119 research outputs found
The Localization Transition of the Two-Dimensional Lorentz Model
We investigate the dynamics of a single tracer particle performing Brownian
motion in a two-dimensional course of randomly distributed hard obstacles. At a
certain critical obstacle density, the motion of the tracer becomes anomalous
over many decades in time, which is rationalized in terms of an underlying
percolation transition of the void space. In the vicinity of this critical
density the dynamics follows the anomalous one up to a crossover time scale
where the motion becomes either diffusive or localized. We analyze the scaling
behavior of the time-dependent diffusion coefficient D(t) including corrections
to scaling. Away from the critical density, D(t) exhibits universal
hydrodynamic long-time tails both in the diffusive as well as in the localized
phase.Comment: 13 pages, 7 figures
Charge Dynamics in the Planar t-J Model
The finite-temperature optical conductivity in the planar
model is analysed using recently introduced numerical method based on the
Lanczos diagonalization of small systems (up to 20 sites), as well as by
analytical approaches, including the method of frequency moments and the
retraceable-path approximation. Results for a dynamical mobility of a single
hole at elevated temperatures reveal a Gaussian-like
spectra, however with a nonanalytical behavior at low . In the single
hole response a difference between the ferromagnetic (J=0) and the
antiferromagnetic () polaron shows up at . At larger dopings
numerical results in studied systems are consistent with the thermodynamical
behavior for . spectra show a non-Drude
falloff at large frequencies. In particular for `optimum' doping
we obtain in the low- regime the relaxation rate with , being consistent with the marginal Fermi
liquid concept and experiments. Within the same regime we reproduce the nearly
linear variation of dc resistivity with . This behavior is weakly
dependent on , provided that .Comment: 21 pages of text plus 17 figures, postscrip
Transport properties of dense fluid argon
We calculate using molecular dynamics simulations the transport properties of
realistically modeled fluid argon at pressures up to and
temperatures up to . In this context we provide a critique of some newer
theoretical predictions for the diffusion coefficients of liquids and a
discussion of the Enskog theory relevance under two different adaptations:
modified Enskog theory (MET) and effective diameter Enskog theory. We also
analyze a number of experimental data for the thermal conductivity of
monoatomic and small diatomic dense fluids.Comment: 8 pages, 6 figure
Detailed Examination of Transport Coefficients in Cubic-Plus-Quartic Oscillator Chains
We examine the thermal conductivity and bulk viscosity of a one-dimensional
(1D) chain of particles with cubic-plus-quartic interparticle potentials and no
on-site potentials. This system is equivalent to the FPU-alpha beta system in a
subset of its parameter space. We identify three distinct frequency regimes
which we call the hydrodynamic regime, the perturbative regime and the
collisionless regime. In the lowest frequency regime (the hydrodynamic regime)
heat is transported ballistically by long wavelength sound modes. The model
that we use to describe this behaviour predicts that as the frequency goes to
zero the frequency dependent bulk viscosity and the frequency dependent thermal
conductivity should diverge with the same power law dependence on frequency.
