161 research outputs found
Perturbation theory for the one-dimensional optical polaron
The one-dimensional optical polaron is treated on the basis of the
perturbation theory in the weak coupling limit. A special matrix diagrammatic
technique is developed. It is shown how to evaluate all terms of the
perturbation theory for the ground-state energy of a polaron to any order by
means of this technique. The ground-state energy is calculated up to the eighth
order of the perturbation theory. The effective mass of an electron is obtained
up to the sixth order of the perturbation theory. The radius of convergence of
the obtained series is estimated. The obtained results are compared with the
results from the Feynman polaron theory.Comment: 9 pages, 2 figures, RevTeX, to be published in Phys. Rev. B (2001)
Ap
Analyticity of The Ground State Energy For Massless Nelson Models
We show that the ground state energy of the translationally invariant Nelson
model, describing a particle coupled to a relativistic field of massless
bosons, is an analytic function of the coupling constant and the total
momentum. We derive an explicit expression for the ground state energy which is
used to determine the effective mass.Comment: 33 pages, 1 figure, added a section on the calculation of the
effective mas
Coarse-Grained Models of Biological Membranes within the Single Chain Mean Field Theory
The Single Chain Mean Field theory is used to simulate the equilibrium
structure of phospholipid membranes at the molecular level. Three levels of
coarse-graining of DMPC phospholipid surfactants are present: the detailed
44-beads double tails model, the 10-beads double tails model and the minimal
3-beads model. We show that all three models are able to reproduce the
essential equilibrium properties of the phospholipid bilayer, while the
simplest 3-beads model is the fastest model which can describe adequately the
thickness of the layer, the area per lipid and the rigidity of the membrane.
The accuracy of the method in description of equilibrium structures of
membranes compete with Monte Carlo simulations while the speed of computation
and the mean field nature of the approach allows for straightforward
applications to systems with great complexity.Comment: Accepted for publication in Soft Matte
Viscosities of the Gay-Berne nematic liquid crystal
We present molecular dynamics simulation measurements of the viscosities of
the Gay-Berne phenomenological model of liquid crystals in the nematic and
isotropic phases. The temperature dependence of the rotational and shear
viscosities, including the nonmonotonic behavior of one shear viscosity are in
good agreement with experimental data. The bulk viscosities are significantly
larger than the shear viscosities, again in agreement with experiment.Comment: 11 pages, 4 Postscript figures, Revte
On the exciton binding energy in a quantum well
We consider a model describing the one-dimensional confinement of an exciton
in a symmetrical, rectangular quantum-well structure and derive upper and lower
bounds for the binding energy of the exciton. Based on these bounds, we
study the dependence of on the width of the confining potential with a
higher accuracy than previous reports. For an infinitely deep potential the
binding energy varies as expected from at large widths to at
small widths. For a finite potential, but without consideration of a mass
mismatch or a dielectric mismatch, we substantiate earlier results that the
binding energy approaches the value for both small and large widths,
having a characteristic peak for some intermediate size of the slab. Taking the
mismatch into account, this result will in general no longer be true. For the
specific case of a quantum-well
structure, however, and in contrast to previous findings, the peak structure is
shown to survive.Comment: 32 pages, ReVTeX, including 9 figure
Pressure Effects and Large Polarons in Layered MgB_2 Superconductor
We consider the dependence of the MgB_2 superconducting critical temperature
on the pressure. Our model exploits the influence of the large polarons on the
band structure of the layered MgB_2 superconductor. Namely, the hole
Pekar-Froehlich polarons form quasi two-dimensional potential wells in the
boron plane which shift the positions of the sigma- and pi-bands. This energy
shift depends on the pressure and the Cooper pairing of the correlated
sigma-electrons happens inside polaron wells. The results obtained are as
follows: dT_c/dp = -\alpha (5.2 \pm 0.9) K/GPa or dT_c/dp = -\alpha (6.9\pm
1.1) K/GPa for a different choice of the Grueneisen parameter. Being compared
with known experimental data they give us a resonable interval for the value of
the Froehlich electron-phonon coupling constant: \alpha = 0.15 - 0.45.Comment: 6 pages, 1 fig, LaTeX, subm. to Phys. Rev.
Phase-ordering dynamics of the Gay-Berne nematic liquid crystal
Phase-ordering dynamics in nematic liquid crystals has been the subject of
much active investigation in recent years in theory, experiments and
simulations. With a rapid quench from the isotropic to nematic phase a large
number of topological defects are formed and dominate the subsequent
equilibration process. We present here the results of a molecular dynamics
simulation of the Gay-Berne model of liquid crystals after such a quench in a
system with 65536 molecules. Twist disclination lines as well as type-1 lines
and monopoles were observed. Evidence of dynamical scaling was found in the
behavior of the spatial correlation function and the density of disclination
lines. However, the behavior of the structure factor provides a more sensitive
measure of scaling, and we observed a crossover from a defect dominated regime
at small values of the wavevector to a thermal fluctuation dominated regime at
large wavevector.Comment: 18 pages, 16 figures, animations available at
http://www.physics.brown.edu/Users/faculty/pelcovits/lc/coarsening.htm
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