253 research outputs found
From extended phase space dynamics to fluid theory
We derive a fluid theory for spin-1/2 particles starting from an extended
kinetic model based on a spin-projected density matrix formalism. The evolution
equation for the spin density is found to contain a pressure-like term. We give
an example where this term is important by looking at a linear mode previously
found in a spin kinetic model.Comment: 4 page
Spin and magnetization effects in plasmas
We give a short review of a number of different models for treating
magnetization effects in plasmas. In particular, the transition between kinetic
models and fluid models is discussed. We also give examples of applications of
such theories. Some future aspects are discussed.Comment: 18 pages, 1 figure. To appear in Plasma Physics and Controlled
Fusion, Special Issue for the 37th ICPP, Santiago, Chil
Structural Relaxation and Mode Coupling in a Simple Liquid: Depolarized Light Scattering in Benzene
We have measured depolarized light scattering in liquid benzene over the
whole accessible temperature range and over four decades in frequency. Between
40 and 180 GHz we find a susceptibility peak due to structural relaxation. This
peak shows stretching and time-temperature scaling as known from
relaxation in glass-forming materials. A simple mode-coupling model provides
consistent fits of the entire data set. We conclude that structural relaxation
in simple liquids and relaxation in glass-forming materials are
physically the same. A deeper understanding of simple liquids is reached by
applying concepts that were originally developed in the context of
glass-transition research.Comment: submitted to New J. Phy
Dielectric and thermal relaxation in the energy landscape
We derive an energy landscape interpretation of dielectric relaxation times
in undercooled liquids, comparing it to the traditional Debye and
Gemant-DiMarzio-Bishop pictures. The interaction between different local
structural rearrangements in the energy landscape explains qualitatively the
recently observed splitting of the flow process into an initial and a final
stage. The initial mechanical relaxation stage is attributed to hopping
processes, the final thermal or structural relaxation stage to the decay of the
local double-well potentials. The energy landscape concept provides an
explanation for the equality of thermal and dielectric relaxation times. The
equality itself is once more demonstrated on the basis of literature data for
salol.Comment: 7 pages, 3 figures, 41 references, Workshop Disordered Systems,
Molveno 2006, submitted to Philosophical Magazin
Spin kinetic theory - quantum kinetic theory in extended phase space
The concept of phase space distribution functions and their evolution is used
in the case of en enlarged phase space. In particular, we include the intrinsic
spin of particles and present a quantum kinetic evolution equation for a scalar
quasi-distribution function. In contrast to the proper Wigner transformation
technique, for which we expect the corresponding quasi-distribution function to
be a complex matrix, we introduce a spin projection operator for the density
matrix in order to obtain the aforementioned scalar quasi-distribution
function. There is a close correspondence between this projection operator and
the Husimi (or Q) function used extensively in quantum optics. Such a function
is based on a Gaussian smearing of a Wigner function, giving a positive
definite distribution function. Thus, our approach gives a Wigner-Husimi
quasi-distribution function in extended phase space, for which the reduced
distribution function on the Bloch sphere is strictly positive. We also discuss
the gauge issue and the fluid moment hierarchy based on such a quantum kinetic
theory.Comment: 10 pages, to appear in Transport Theory and Statistical Physics,
proceedings of Vlasovia III, 200
The nature of the short wavelength excitations in vitreous silica: X-Rays Brillouin scattering study
The dynamical structure factor (S(Q,E)) of vitreous silica has been measured
by Inelastic X-ray Scattering varying the exchanged wavevector (Q) at fixed
exchanged energy (E) - an experimental procedure that, contrary to the usual
one at constant Q, provides spectra with much better identified inelastic
features. This allows the first direct evidence of Brillouin peaks in the
S(Q,E) of SiO_2 at energies above the Boson Peak (BP) energy, a finding that
excludes the possibility that the BP marks the transition from propagating to
localised dynamics in glasses.Comment: 4 pages, 3 Postscript figures. To appear in Physical Review Letter
Melting of 2D liquid crystal colloidal structure
Using video microscopy, we investigated melting of a two-dimensional colloidal system, formed by glycerol droplets at the free surface of a nematic liquid crystalline layer. Analyzing different structure correlation functions, we conclude that melting occurs through an intermediate hexatic phase, as predicted by the Kosterlitz-Thouless-Halperin-Nelson-Young(KTHNY) theory. However, the temperature range of the intermediate phase is rather narrow, <1°C, and the characteristic critical power law decays of the correlation functions are not fully developed. We conclude that the melting of our 2D systems qualitatively occurs according to KTHNY, although quantitative details of the transition scenario may partly depend on the details of interparticle interaction
Scalar quantum kinetic theory for spin-1/2 particles: mean field theory
Starting from the Pauli Hamiltonian operator, we derive a scalar quantum
kinetic equations for spin-1/2 systems. Here the regular Wigner two-state
matrix is replaced by a scalar distribution function in extended phase space.