Thus, we can define the bulk Prandtl number as the ratio of the bulk viscosity
to the thermal conductivity (with suitable prefactors to render it
dimensionless). This dimensionless ratio should approach a constant value as
frequency goes to zero. We use mode-coupling theory to predict the zero
frequency limit. Values of the bulk Prandtl number from simulations are in
agreement with these predictions over a wide range of system parameters. In the
middle frequency regime, which we call the perturbative regime, heat is
transported by sound modes which are damped by four-phonon processes. We call
the highest frequency regime the collisionless regime since at these
frequencies the observing times are much shorter than the characteristic
relaxation times of phonons. The perturbative and collisionless regimes are
discussed in detail in the appendices.Comment: Latex with references in .bib file. 36 pages, 8 figures. Submitted to
J. Stat. Phys. on Sept. 2
Glass Transition of Hard Sphere Systems: Molecular Dynamics and Density Functional Theory
The glass transition of a hard sphere system is investigated within the
framework of the density functional theory (DFT). Molecular dynamics (MD)
simulations are performed to study dynamical behavior of the system on the one
hand and to provide the data to produce the density field for the DFT on the
other hand. Energy landscape analysis based on the DFT shows that there appears
a metastable (local) free energy minimum representing an amorphous state as the
density is increased. This state turns out to become stable, compared with the
uniform liquid, at some density, around which we also observe sharp slowing
down of the relaxation in MD simulations.Comment: 5 pages, 5 figure
Random close packing of granular matter
We propose an interpretation of the random close packing of granular
materials as a phase transition, and discuss the possibility of experimental
verification.Comment: 6 page
Self-consistent Overhauser model for the pair distribution function of an electron gas in dimensionalities D=3 and D=2
We present self-consistent calculations of the spin-averaged pair
distribution function for a homogeneous electron gas in the paramagnetic
state in both three and two dimensions, based on an extension of a model that
was originally proposed by A. W. Overhauser [Can. J. Phys. {\bf 73}, 683
(1995)] and further evaluated by P. Gori-Giorgi and J. P. Perdew [Phys. Rev. B
{\bf 64}, 155102 (2001)]. The model involves the solution of a two-electron
scattering problem via an effective Coulombic potential, that we determine
within a self-consistent Hartree approximation. We find numerical results for
that are in excellent agreement with Quantum Monte Carlo data at low and
intermediate coupling strength , extending up to in
dimensionality D=3. However, the Hartree approximation does not properly
account for the emergence of a first-neighbor peak at stronger coupling, such
as at in D=2, and has limited accuracy in regard to the spin-resolved
components and . We also
report calculations of the electron-electron s-wave scattering length, to test
an analytical expression proposed by Overhauser in D=3 and to present new
results in D=2 at moderate coupling strength. Finally, we indicate how this
approach can be extended to evaluate the pair distribution functions in
inhomogeneous electron systems and hence to obtain improved
exchange-correlation energy functionals.Comment: 14 pages, 7 figuers, to apear in Physical Review
Dynamics and Scaling of 2D Polymers in a Dilute Solution
The breakdown of dynamical scaling for a dilute polymer solution in 2D has
been suggested by Shannon and Choy [Phys. Rev. Lett. {\bf 79}, 1455 (1997)].
However, we show here both numerically and analytically that dynamical scaling
holds when the finite-size dependence of the relevant dynamical quantities is
properly taken into account. We carry out large-scale simulations in 2D for a
polymer chain in a good solvent with full hydrodynamic interactions to verify
dynamical scaling. This is achieved by novel mesoscopic simulation techniques
Which mechanism underlies the water-like anomalies in core-softened potentials?
Using molecular dynamics simulations we investigate the thermodynamic of
particles interacting with a continuous and a discrete versions of a
core-softened (CS) intermolecular potential composed by a repulsive shoulder.
Dynamic and structural properties are also analyzed by the simulations. We show
that in the continuous version of the CS potential the density at constant
pressure has a maximum for a certain temperature. Similarly the diffusion
constant, , at a constant temperature has a maximum at a density
and a minimum at a density
, and structural properties are also
anomalous. For the discrete CS potential none of these anomalies are observed.
The absence of anomalies in the discrete case and its presence in the
continuous CS potential are discussed in the framework of the excess entropy.Comment: 8 page
A cluster theory for a Janus fluid
Recent Monte Carlo simulations on the Kern and Frenkel model of a Janus fluid
have revealed that in the vapour phase there is the formation of preferred
clusters made up of a well-defined number of particles: the micelles and the
vesicles. A cluster theory is developed to approximate the exact clustering
properties stemming from the simulations. It is shown that the theory is able
to reproduce the micellisation phenomenon.Comment: 27 pages, 8 figures, 6 table
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