42 research outputs found
Properties of two - dimensional dusty plasma clusters
Two-dimensional classical cluster of particles interacting through a screened
Coulomb potential is studied. This system can be used as a model for "dusty
particles" in high-frequency discharge plasma. For systems consisting of N = 2
- 40 particles and confined by a harmonic potential we find ground-state
configurations, eigenfrequencies and eigenvectors for the normal modes as a
function of the Debye screening length R_D in plasma. Variations in R_D cause
changes in the ground-state structure of clusters, each structural
rearrangement can be considered as a phase transition of first or second order
(with respect to parameter R_D). Monte Carlo and molecular dynamics are used to
study in detail the melting of the clusters as the temperature is increased. By
varying the density and the temperature of plasma, to which the particles are
immersed, one can modulate thermodynamical properties of the system,
transforming it in a controllable way to an ordered (crystal-like),
orientationaly disordered or totally disordered (liquid-like) states. The
possibility of dynamical coexistence phenomena in small clusters is discussed.Comment: 5 pages, 6 Postscript figures; to appear in Phys.Lett.
"Shaking" of an atom in a non-stationary cavity
We consider an atom interacting with a quantized electromagnetic field inside
a cavity with variable parameters. The atom in the ground state located in the
initially empty cavity can be excited by variation of cavity parameters. We
have discovered two mechanisms of atomic excitation. The first arises due to
the interaction of the atom with the non-stationary electromagnetic field
created by modulation of cavity parameters. If the characteristic time of
variation of cavity parameters is of the order of the atomic transition time,
the processes of photon creation and atomic excitation are going on
simultaneously and hence excitation of the atom cannot be reduced to trivial
absorption of the photons produced by the dynamical Casimir effect. The second
mechanism is "shaking" of the atom due to fast modulation of its ground state
Lamb shift which takes place as a result of fast variation of cavity arameters.
The last mechanism has no connection with the vacuum dynamical Casimir effect.
Moreover, it opens a new channel of photon creation in the non-stationary
cavity. Nevertheless, the process of photon creation is altered by the presence
of the atom in the cavity, even if one disregards the existence of the new
channel. In particular, it removes the restriction for creation of only even
number of photons and also changes the expectation value for the number of
created photons. Our consideration is based on a simple model of a two-level
atom interacting with a single mode of the cavity field. Qualitatively our
results are valid for a real atom in a physical cavity.Comment: 12 pages,4 *.eps figures, this version is identical to the one to be
published in Physics Letters A (in print
Interaction potential between dynamic dipoles: polarized excitons in strong magnetic fields
The interaction potential of a two-dimensional system of excitons with
spatially separated electron-hole layers is considered in the strong magnetic
field limit. The excitons are assumed to have free dynamics in the -
plane, while being constrained or `polarized' in the direction. The model
simulates semiconductor double layer systems under strong magnetic field normal
to the layers. The {\em residual} interaction between excitons exhibits
interesting features, arising from the coupling of the center-of-mass and
internal degrees of freedom of the exciton in the magnetic field. This coupling
induces a dynamical dipole moment proportional to the center-of-mass magnetic
moment of the exciton. We show the explicit dependence of the inter-exciton
potential matrix elements, and discuss the underlying physics. The unusual
features of the interaction potential would be reflected in the collective
response and non-equilibrium properties of such system.Comment: REVTEX - 11 pages - 1 fi
Orientational melting of two-shell carbon nanoparticles: molecular dynamics study
The energetic characteristics of two-shell carbon nanoparticles ("onions")
with different shapes of second shell are calculated. The barriers of relative
rotation of shells are found to be surprisingly small; therefore, free relative
rotation of shells can take place at room temperature. The intershell
orientational melting of the nanoparticle is studied by
molecular dynamics. The parameters of Arrhenius formula for jump rotational
intershell diffusion are calculated. The definition of orientational melting
temperature is proposed as the temperature when the transition probability over
barrier between equivalent potential minima is equal to 1/2. The temperature of
orientational melting of the nanoparticle is about 60 K.Comment: 9 pages, 10 figures, some new simulation results and formulations
introduce
Theory, Simulation and Nanotechnological Applications of Adsorption on a Surface with Defects
Theory of adsorption on a surface with nanolocal defects is proposed. Two
efficacy parameters of surface modification for nanotechnological purposes are
introduced, where the modification is a creation of nanolocal artificial
defects. The first parameter corresponds to applications where it is necessary
to increase the concentration of certain particles on the modified surface. And
the second one corresponds to the pattern transfer with the help of particle
self-organization on the modified surface. The analytical expressions for both
parameters are derived with the help of the thermodynamic and the kinetic
approaches for two cases: jump diffusion and free motion of adsorbed particles
over the surface. The possibility of selective adsorption of molecules is shown
with the help of simulation of the adsorption of acetylene and benzene
molecules in the pits on the graphite surface. The process of particle
adsorption from the surface into the pit is theoretically studied by molecular
dynamic technique. Some possible nanotechnological applications of adsorption
on the surface with artificial defects are considered: fabrication of sensors
for trace molecule detection, separation of isomers, and pattern transfer.Comment: 12 pages, 2 Postscript figures. Submitted to Surface Science (1998
Can Barrier to Relative Sliding of Carbon Nanotube Walls Be Measured?
