共查询到20条相似文献,搜索用时 31 毫秒
1.
The Frank elasticity constants which describe splay (K
1), twist (K
2), and bend (K
3) distortion modes are investigated for 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic liquid crystal. The calculations
rest on statistical-mechanical approaches where the absolute values of K
i
(i=1,2,3) are dependent on the direct correlation function (DCF) of the corresponding nematic state. The DCF was determined
using the pair correlation function by solving the Ornstein-Zernike equation. The pair correlation function, in turn, was
obtained from molecular dynamics (MD) trajectory. Three different approaches for calculations of the elasticity constants
were employed based on different level of approximation about the orientational order and molecular correlations. The best
agreement with experimental values of elasticity constants was obtained in a model where the full orientational distribution
function was used. In addition we have investigated the approximation about spherical distribution of the intermolecular vectors
in the nematic phase, often used in derivation of various mean-field theories and employed here for the construction of the
DCF. We found that this assumption is not strictly valid, in particular a strong deviation from the isotropic distribution
is observed for short intermolecular distances.
Received 22 March 2000 and Received in final form 9 June 2000 相似文献
2.
3.
M. Holzmann P. Grüter F. Laloë 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(4):739-760
We study the occurrence of a Bose-Einstein transition in a dilute gas with repulsive interactions, starting from temperatures
above the transition temperature. The formalism, based on the use of Ursell operators, allows us to evaluate the one-particle
density operator with more flexibility than in mean-field theories, since it does not necessarily coincide with that of an
ideal gas with adjustable parameters (chemical potential, etc.). In a first step, a simple approximation is used (Ursell-Dyson
approximation), which allow us to recover results which are similar to those of the usual mean-field theories. In a second
step, a more precise treatment of the correlations and velocity dependence of the populations in the system is elaborated.
This introduces new physical effects, such as a change of the velocity profile just above the transition: the proportion of
atoms with low velocities is higher than in an ideal gas. A consequence of this distortion is an increase of the critical
temperature (at constant density) of the Bose gas, in agreement with those of recent path integral Monte-Carlo calculations
for hard spheres.
Received 13 November 1998 相似文献
4.
A Green function formalism is applied to study the dielectric function spectra and the small polaron hopping conduction in organic molecular crystals. In the calculations, the electron-phonon interaction is considered within the Hartree-Fock approximation, and the temperature effect is taken into account. Our theoretical approach is based on the polar electron-phonon interaction (Fröhlich type) to characterize the non-degenerate polaron gas, with the assumption of the electronic hopping between the first-neighbor. 相似文献
5.
R. Jursa S. Wermbter G. Czycholl 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(3):387-391
We study the influence of the short-ranged Hubbard correlation U between the conduction electrons on the Cooper pair formation in normal (s-wave) superconductors. The Coulomb correlation is considered within the standard second order perturbation theory, which
becomes exact in the weak coupling limit but goes beyond the simple Hartree-Fock treatment by yielding a finite lifetime of
the quasiparticles at finite temperature. An attractive pairing interaction V, which may be mediated by the standard electron-phonon mechanism, is considered between nearest neighbor sites. A critical
value for the attractive interaction is required to obtain a superconducting state. For finite temperature a gapless superconductivity is obtained due to the finite lifetime of the quasiparticles, i.e. the Coulomb correlation has a pair-breaking influence. The energy gap and depend very sensitively on U, V and band filling n and develop a maximum away from half filling as function of n. The ratio varies with n, being higher than the BCS value near half filling and reaching the BCS value for lower n.
Received 17 February 1999 相似文献
6.
V. Meden W. Metzner U. Schollwöck O. Schneider T. Stauber K. Schönhammer 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(4):631-646
We present a study of the one-particle spectral properties for a variety of models of Luttinger liquids with open boundaries.
