds KC8, KC24 and RbC24 by means of neutron inelastic scattering. The acoustic branches show at low q a behavior ω2= Aq2+Bq4 characteristic of layered materials. The optical branches are derived qualitatively from graphite-like optical branches hybridized with new alkali-like branches. In stage 2 compounds, the shear constant C44, which can be obtained by extrapolating the acoustic branch towards q=0, is appreciably smaller than in stage-1 compounds or in pure graphite. At low temperatures, it was noted in KC24 that a frequency gap in the acoustic branch opens up near q=0.6 A-1, corresponding to the Brillouin zone boundary of the low temperature alkali superstructure.

Observation of the structural phase transition in SrFio3 by diffuse electron scattering

1990 ◽  
Vol 48 (4) ◽  
pp. 420-421
Author(s):  
J.M. Zuo ◽  
Peter Rez
Keyword(s):  
Phase Transition ◽  
Transition Temperature ◽  
Electron Diffraction ◽  
Structural Phase Transition ◽  
Acoustic Waves ◽  
Phonon Dispersion ◽  
Structural Phase ◽  
Low Frequency ◽  
Brillouin Zone Boundary ◽  
Structure Changes

Thermal streaks were first observed in electron diffraction patterns by Honjo et al and were attributed to low frequency acoustic waves. Recently Hua et al have also used electron diffraction to invetsigate phonons in NiAs and KCP.Strontium titanate (SrTiO3) has a perovskite structure, and is of interest because it undergoes a second order displacive structural phase transition at above 110K. Below the transition temperature, the structure changes to tetragonal with a unit cell (a, a, 2 c) , the c-axis being slightly elongated with c/a=1.0006. This structural phase transition is believed to be due to the softening of the phonon at the (111) Brillouin zone boundary, i.e. q=(lll)/2. The detailed phonon dispersion has been measured by neutron workers (for example, see 5), and is shown in Fig. 1. The phonon dispersion shows a characteristic dip at q=( 111 )/2, which is further lowered (softening) as the temperature decreases till the transition temperature is reached.

Helicity waves propagating in a plasma

1991 ◽  
Vol 45 (3) ◽  
pp. 481-488 ◽  
Author(s):  
Z. Yoshida
Keyword(s):  
Magnetic Field ◽  
Faraday Effect ◽  
Low Frequency ◽  
Plasma Waves ◽  
Frequency Limit ◽  
High Frequencies ◽  
Transverse Modes ◽  
Longitudinal Part ◽  
The Waves ◽  
Perturbed Magnetic Field

There exist plasma waves that transport helicity although they do not propagate electromagnetic energy. The dispersion relations of such helicity waves are studied. The electric field of the waves is parallel to the perturbed magnetic field, and both are perpendicular to the perturbed current. In cross-field propagation, a helicity wave is decomposed into two transverse modes with different polarizations and a longitudinal part. The helicity waves are principally Alfvénic in the low-frequency limit. At high frequencies, the Faraday effect comes into the polarization.

Elastic and photo-elastic constants of CH4 and CD4 obtained by Brillouin scattering

1982 ◽  
Vol 60 (3) ◽  
pp. 287-298 ◽  
Author(s):  
S. C. Rand ◽  
B. P. Stoicheff
Keyword(s):  
High Resolution ◽  
Single Crystals ◽  
Elastic Constants ◽  
Brillouin Scattering ◽  
Low Frequency ◽  
Mass Ratio ◽  
Translational Coupling ◽  
Brillouin Spectroscopy ◽  
Transverse Modes ◽  
Triple Points

The elastic constants and Pockels coefficients of CH4 and CD4 single crystals near their triple points have been determined using high-resolution Brillouin spectroscopy. For CH4 at 90.4 K, the elastic constants are C11 = 19.57 ± 0.30, C12 = 14.46 ± 0.20, C44 = 9.20 ± 0.15 kbar, and the ratios of the Pockels coefficients are p12/p11 = 1.031 ± 0.035 and p44/p11 = 0.069 ± 0.010. For CD4 at 89.2 K, C12 = 20.04 ± 0.30, C12 = 15.00 ± 0.25, C44 = 9.15 ± 0.15 kbar, and p12/p11 = 1.027 ± 0.020, P44/p11 = 0.138 ± 0.010. The velocities of the low-frequency transverse modes in CH4 and CD4 are anomalously slow in the [Formula: see text] direction, indicative of rotational–translational coupling of molecules. Isotopic differences are also evident: the Pockels coefficient p44 in CD4 is double that in CH4; and the velocities of both transverse modes in the principle directions [Formula: see text], [Formula: see text], and [Formula: see text] are in the ratio V(CH4)/V(CD4) = 1.130 instead of 1.118 as expected from the isotopic mass ratio.

