517 research outputs found
Role of structural relaxations and vibrational excitations in the high-frequency dynamics of liquids and glasses
We present theoretical investigation on the high-frequency collective
dynamics in liquids and glasses at microscopic length scales and terahertz
frequency region based on the mode-coupling theory for ideal liquid-glass
transition. We focus on recently investigated issues from
inelastic-X-ray-scattering and computer-simulation studies for dynamic
structure factors and longitudinal and transversal current spectra: the
anomalous dispersion of the high-frequency sound velocity and the nature of the
low-frequency excitation called the boson peak. It will be discussed how the
sound mode interferes with other low-lying modes present in the system.
Thereby, we provide a systematic explanation of the anomalous sound-velocity
dispersion in systems -- ranging from high temperature liquid down to deep
inside the glass state -- in terms of the contributions from the
structural-relaxation processes and from vibrational excitations called the
anomalous-oscillation peak (AOP). A possibility of observing negative
dispersion -- the {\em decrease} of the sound velocity upon increase of the
wave number -- is argued when the sound-velocity dispersion is dominated by the
contribution from the vibrational dynamics. We also show that the low-frequency
excitation, observable in both of the glass-state longitudinal and transversal
current spectra at the same resonance frequency, is the manifestation of the
AOP. As a consequence of the presence of the AOP in the transversal current
spectra, it is predicted that the transversal sound velocity also exhibits the
anomalous dispersion. These results of the theory are demonstrated for a model
of the Lennard-Jones system.Comment: 25 pages, 22 figure
Anharmonic vs. relaxational sound damping in glasses: II. Vitreous silica
The temperature dependence of the frequency dispersion in the sound velocity
and damping of vitreous silica is reanalyzed. Thermally activated relaxation
accounts for the sound attenuation observed above 10 K at sonic and ultrasonic
frequencies. Its extrapolation to the hypersonic regime reveals that the
anharmonic coupling to the thermal bath becomes important in
Brillouin-scattering measurements. At 35 GHz and room temperature, the damping
due to this anharmonicity is found to be nearly twice that produced by
thermally activated relaxation. The analysis also reveals a sizeable velocity
increase with temperature which is not related with sound dispersion. This
suggests that silica experiences a gradual structural change that already
starts well below room temperature.Comment: 13 pages with 8 figure
Effect of Injector Exit Geometry on Atomization of a Liquid-Liquid Double Swirl Coaxial Injector using Non-Invasive Laser, Optical, and X-ray Techniques
The spray characteristics of a liquid-liquid double swirl coaxial injector were studied using non-invasive optical, laser, and X-ray diagnostics. Phase Doppler interferometry was used to characterize droplet statistics and non-dimensional droplet parameters over a range of inlet conditions and for various fluids allowing for a study on the role of fluid properties on atomization. Based on the atomization statistics and observed trends from high-speed images, a description of breakup regimes over a range of Reynolds and Weber numbers was created. Next, X-ray computed tomography scans revealed that the spray cone was distinctively non-uniform and comprised of several pockets of increased mass concentration which varied with injection pressure. Finally, a parametric study of injector exit geometry demonstrated that spray breakup time, breakup type, and sheet stability could be controlled with exit geometry. Implications for these data on injector stability and atomization efficiency are discussed considering the desired performance characteristics of liquid-liquid rocket injectors
Influence of Mg, Ag and Al substitutions on the magnetic excitations in the triangular-lattice antiferromagnet CuCrO2
Magnetic excitations in CuCrO, CuCrMgO,
CuAgCrO, and CuCrAlO have been
studied by powder inelastic neutron scattering to elucidate the element
substitution effects on the spin dynamics in the Heisenberg triangular-lattice
antiferromagnet CuCrO. The magnetic excitations in
CuCrMgO consist of a dispersive component and a flat
component. Though this feature is apparently similar to CuCrO, the energy
structure of the excitation spectrum shows some difference from that in
CuCrO. On the other hand, in CuAgCrO and
CuCrAlO the flat components are much reduced, the
low-energy parts of the excitation spectra become intense, and additional
low-energy diffusive spin fluctuations are induced. We argued the origins of
these changes in the magnetic excitations are ascribed to effects of the doped
holes or change of the dimensionality in the magnetic correlations.Comment: 7 pages, 5 figure
Inhibition of activin/nodal signalling is necessary for pancreatic differentiation of human pluripotent stem cells
Peer reviewedPublisher PD
Anharmonicity, vibrational instability and Boson peak in glasses
We show that a {\em vibrational instability} of the spectrum of weakly
interacting quasi-local harmonic modes creates the maximum in the inelastic
scattering intensity in glasses, the Boson peak. The instability, limited by
anharmonicity, causes a complete reconstruction of the vibrational density of
states (DOS) below some frequency , proportional to the strength of
interaction. The DOS of the new {\em harmonic modes} is independent of the
actual value of the anharmonicity. It is a universal function of frequency
depending on a single parameter -- the Boson peak frequency, which
is a function of interaction strength. The excess of the DOS over the Debye
value is at low frequencies and linear in in the
interval . Our results are in an excellent
agreement with recent experimental studies.Comment: LaTeX, 8 pages, 6 figure
- …