311 research outputs found
Exploring, tailoring, and traversing the solution landscape of a phase-shaped CARS process
Pulse shaping techniques are used to improve the selectivity of broadband CARS experiments, and to reject the overwhelming background. Knowledge about the fitness landscape and the capability of tailoring it is crucial for both fundamental insight and performing an efficient optimization of phase shapes. We use an evolutionary algorithm to find the optimal spectral phase of the broadband pump and probe beams in a background-suppressed shaped CARS process. We then investigate the shapes, symmetries, and topologies of the landscape contour lines around the optimal solution and also around the point corresponding to zero phase. We demonstrate the significance of the employed phase bases in achieving convex contour lines, suppressed local optima, and high optimization fitness with a few (and even a single) optimization parameter
The Study of Shocks in Three-States Driven-Diffusive Systems: A Matrix Product Approach
We study the shock structures in three-states one-dimensional
driven-diffusive systems with nearest neighbors interactions using a matrix
product formalism. We consider the cases in which the stationary probability
distribution function of the system can be written in terms of superposition of
product shock measures. We show that only three families of three-states
systems have this property. In each case the shock performs a random walk
provided that some constraints are fulfilled. We calculate the diffusion
coefficient and drift velocity of shock for each family.Comment: 15 pages, Accepted for publication in Journal of Statistical
Mechanics: Theory and Experiment (JSTAT
Construction of a matrix product stationary state from solutions of finite size system
Stationary states of stochastic models, which have states per site, in
matrix product form are considered. First we give a necessary condition for the
existence of a finite -dimensional matrix product state for any .
Second, we give a method to construct the matrices from the stationary states
of small size system when the above condition and are satisfied.
Third, the method by which one can check that the obtained matrices are valid
for any system size is presented for the case where is satisfied. The
application of our methods is explained using three examples: the asymmetric
exclusion process, a model studied in [F. H. Jafarpour: J. Phys. A: Math. Gen.
36 (2003) 7497] and a hybrid of both of the models.Comment: 22 pages, no figure. Major changes: sec.3 was shortened; the list of
references were changed. This is the final version, which will appear in
J.Phys.
The optimized Rayleigh-Ritz scheme for determining the quantum-mechanical spectrum
The convergence of the Rayleigh-Ritz method with nonlinear parameters
optimized through minimization of the trace of the truncated matrix is
demonstrated by a comparison with analytically known eigenstates of various
quasi-solvable systems. We show that the basis of the harmonic oscillator
eigenfunctions with optimized frequency ? enables determination of boundstate
energies of one-dimensional oscillators to an arbitrary accuracy, even in the
case of highly anharmonic multi-well potentials. The same is true in the
spherically symmetric case of V (r) = {\omega}2r2 2 + {\lambda}rk, if k > 0.
For spiked oscillators with k < -1, the basis of the pseudoharmonic oscillator
eigenfunctions with two parameters ? and {\gamma} is more suitable, and
optimization of the latter appears crucial for a precise determination of the
spectrum.Comment: 22 pages,8 figure
Operator Method for Nonperturbative Calculation of the Thermodynamic Values in Quantum Statistics. Diatomic Molecular Gas
Operator method and cumulant expansion are used for nonperturbative
calculation of the partition function and the free energy in quantum
statistics. It is shown for Boltzmann diatomic molecular gas with some model
intermolecular potentials that the zeroth order approximation of the proposed
method interpolates the thermodynamic values with rather good accuracy in the
entire range of both the Hamiltonian parameters and temperature. The systematic
procedure for calculation of the corrections to the zeroth order approximation
is also considered.Comment: 22 pages, 7 Postscript figures, accepted for publication in Journal
of Physics
Partially Asymmetric Simple Exclusion Model in the Presence of an Impurity on a Ring
We study a generalized two-species model on a ring. The original model [1]
describes ordinary particles hopping exclusively in one direction in the
presence of an impurity. The impurity hops with a rate different from that of
ordinary particles and can be overtaken by them. Here we let the ordinary
particles hop also backward with the rate q. Using Matrix Product Ansatz (MPA),
we obtain the relevant quadratic algebra. A finite dimensional representation
of this algebra enables us to compute the stationary bulk density of the
ordinary particles, as well as the speed of impurity on a set of special
surfaces of the parameter space. We will obtain the phase structure of this