We first consider the Tomonaga-Luttinger model using bosonization. For weak interactions the boundary exponent of the power-law
suppression of the spectral weight close to the chemical potential is dominated by a term linear in the interaction. This
motivates us to study the spectral properties also within the Hartree-Fock approximation. It already gives power-law behavior
and qualitative agreement with the exact spectral function. For the lattice model of spinless fermions and the Hubbard model
we present numerically exact results obtained using the density-matrix renormalization-group algorithm. We show that many
aspects of the behavior of the spectral function close to the boundary can again be understood within the Hartree-Fock approximation.
For the repulsive Hubbard model with interaction U the spectral weight is enhanced in a large energy range around the chemical potential. At smaller energies a power-law suppression,
as predicted by bosonization, sets in. We present an analytical discussion of the crossover and show that for small U it occurs at energies exponentially (in -1/U) close to the chemical potential, i.e. that bosonization only holds on exponentially small energy scales. We show that such a crossover can also be found in other
models.
Received 8 February 2000 and Received in final form 25 April 2000 相似文献
7.
8.
W. Kob J.-L. Barrat 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,13(2):319-333
By means of molecular dynamics computer simulations we investigate the out of equilibrium relaxation dynamics of a simple
glass former, a binary Lennard-Jones system, after a quench to low temperatures. We find that one-time quantities, such as
the energy or the structure factor, show only a weak time dependence. By comparing the out of equilibrium structure factor
with equilibrium data we find evidence that during the aging process the system remains in that part of phase space that mode-coupling
theory classifies as liquid like. Two-times correlation functions show a strong time and waiting time dependence. For large and times corresponding to the early -relaxation regime the correlators approach the Edwards-Anderson value by means of a power-law in time. For large but fixed
values of the relaxation dynamics in the -relaxation regime seems to be independent of the observable and temperature. The -relaxation shows a power-law dependence on time with an exponent which is independent of but depends on the observable. We find that at long times the correlation functions can be expressed as and compute the function h(t). This function is found to show a t-dependence which is a bit stronger than a logarithm and to depend on the observable considered. If the system is quenched
to very low temperatures the relaxation dynamics at long times shows fast drops as a function of time. We relate these drops
to relatively local rearrangements in which part of the sample relaxes its stress by a collective motion of 50-100 particles.
Finally we discuss our measurements of the time dependent response function. We find that at long times the correlation functions
and the response are not related by the usual fluctuation dissipation theorem but that this relation is similar to the one
found for spin glasses with one step replica symmetry breaking.
Received 17 May 1999 相似文献
9.
The linear relaxation of a one-dimensional bonded fluid model is studied in the dynamical mean field approximation at the pair level. The results are compared with simulation data. Relaxation at low temperatures shows a cooperative character that can be understood in terms of a simple diffusion picture. 相似文献
10.
K.L. Yao L. Zhao 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(3):411-417
Based on a theoretical model proposed for quasi-one-dimensional organic polymer ferromagnets, the ground state and low-lying
magnetic excitation are studied. Within Hartree-Fock approximation, the ground state of the system is shown to be a stable
ferromagnetic state due to the electron-electron correlation and topological structure of the system. The random-phase approximation
is employed to explore the magnon excitation and the excitation spectrum is obtained, including an acoustic mode and four
optical modes. It is found that the acoustic mode possesses the characteristic of the ferromagnetic magnon.
Received 9 July 1999 相似文献
11.
Molecular dynamics simulations of water, liquid beryllium fluoride and silica melt are used to study the accuracy with which the entropy of ionic and molecular liquids can be estimated from atom–atom radial distribution function data. The pair correlation entropy is demonstrated to be sufficiently accurate that the density–temperature regime of anomalous behaviour as well as the strength of the entropy anomaly can be predicted reliably for both ionic melts as well as different rigid-body pair potentials for water. Errors in the total thermodynamic entropy for ionic melts due to the pair correlation approximation are of the order of 10% or less for most state points, but can be significantly larger in the anomalous regime at very low temperatures. In the case of water, the rigid-body constraints result in larger errors in the pair correlation approximation, between 20 and 30%, for most state points. Comparison of the excess entropy, S e, of ionic melts with the pair correlation entropy, S 2, shows that the temperature dependence of S e is well described by T ?2/5 scaling across both the normal and anomalous regimes, unlike in the case of S 2. The residual multiparticle entropy, ΔS = S e ? S 2, shows a strong negative correlation with tetrahedral order in the anomalous regime. 相似文献
12.
