3,220 research outputs found
Sound modes in composite incommensurate crystals
We propose a simple phenomenological model describing composite crystals,
constructed from two parallel sets of periodic inter-penetrating chains. In the
harmonic approximation and neglecting thermal fluctuations we find the
eigenmodes of the system. It is shown that at high frequencies there are two
longitudinal sound modes with standard attenuation, while in the low frequency
region there is one propagating sound mode and an over-damped phase mode. The
crossover between these two regions is analyzed numerically and the dynamical
structure factor is calculated. It is shown that the qualitative features of
the experimentally observed spectra can be consistently described by our model.Comment: 12 pages, 2 eps figures, Revtex, accepted to European Physics Journal
B, (2002
Hydrodynamic model for electron-hole plasma in graphene
We propose a hydrodynamic model describing steady-state and dynamic electron
and hole transport properties of graphene structures which accounts for the
features of the electron and hole spectra. It is intended for electron-hole
plasma in graphene characterized by high rate of intercarrier scattering
compared to external scattering (on phonons and impurities), i.e., for
intrinsic or optically pumped (bipolar plasma), and gated graphene (virtually
monopolar plasma). We demonstrate that the effect of strong interaction of
electrons and holes on their transport can be treated as a viscous friction
between the electron and hole components. We apply the developed model for the
calculations of the graphene dc conductivity, in particular, the effect of
mutual drag of electrons and holes is described. The spectra and damping of
collective excitations in graphene in the bipolar and monopolar limits are
found. It is shown that at high gate voltages and, hence, at high electron and
low hole densities (or vice-versa), the excitations are associated with the
self-consistent electric field and the hydrodynamic pressure (plasma waves). In
intrinsic and optically pumped graphene, the waves constitute quasineutral
perturbations of the electron and hole densities (electron-hole sound waves)
with the velocity being dependent only on the fundamental graphene constants.Comment: 11 pages, 6 figure
Soft phonon modes in rutile TiO
The lattice dynamics of TiO in the rutile crystal structure was studied
by a combination of thermal diffuse scattering, inelastic x-ray scattering and
density functional perturbation theory. We experimentally confirm the existence
of an anomalous soft transverse acoustic mode with energy minimum at q = (1/2
1/2 1/4). The phonon energy landscape of this particular branch is reported and
compared to the calculation. The harmonic calculation underestimates the phonon
energies but despite this the shape of both the energy landscape and the
scattering intensities are well reproduced. We find a significant temperature
dependence in energy of this transverse acoustic mode over an extended region
in reciprocal space which is in line with a substantially anharmonic mode
potential-energy surface. The reported low energy branch is quite different
from the ferroelectric mode that softens at the Brillouin zone center and may
help explain anomalous convergence behavior in calculating TiO surface
properties
A perturbation solution for long wavelength thermoacoustic propagation in dispersions
AbstractIn thermoacoustic scattering the scattered field consists of a propagating acoustic wave together with a non-propagational “thermal” wave of much shorter wavelength. Although the scattered field may be obtained from a Rayleigh expansion, in cases where the particle radius is small compared with the acoustic wave length, these solutions are ill-conditioned. For this reason asymptotic or perturbation solutions are sought. In many situations the radius of the scatter is comparable to the length of the thermal wave. By exploiting the non-propagational character of the thermal field we obtain an asymptotic solution for long acoustic waves that is valid over a wide range of thermal wavelengths, on both sides of the thermal resonance condition. We show that this solution gives excellent agreement with both the full solution of the coupled Helmholtz equations and experimental measurements. This treatment provides a bridge between perturbation theory approximations in the long wavelength limit and high frequency solutions to the thermal field employing the geometric theory of diffraction
Acoustic propagation in dispersions in the long wavelength limit
The problem of scattering of ultrasound by particles in the long wavelength limit
has a well-established solution in terms of Rayleigh expansions of the scattered fields. However,
this solution is ill-conditioned numerically, and recent work has attempted to identify an alternative
method. The scattered fields have been expressed as a perturbation expansion in the parameter Ka
(the wavenumber multiplied by the particle radius), which is small in the long wavelength region.
In the work reported here the problem has been formulated so as to be valid for all values of the
thermal wavelength, which varies in order of magnitude, from much smaller to much larger than
the particle size in the long wavelength region. Thus the present solution overlaps the limiting
solutions for very small thermal wavelength (geometric theory) and very large thermal wavelength
(low frequency) previously reported. Close agreement is demonstrated with the established Rayleigh
expansion solution
Ab-initio study of structure and dynamics properties of crystalline ice
We investigated the structural and dynamical properties of a tetrahedrally
coordinated crystalline ice from first principles based on density functional
theory within the generalized gradient approximation with the projected
augmented wave method. First, we report the structural behaviour of ice at
finite temperatures based on the analysis of radial distribution functions
obtained by molecular dynamics simulations. The results show how the ordering
of the hydrogen bonding breaks down in the tetrahedral network of ice with
entropy increase in agreement with the neutron diffraction data. We also
calculated the phonon spectra of ice in a 3x1x1 supercell by using the direct
method. So far, due to the direct method used in this calculation, the phonon
spectra is obtained without taking into account the effect of polarization
arising from dipole-dipole interactions of water molecules which is expected to
yield the splitting of longitudinal and transverse optic modes at the
Gamma-point. The calculated longitudinal acoustic velocities from the initial
slopes of the acoustic mode is in a reasonable agreement with the neutron
scatering data. The analysis of the vibrational density of states shows the
existence of a boson peak at low energy of translational region a
characteristic common to amorphous systems.Comment: International symposium on structure and dynamics of heterogeneous
system SDHS'0
- …