169 research outputs found
A non-perturbative analysis of symmetry breaking in two-dimensional phi^4 theory using periodic field methods
We describe the generalization of spherical field theory to other modal
expansion methods. The main approach remains the same, to reduce a
d-dimensional field theory into a set of coupled one-dimensional systems. The
method we discuss here uses an expansion with respect to periodic-box modes. We
apply the method to phi^4 theory in two dimensions and compute the critical
coupling and critical exponents. We compare with lattice results and
predictions via universality and the two-dimensional Ising model.Comment: 12 pages, 4 figures, version to appear in Physics Letters
Theoretical prediction of spectral and optical properties of bacteriochlorophylls in thermally disordered LH2 antenna complexes
A general approach for calculating spectral and optical properties of
pigment-protein complexes of known atomic structure is presented. The method,
that combines molecular dynamics simulations, quantum chemistry calculations
and statistical mechanical modeling, is demonstrated by calculating the
absorption and circular dichroism spectra of the B800-B850 BChls of the LH2
antenna complex from Rs. molischianum at room temperature. The calculated
spectra are found to be in good agreement with the available experimental
results. The calculations reveal that the broadening of the B800 band is mainly
caused by the interactions with the polar protein environment, while the
broadening of the B850 band is due to the excitonic interactions. Since it
contains no fitting parameters, in principle, the proposed method can be used
to predict optical spectra of arbitrary pigment-protein complexes of known
structure.Comment: ReVTeX4, 11 pages, 9 figures, submitted to J. Chem. Phy
Calculating potentials of mean force and diffusion coefficients from nonequilibirum processes without Jarzynski's equality
In general, the direct application of the Jarzynski equality (JE) to
reconstruct potentials of mean force (PMFs) from a small number of
nonequilibrium unidirectional steered molecular dynamics (SMD) paths is
hindered by the lack of sampling of extremely rare paths with negative
dissipative work. Such trajectories, that transiently violate the second law,
are crucial for the validity of JE. As a solution to this daunting problem, we
propose a simple and efficient method, referred to as the FR method, for
calculating simultaneously both the PMF U(z) and the corresponding diffusion
coefficient D(z) along a reaction coordinate z for a classical many particle
system by employing a small number of fast SMD pullings in both forward (F) and
time reverse (R) directions, without invoking JE. By employing Crook's
transient fluctuation theorem (that is more general than JE) and the stiff
spring approximation, we show that: (i) the mean dissipative work W_d in the F
and R pullings are equal, (ii) both U(z) and W_d can be expressed in terms of
the easily calculable mean work of the F and R processes, and (iii) D(z) can be
expressed in terms of the slope of W_d. To test its viability, the FR method is
applied to determine U(z) and D(z) of single-file water molecules in
single-walled carbon nanotubes (SWNTs). The obtained U(z) is found to be in
very good agreement with the results from other PMF calculation methods, e.g.,
umbrella sampling. Finally, U(z) and D(z) are used as input in a stochastic
model, based on the Fokker-Planck equation, for describing water transport
through SWNTs on a mesoscopic time scale that in general is inaccessible to MD
simulations.Comment: ReVTeX4, 13 pages, 6 EPS figures, Submitted to Journal of Chemical
Physic
Phase Fluctuations and Pseudogap Properties: Influence of Nonmagnetic Impurities
The presence of nonmagnetic impurities in a 2D ``bad metal'' depresses the
superconducting Berezinskii-Kosterlitz-Thouless transition temperature, while
leaving the pairing energy scale unchanged. Thus the region of the pseudogap
non-superconducting phase, where the modulus of the order parameter is non-zero
but its phase is random, and which opens at the pairing temperature is
substantially bigger than for the clean system. This supports the premise that
fluctuations in the phase of the order parameter can in principle describe the
pseudogap phenomena in high- materials over a rather wide range of
temperatures and carrier densities. The temperature dependence of the bare
superfluid density is also discussed.Comment: 11 pages, LaTeX, 1 EPS figure; final version to appear in
Low.Temp.Phy
Pairing Fluctuation Theory of Superconducting Properties in Underdoped to Overdoped Cuprates
We propose a theoretical description of the superconducting state of under-
to overdoped cuprates, based on the short coherence length of these materials
and the associated strong pairing fluctuations. The calculated and the
zero temperature excitation gap , as a function of hole
concentration , are in semi-quantitative agreement with experiment. Although
the ratio has a strong dependence, different from the
universal BCS value, and deviates significantly from the BCS
prediction, we obtain, quite remarkably, quasi-universal behavior, for the
normalized superfluid density and the Josephson critical
current , as a function of . While experiments on
are consistent with these results, future measurements on
are needed to test this prediction.Comment: 4 pages, 3 figures, REVTeX, submitted to Phys. Rev. Let
Comment on "Is the nonlinear Meissner effect unobservable?"