The correlation phenomena in two-dimensional semiconductors in high transverse magnetic fields are considered. A diagram expansion convergent at any temperatures T is obtained due to the special choice of an initial approximation. The expansion parameter appears equal to (Eo/T)exp(-Eo/2T) where Eo is the exciton binding energy. The expansion parameter of the usual Matsubara diagram technique would be proportional to 1/T, if the noninteracting electron-hole system were choosen as the initial approximation, due to the infinite degree of degeneracy of the ground state. The system proves equivalent to the slightly non-ideal exciton gas with the same long-wave properties as the two-dimensional Bose gas. The Hartree-Fock approximation is exact at T = 0, but there is no phase transition with a long-range order creation at T≠0 in contrast to the HFA results (although there remain some properties of the transition such as the maximum of specific heat etc.). The Berezinsky-Kosterlitz-Thouless transition is predicted in the system for very low temperatures. The diagram technique developed may be employed to treat the thermodynamics of other infinitely-fold degenerated systems. 相似文献
13.
14.
15.
16.
We study the dynamical correlation functions and heat conduction for the simplest model of quasi one-dimensional (1d) dielectric
crystal i.e. a chain of classical particles coupled by quadratic and cubic intersite potential. Even in the weakly nonlinear regime, numerical
simulation on long enough chains reveal sizeable deviations from the perturbative results in the form of a slower decay of
correlations in equilibrium. Their origin can be traced back to the subtle nonlinear effects described by mode-coupling theories.
Measures of thermal conductivity with nonequilibrium molecular-dynamics method confirm the relevance of such effects for low-dimensional
lattices even at very low temperatures.
Received 20 April 2000 相似文献
17.
V. Drchal 《Journal of Physics and Chemistry of Solids》1979,40(5):393-403
Dynamical electron-electron correlation in paramagnetic disordered binary alloys described by the Hubbard-type Hamiltonian is studied using the T-matrix approximation (TMA) valid for low concentration of particles ( 0.6/atom). We introduce the terminal-point approximation for the many-body quantities, which allows us to solve the random part of the problem within the single-site approximation (CPA). The one-centre version of the TMA is used in numerical calculations. Results are compared with those of the Hartree-Fock approximation, and with the model of a pair of interacting electrons in a random alloy (CPA II). 相似文献
18.
C. Rummel J. Ankerhold 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(1):105-115
Based on the path integral approach the partition function of a many body system with separable two body interaction is calculated
in the sense of a semiclassical approximation. The commonly used Gaussian type of approximation, known as the perturbed static
path approximation (PSPA), breaks down near a crossover temperature due to instabilities of the classical mean field solution.
It is shown how the PSPA is systematically improved within the crossover region by taking into account large non-Gaussian
fluctuations and an approximation applicable down to very low temperatures is carried out. These findings are tested against
exact results for the archetypical cases of a particle moving in a one dimensional double well and the exactly solvable Lipkin-Meshkov-Glick
model. The extensions should have applications in finite systems at low temperatures as in nuclear physics and mesoscopic
systems, e.g. for gap fluctuations in nanoscale superconducting devices previously studied within a PSPA type of approximation.
Received 28 March 2002 Published online 17 September 2002 相似文献
19.
We investigate the dynamic response of population transfer between two components of a finite temperature spinor Bose condensed gas to a time-dependent coupling potential. Comparison between the results obtained in the Bogoliubov–Popov approximation (BPA) and in the generalized random phase approximation (GRPA) shows noticeable discrepancies. In particular, the inter-component current response function calculated in the GRPA displays a gapped spectrum due to the exchange interaction energy whereas the corresponding density response function is gapless. We verify that, contrary to the BPA, the GRPA preserves the SU(2) symmetry and the f-sum rule associated to the spinor gas. In order to validate the approximation, we propose an experimental setup that allows the observation of the predicted gap. 相似文献