13,428 research outputs found
coupling constant in light cone QCD sum rules
We employ the light cone QCD sum rules to calculate coupling
constant by studying the two point correlation function between the vacuum and
the pion state. Our result is consistent with the traditional QCD sum rules
calculations and it is in agreement with the experimental value.Comment: 8 pages, latex, 2 figure
Solitonic-exchange mechanism of surface~diffusion
We study surface diffusion in the framework of a generalized
Frenkel-Kontorova model with a nonconvex transverse degree of freedom. The
model describes a lattice of atoms with a given concentration interacting by
Morse-type forces, the lattice being subjected to a two-dimensional substrate
potential which is periodic in one direction and nonconvex (Morse) in the
transverse direction. The results are used to describe the complicated
exchange-mediated diffusion mechanism recently observed in MD simulations [J.E.
Black and Zeng-Ju Tian, Phys. Rev. Lett. {\bf 71}, 2445-2448(1993)].Comment: 22 Revtex pages, 9 figures to appear in Phys. Rev.
Dynamical phase diagram of the dc-driven underdamped Frenkel-Kontorova chain
Multistep dynamical phase transition from the locked to the running state of
atoms in response to a dc external force is studied by MD simulations of the
generalized Frenkel-Kontorova model in the underdamped limit. We show that the
hierarchy of transition recently reported [Braun et al, Phys. Rev. Lett. 78,
1295 (1997)] strongly depends on the value of the friction constant. A simple
phenomenological explanation for the friction dependence of the various
critical forces separating intermediate regimes is given.Comment: 12 Revtex Pages, 4 EPS figure
Parallel updating cellular automaton models of driven diffusive Frenkel-Kontorova-type systems
Three cellular automaton (CA) models of increasing complexity are introduced
to model driven diffusive systems related to the generalized Frenkel-Kontorova
(FK) models recently proposed by Braun [Phys.Rev.E58, 1311 (1998)]. The models
are defined in terms of parallel updating rules. Simulation results are
presented for these models. The features are qualitatively similar to those
models defined previously in terms of sequentially updating rules. Essential
features of the FK model such as phase transitions, jamming due to atoms in the
immobile state, and hysteresis in the relationship between the fraction of
atoms in the running state and the bias field are captured. Formulating in
terms of parallel updating rules has the advantage that the models can be
treated analytically by following the time evolution of the occupation on every
site of the lattice. Results of this analytical approach are given for the two
simpler models. The steady state properties are found by studying the stable
fixed points of a closed set of dynamical equations obtained within the
approximation of retaining spatial correlations only upto two nearest
neighboring sites. Results are found to be in good agreement with numerical
data.Comment: 26 pages, 4 eps figure
On the driven Frenkel-Kontorova model: II. Chaotic sliding and nonequilibrium melting and freezing
The dynamical behavior of a weakly damped harmonic chain in a spatially
periodic potential (Frenkel-Kontorova model) under the subject of an external
force is investigated. We show that the chain can be in a spatio-temporally
chaotic state called fluid-sliding state. This is proven by calculating
correlation functions and Lyapunov spectra. An effective temperature is
attributed to the fluid-sliding state. Even though the velocity fluctuations
are Gaussian distributed, the fluid-sliding state is clearly not in equilibrium
because the equipartition theorem is violated. We also study the transition
between frozen states (stationary solutions) and=7F molten states
(fluid-sliding states). The transition is similar to a first-order phase
transition, and it shows hysteresis. The depinning-pinning transition
(freezing) is a nucleation process. The frozen state contains usually two
domains of different particle densities. The pinning-depinning transition
(melting) is caused by saddle-node bifurcations of the stationary states. It
depends on the history. Melting is accompanied by precursors, called
micro-slips, which reconfigurate the chain locally. Even though we investigate
the dynamics at zero temperature, the behavior of the Frenkel-Kontorova model
is qualitatively similar to the behavior of similar models at nonzero
temperature.Comment: Written in RevTeX, 13 figures in PostScript, appears in PR
Discrete soliton ratchets driven by biharmonic fields
Directed motion of topological solitons (kinks or antikinks) in the damped
and AC-driven discrete sine-Gordon system is investigated. We show that if the
driving field breaks certain time-space symmetries, the soliton can perform
unidirectional motion. The phenomenon resembles the well known effects of
ratchet transport and nonlinear harmonic mixing. Direction of the motion and
its velocity depends on the shape of the AC drive. Necessary conditions for the
occurrence of the effect are formulated. In comparison with the previously
studied continuum case, the discrete case shows a number of new features:
non-zero depinning threshold for the driving amplitude, locking to the rational
fractions of the driving frequency, and diffusive ratchet motion in the case of
weak intersite coupling.Comment: 13 pages, 13 figure
Soliton Staircases and Standing Strain Waves in Confined Colloidal Crystals
We show by computer simulation of a two-dimensional crystal confined by
corrugated walls that confinement can be used to impose a controllable
mesoscopic superstructure of predominantly mechanical elastic character. Due to
an interplay of the particle density of the system and the width D of the
confining channel, "soliton staircases" can be created along both parallel
confining boundaries, that give rise to standing strain waves in the entire
crystal. The periodicity of these waves is of the same order as D. This
mechanism should be useful for structure formation in the self-assembly of
various nanoscopic materials.Comment: 22 pages, 5 figure
Stokes' Drift of linear Defects
A linear defect, viz. an elastic string, diffusing on a planar substrate
traversed by a travelling wave experiences a drag known as Stokes' drift. In
the limit of an infinitely long string, such a mechanism is shown to be
characterized by a sharp threshold that depends on the wave parameters, the
string damping constant and the substrate temperature. Moreover, the onset of
the Stokes' drift is signaled by an excess diffusion of the string center of
mass, while the dispersion of the drifting string around its center of mass may
grow anomalous.Comment: 14 pages, no figures, to be published in Phys.Rev.
Particle Number Fluctuations in the Microcanonical Ensemble
Particle number fluctuations are studied in the microcanonical ensemble. For
the Boltzmann statistics we deduce exact analytical formulae for the
microcanonical partition functions in the case of non-interacting massless
neutral particles and charged particles with zero net charge. The particle
number fluctuations are calculated and we find that in the microcanonical
ensemble they are suppressed in comparison to the fluctuations in the canonical
and grand canonical ensembles. This remains valid in the thermodynamic limit
too, so that the well-known equivalence of all statistical ensembles refers to
average quantities, but does not apply to fluctuations. In the thermodynamic
limit we are able to calculate the particle number fluctuations in the system
of massive bosons and fermions when the exact conservation laws of both the
energy and charge are taken into account.Comment: REVTeX, 17 pages, 9 figures, v3: misprints a correcte
Yukawa particles confined in a channel and subject to a periodic potential: ground state and normal modes
We consider a classical system of two-dimensional (2D) charged particles,
which interact through a repulsive Yukawa potential ,
confined in a parabolic channel which limits the motion of the particles in the
-direction. Along the -direction, the particles are also subject to a
periodic potential substrate. The ground state configurations and the normal
mode spectra of the system are obtained as function of the periodicity and
strength of the periodic potential (), and density. An interesting set of
tunable ground state configurations are found, with first and second order
structural transitions between them. A magic configuration with particles
aligned in each minimum of the periodic potential is obtained for V_0 larger
than some critical value which has a power law dependence on the density. The
phonon spectrum of different configurations were also calculated. A
localization of the modes into a small frequency interval is observed for a
sufficient strength of the periodic potential. A tunable band-gap is found as a
function of . This model system can be viewed as a generalization of the
Frenkel and Kontorova model.Comment: Submmited to PR
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