In a recent Letter (Phys. Rev. Lett. 81, p.5640 (1998), cond-mat/9808249 v3),
it was suggested that nonlocal effects may prevent observation of the nonlinear
Meissner effect in YBCO. We argue that this claim is incorrect with regards to
measurements of the nonlinear transverse magnetic moment, and that the most
likely reason for a null result lies elsewhere.Comment: 1 pag
Kinetic Monte Carlo and Cellular Particle Dynamics Simulations of Multicellular Systems
Computer modeling of multicellular systems has been a valuable tool for
interpreting and guiding in vitro experiments relevant to embryonic
morphogenesis, tumor growth, angiogenesis and, lately, structure formation
following the printing of cell aggregates as bioink particles. Computer
simulations based on Metropolis Monte Carlo (MMC) algorithms were successful in
explaining and predicting the resulting stationary structures (corresponding to
the lowest adhesion energy state). Here we present two alternatives to the MMC
approach for modeling cellular motion and self-assembly: (1) a kinetic Monte
Carlo (KMC), and (2) a cellular particle dynamics (CPD) method. Unlike MMC,
both KMC and CPD methods are capable of simulating the dynamics of the cellular
system in real time. In the KMC approach a transition rate is associated with
possible rearrangements of the cellular system, and the corresponding time
evolution is expressed in terms of these rates. In the CPD approach cells are
modeled as interacting cellular particles (CPs) and the time evolution of the
multicellular system is determined by integrating the equations of motion of
all CPs. The KMC and CPD methods are tested and compared by simulating two
experimentally well known phenomena: (1) cell-sorting within an aggregate
formed by two types of cells with different adhesivities, and (2) fusion of two
spherical aggregates of living cells.Comment: 11 pages, 7 figures; submitted to Phys Rev
Chaos in Andreev Billiards
A new type of classical billiard - the Andreev billiard - is investigated
using the tangent map technique. Andreev billiards consist of a normal region
surrounded by a superconducting region. In contrast with previously studied
billiards, Andreev billiards are integrable in zero magnetic field, {\it
regardless of their shape}. A magnetic field renders chaotic motion in a
generically shaped billiard, which is demonstrated for the Bunimovich stadium
by examination of both Poincar\'e sections and Lyapunov exponents. The issue of
the feasibility of certain experimental realizations is addressed.Comment: ReVTeX3.0, 4 pages, 3 figures appended as postscript file (uuencoded
with uufiles
Algorithm for obtaining the gradient expansion of the local density of states and the free energy of a superconductor
We present an efficient algorithm for obtaining the gauge-invariant gradient
expansion of the local density of states and the free energy of a clean
superconductor. Our method is based on a new mapping of the semiclassical
linearized Gorkov equations onto a pseudo-Schroedinger equation for a
three-component wave-function psi(x), where one component is directly related
to the local density of states. Because psi(x) satisfies a linear equation of
motion, successive terms in the gradient expansion can be obtained by simple
linear iteration. Our method works equally well for real and complex order
parameter, and in the presence of arbitrary external fields. We confirm a
recent calculation of the fourth order correction to the free energy by
Kosztin, Kos, Stone and Leggett [Phys. Rev. B 58, 9365 (1998)], who obtained a
discrepancy with an earlier result by Tewordt [Z. Phys. 180, 385 (1964)]. We
also give the fourth order correction to the local density of states, which has
not been published before.Comment: 12 preprint pages, added remark concerning Eilenberger equation,
accepted for publication in Phys. Rev.
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