Apart from being a formulation of principal interest, such scalar quantum
kinetic equation makes the comparison to classical kinetic theory
straightforward, and lends itself naturally to currently available numerical
Vlasov and Boltzmann schemes. Moreover, while the quasi-distribution is a
Wigner function in regular phase space, it is given by a Q-function in spin
space. As such, nonlinear and dynamical quantum plasma problems are readily
handled. Moreover, the issue of gauge invariance is treated. Applications (e.g.
ultra-dense laser compressed targets and their diagnostics), possible
extensions, and future improvements of the presented quantum statistical model
are discussed.Comment: 21 pages, 2 figure
Dielectric and conductivity relaxation in mixtures of glycerol with LiCl
We report a thorough dielectric characterization of the alpha relaxation of
glass forming glycerol with varying additions of LiCl. Nine salt concentrations
from 0.1 - 20 mol% are investigated in a frequency range of 20 Hz - 3 GHz and
analyzed in the dielectric loss and modulus representation. Information on the
dc conductivity, the dielectric relaxation time (from the loss) and the
conductivity relaxation time (from the modulus) is provided. Overall, with
increasing ion concentration, a transition from reorientationally to
translationally dominated behavior is observed and the translational ion
dynamics and the dipolar reorientational dynamics become successively coupled.
This gives rise to the prospect that by adding ions to dipolar glass formers,
dielectric spectroscopy may directly couple to the translational degrees of
freedom determining the glass transition, even in frequency regimes where
usually strong decoupling is observed.Comment: 8 pages, 7 figure
Flow-volume loops derived from three-dimensional echocardiography: a novel approach to the assessment of left ventricular hemodynamics
BACKGROUND: This study explores the feasibility of non-invasive evaluation of left ventricular (LV) flow-volume dynamics using 3-dimensional (3D) echocardiography, and the capacity of such an approach to identify altered LV hemodynamic states caused by valvular abnormalities. METHODS: Thirty-one patients with moderate-severe aortic (AS) and mitral (MS) stenoses (21 and 10 patients, respectively) and 10 healthy volunteers underwent 3D echocardiography with full volume acquisition using Philips Sonos 7500 equipment. The digital 3D data were post- processed using TomTec software. LV flow-volume loops were subsequently constructed for each subject by plotting instantaneous LV volume data sampled throughout the cardiac cycle vs. their first derivative representing LV flow. After correction for body surface area, an average flow-volume loop was calculated for each subject group. RESULTS: Flow-volume loops were obtainable in all subjects, except 3 patients with AS. The flow-volume diagrams displayed clear differences in the form and position of the loops between normal individuals and the respective patient groups. In patients with AS, an "obstructive" pattern was observed, with lower flow values during early systole and larger end-systolic volume. On the other hand, patients with MS displayed a "restrictive" flow-volume pattern, with reduced diastolic filling and smaller end-diastolic volume. CONCLUSION: Non-invasive evaluation of LV flow-volume dynamics using 3D-echocardiographic data is technically possible and the approach has a capacity to identify certain specific types of alteration of LV flow-volume pattern caused by valvular abnormalities, thus reflecting underlying hemodynamic states specific for these abnormalities
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