PHONON BEHAVIOR OF Y-Ba-Cu-O SUPERCONDUCTOR AND SEMICONDUCTOR

1987 ◽  
Vol 01 (02) ◽  
pp. 409-412 ◽  
Author(s):  
Jinghui Ruan ◽  
Jizhou Li ◽  
Ansun Yu ◽  
Shu Gan ◽  
Tonghua Yang ◽  
...  
Keyword(s):  
Transition Temperature ◽  
Density Of States ◽  
High Frequency ◽  
Superconducting Transition Temperature ◽  
Low Frequency ◽  
Scattering Method ◽  
Phonon Density ◽  
Superconducting Transition ◽  
Phonon Density Of States ◽  
Neutron Inelastic Scattering

The generalized phonon density of states of Y-Ba-Cu-O superconductor and semiconductor has been measured by neutron inelastic scattering method. The results show that the decrease or disappearance of high frequency modes and the increase of low frequency modes may have some relations to the high superconducting transition temperature

Tilt and acoustic instabilities in ABX 4, A 2 BX 4 and ABX 3 perovskite structure types: their role in the incommensurate phases of the organic–inorganic perovskites

2005 ◽  
Vol 61 (6) ◽  
pp. 616-626 ◽  
Author(s):  
Ian Peter Swainson
Keyword(s):  
Brillouin Zone ◽  
Three Dimensional ◽  
Low Frequency ◽  
Zone Boundary ◽  
Brillouin Zone Boundary ◽  
Perovskite Layer ◽  
Modulated Phases ◽  
Acoustic Instabilities ◽  
Incommensurate Phases ◽  
Zero Frequency

An examination of the tilt modes and other low-frequency modes is made for an isolated, untilted perovskite layer, which maps very simply to the ABX 4 perovskites. A sheet of pure tilts exists at the Brillouin Zone boundary at {ξ, ½, ζ}. An instability is also found at all wavevectors which can be described as continuously varying from pure tilts to pure layer displacements as a function of the wavevector. Analysis is extended by considering the stacking of layers in the I-centered A 2 BX 4-layer perovskites. The effect of freezing in the commensurate tilt required to generate the Cmca tilt system, pertinent to the modulated phases of propylammonium salts, is examined. The zero-frequency modes are restricted to two planes in the Brillouin Zone. All of the observed wavevectors associated with modulated phases, and the commensurately tilted propylammonium tetrachlorocadmate, are consistent with this calculation. The effect of full three-dimensional connectivity is briefly reviewed for the true ABX 3 perovskites. While pure tilt incommensurates appear to be hypothetically possible, they do not appear to have been observed to date.

scholarly journals A computational study of the molecular and crystal structure and selected physical properties of octahydridosilasequioxane, (Si 2 O 3 H 2 ) 4 . II. Vibrational analysis

2011 ◽  
Vol 468 (2139) ◽  
pp. 851-870 ◽  
Author(s):  
C. J. H. Schutte ◽  
J. A. Pretorius
Keyword(s):  
Ir Spectra ◽  
Phonon Dispersion ◽  
Computational Study ◽  
Point Group ◽  
Low Frequency ◽  
Vibrational Analysis ◽  
Unit Cell ◽  
Molecular Spectra ◽  
Vibrational Modes ◽  
Free Molecule

A computational study of octahydridosilasequioxane, Si 8 O 12 H 8 , as a free molecule and when embedded in the unit cell R -3, Z =3, showed that the point group of the free molecule is indeed O h , but that its crystal symmetry is reduced to C 3i . Since the molecular and site-group symmetries influence the vibrational structure of a molecule, a full computational vibrational analysis of the isolated molecule and when embedded in the crystal lattice, is reported here. The analysis of the free molecular spectra given here agrees with that of its experimental infra-red (IR)-spectra and allows the assignment of all the vibrational modes, while the computed phonon dispersion of the crystal confirms the assignment of the internal vibrational modes of the molecule in the crystal. The computed and experimental IR spectra as well as Raman spectra show no indication of serious vibrational intermolecular coupling owing to the presence of multiple molecules in the unit cell. This may be the result of a weak intermolecular vibrational coupling in the solid state, which may feature in the low-frequency modes.

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