Interwall interaction energies, as well as barriers to relative sliding of
the walls along the nanotube axis, are first calculated for pairs of both
armchair or both zigzag adjacent walls of carbon nanotubes with a wide range of
radiuses. It is found that for the pairs with the radius of the outer wall
greater than 5 nm both the interwall interaction energy and barriers to the
relative sliding per one atom of the outer wall only slightly depends on the
wall radius. A wide set of the measurable physical quantities determined by
these barriers are estimated as a function of the wall radius: shear strengths
and diffusion coefficients for relative sliding of the walls along the axis, as
well as frequencies of relative axial oscillations of the walls. For
nonreversible telescopic extension of the walls, maximum overlap of the walls
for which threshold static friction forces are greater than capillary forces is
estimated. Possibility of experimental verification of the calculated barriers
by measurements of the estimated physical quantities is discussed.Comment: 16 pages, 8 figure
Modeling of graphene-based NEMS
The possibility of designing nanoelectromechanical systems (NEMS) based on
relative motion or vibrations of graphene layers is analyzed. Ab initio and
empirical calculations of the potential relief of interlayer interaction energy
in bilayer graphene are performed. A new potential based on the density
functional theory calculations with the dispersion correction is developed to
reliably reproduce the potential relief of interlayer interaction energy in
bilayer graphene. Telescopic oscillations and small relative vibrations of
graphene layers are investigated using molecular dynamics simulations. It is
shown that these vibrations are characterized with small Q-factor values. The
perspectives of nanoelectromechanical systems based on relative motion or
vibrations of graphene layers are discussed.Comment: 19 pages, 4 figure
Charge transport and phase transition in exciton rings
The macroscopic exciton rings observed in the photoluminescence (PL) patterns
of excitons in coupled quantum wells (CQWs) are explained by a series of
experiments and a theory based on the idea of carrier imbalance, transport and
recombination. The rings are found to be a source of cold excitons with
temperature close to that of the lattice. We explored states of excitons in the
ring over a range of temperatures down to 380 mK. These studies reveal a sharp,
albeit continuous, second order phase transition to a low-temperature ordered
exciton state, characterized by ring fragmentation into a periodic array of
aggregates. An instability at the onset of degeneracy in the cold exciton
system, due to stimulated exciton formation, is proposed as the transition
mechanism.Comment: 8 pages including 4 figure
Simulation of wavepacket tunneling of interacting identical particles
We demonstrate a new method of simulation of nonstationary quantum processes,
considering the tunneling of two {\it interacting identical particles},
represented by wave packets. The used method of quantum molecular dynamics
(WMD) is based on the Wigner representation of quantum mechanics. In the
context of this method ensembles of classical trajectories are used to solve
quantum Wigner-Liouville equation. These classical trajectories obey
Hamilton-like equations, where the effective potential consists of the usual
classical term and the quantum term, which depends on the Wigner function and
its derivatives. The quantum term is calculated using local distribution of
trajectories in phase space, therefore classical trajectories are not
independent, contrary to classical molecular dynamics. The developed WMD method
takes into account the influence of exchange and interaction between particles.
The role of direct and exchange interactions in tunneling is analyzed. The
tunneling times for interacting particles are calculated.Comment: 11 pages, 3 figure
Charged vortices in superfluid systems with pairing of spatially separated carriers
It is shown that in a magnetic field the vortices in superfluid electron-hole
systems carry a real electrical charge. The charge value depends on the
relation between the magnetic length and the Bohr radiuses of electrons and
holes. In double layer systems at equal electron and hole filling factors in
the case of the electron and hole Bohr radiuses much larger than the magnetic
length the vortex charge is equal to the universal value (electron charge times
the filling factor).Comment: 4 page