9 research outputs found
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
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
Topological Defects and Non-homogeneous Melting of Large 2D Coulomb Clusters
The configurational and melting properties of large two-dimensional clusters
of charged classical particles interacting with each other via the Coulomb
potential are investigated through the Monte Carlo simulation technique. The
particles are confined by a harmonic potential. For a large number of particles
in the cluster (N>150) the configuration is determined by two competing
effects, namely in the center a hexagonal lattice is formed, which is the
groundstate for an infinite 2D system, and the confinement which imposes its
circular symmetry on the outer edge. As a result a hexagonal Wigner lattice is
formed in the central area while at the border of the cluster the particles are
arranged in rings. In the transition region defects appear as dislocations and
disclinations at the six corners of the hexagonal-shaped inner domain. Many
different arrangements and type of defects are possible as metastable
configurations with a slightly higher energy. The particles motion is found to
be strongly related to the topological structure. Our results clearly show that
the melting of the clusters starts near the geometry induced defects, and that
three different melting temperatures can be defined corresponding to the
melting of different regions in the cluster.Comment: 7 pages, 11 figures, submitted to Phys. Rev.
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
Critical Currents of Ideal Quantum Hall Superfluids
Filling factor bilayer electron systems in the quantum Hall regime
have an excitonic-condensate superfluid ground state when the layer separation
is less than a critical value . On a quantum Hall plateau current
injected and removed through one of the two layers drives a dissipationless
edge current that carries parallel currents, and a dissipationless bulk
supercurrent that carries opposing currents in the two layers. In this paper we
discuss the theory of finite supercurrent bilayer states, both in the presence
and in the absence of symmetry breaking inter-layer hybridization. Solutions to
the microscopic mean-field equations exist at all condensate phase winding
rates for zero and sufficiently weak hybridization strengths. We find, however,
that collective instabilities occur when the supercurrent exceeds a critical
value determined primarily by a competition between direct and exchange
inter-layer Coulomb interactions. The critical current is estimated using a
local stability criterion and varies as when approaches
from below. For large inter-layer hybridization, we find that the
critical current is limited by a soliton instability of microscopic origin.Comment: 18 RevTeX pgs, 21 eps figure
Phase transitions of electron-hole and unbalanced electron systems in coupled quantum wells in high magnetic fields
Static and dynamic behaviours of multivortex states in a superconducting sample with mesoscopic pinning sites
This preliminary work has focused on the static
transitions between the multivortex states interacting with square
arrays of the mesoscopic pinning sites in superconducting samples.
Our results were obtained from an extensive series of numerical
simulations as functions of the magnetic field, pinning radius,
and sample size. We have presented a wide range of multivortex
configurations from commensurate dimer states to more concentric
vortex shells at the matching fields. The stability of these
states was also studied by means of the current-voltage V(I)
curves which illustrate dynamic phase transitions as a function of
applied driving force. These transitions manifested themselves as
either a sudden jump in velocity or a nonlinear increase with
velocity fluctuations in V(I) curves. We have investigated
whether that the phase transitions between the pinned regime and
the elastic flow regime are indicative of the stability of the
initial vortex states. The variety of intermediate flow phases is
attributed to large pinning size (reentrant behavior), strong
commensurability and caging effects. In particular, three-shell
vortex structures were obtained in the presence of larger pinning
sites at adequate matching magnetic fields