model in the accessible region and show how the phase structure of the original
model is modified. In the infinite-volume limit this model presents a shock in
one of its phases.Comment: Adding more references and doing minor corrections, 16 pages and 3
Eps figure
Nonequilibrium Steady States of Matrix Product Form: A Solver's Guide
We consider the general problem of determining the steady state of stochastic
nonequilibrium systems such as those that have been used to model (among other
things) biological transport and traffic flow. We begin with a broad overview
of this class of driven diffusive systems - which includes exclusion processes
- focusing on interesting physical properties, such as shocks and phase
transitions. We then turn our attention specifically to those models for which
the exact distribution of microstates in the steady state can be expressed in a
matrix product form. In addition to a gentle introduction to this matrix
product approach, how it works and how it relates to similar constructions that
arise in other physical contexts, we present a unified, pedagogical account of
the various means by which the statistical mechanical calculations of
macroscopic physical quantities are actually performed. We also review a number
of more advanced topics, including nonequilibrium free energy functionals, the
classification of exclusion processes involving multiple particle species,
existence proofs of a matrix product state for a given model and more
complicated variants of the matrix product state that allow various types of
parallel dynamics to be handled. We conclude with a brief discussion of open
problems for future research.Comment: 127 pages, 31 figures, invited topical review for J. Phys. A (uses
IOP class file
Density Profile of the One-Dimensional Partially Asymmetric Simple Exclusion Process with Open Boundaries
The one-dimensional partially asymmetric simple exclusion process with open
boundaries is considered. The stationary state, which is known to be
constructed in a matrix product form, is studied by applying the theory of
q-orthogonal polynomials. Using a formula of the q-Hermite polynomials, the
average density profile is computed in the thermodynamic limit. The phase
diagram for the correlation length, which was conjectured in the previous
work[J. Phys. A {\bf 32} (1999) 7109], is confirmed.Comment: 24 pages, 6 figure
Construction of a Coordinate Bethe Ansatz for the asymmetric simple exclusion process with open boundaries
The asymmetric simple exclusion process with open boundaries, which is a very
simple model of out-of-equilibrium statistical physics, is known to be
integrable. In particular, its spectrum can be described in terms of Bethe
roots. The large deviation function of the current can be obtained as well by
diagonalizing a modified transition matrix, that is still integrable: the
spectrum of this new matrix can be also described in terms of Bethe roots for
special values of the parameters. However, due to the algebraic framework used
to write the Bethe equations in the previous works, the nature of the
excitations and the full structure of the eigenvectors were still unknown. This
paper explains why the eigenvectors of the modified transition matrix are
physically relevant, gives an explicit expression for the eigenvectors and
applies it to the study of atypical currents. It also shows how the coordinate
Bethe Ansatz developped for the excitations leads to a simple derivation of the
Bethe equations and of the validity conditions of this Ansatz. All the results
obtained by de Gier and Essler are recovered and the approach gives a physical
interpretation of the exceptional points The overlap of this approach with
other tools such as the matrix Ansatz is also discussed. The method that is
presented here may be not specific to the asymmetric exclusion process and may
be applied to other models with open boundaries to find similar exceptional
points.Comment: references added, one new subsection and corrected typo
Microscopic structure of travelling wave solutions in a class of stochastic interacting particle systems
We obtain exact travelling wave solutions for three families of stochastic
one-dimensional nonequilibrium lattice models with open boundaries. These
solutions describe the diffusive motion and microscopic structure of (i) of
shocks in the partially asymmetric exclusion process with open boundaries, (ii)
of a lattice Fisher wave in a reaction-diffusion system, and (iii) of a domain
wall in non-equilibrium Glauber-Kawasaki dynamics with magnetization current.
For each of these systems we define a microscopic shock position and calculate
the exact hopping rates of the travelling wave in terms of the transition rates
of the microscopic model. In the steady state a reversal of the bias of the
travelling wave marks a first-order non-equilibrium phase transition, analogous
to the Zel'dovich theory of kinetics of first-order transitions. The stationary
distributions of the exclusion process with shocks can be described in
terms of -dimensional representations of matrix product states.Comment: 27 page
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