3,277 research outputs found
Floating Wigner molecules and possible phase transitions in quantum dots
A floating Wigner crystal differs from the standard one by a spatial
averaging over positions of the Wigner-crystal lattice. It has the same
internal structure as the fixed crystal, but contrary to it, takes into account
rotational and/or translational symmetry of the underlying jellium background.
We study properties of a floating Wigner molecule in few-electron
spin-polarized quantum dots, and show that the floating solid has the lower
energy than the standard Wigner crystal with fixed lattice points. We also
argue that internal rotational symmetry of individual dots can be broken in
arrays of quantum dots, due to degenerate ground states and inter-dot Coulomb
coupling.Comment: 6 pages incl 3 figure
Ridges and Soft Jet Components in Untriggered Di-hadron Correlations in Pb+Pb Collisions at 2.76 TeV
We study untriggered di-hadron correlations in Pb+Pb at 2.76 TeV, based on an
event-by-event simulation of a hydrodynamic expansion starting from flux tube
initial conditions. The correlation function shows interesting structures as a
function of the pseudorapidity difference and the azimuthal angle
difference , in particular comparing different centralities. We can
clearly identify a peak-like nearside structure associated with very low
momentum components of jets for peripheral collisions, which disappears towards
central collisions. On the other hand, a very broad ridge structure from
asymmetric flow seen at central collisions, gets smaller and finally disappears
towards peripheral collisions
Bose-Einstein Correlations in a Fluid Dynamical Scenario for Proton-Proton Scattering at 7 TeV
Using a fluid dynamical scenario for scattering at 7 TeV, we compute
correlation functions for pairs. Femtoscopic radii are extracted
based on three-dimensional parametrizations of the correlation functions. We
study the radii as a function of the transverse momenta of the pairs, for
different multiplicity classes, corresponding to recent experimental results
from ALICE. We find the same decrease of the radii with , more and more
pronounced with increasing multiplicity, but absent for the lowest
multiplicities. In the model we understand this as transition from string
expansion (low multiplicity) towards a three-dimensional hydrodynamical
expansion (high multiplicity)
Dielectric function and plasmons in graphene
The electromagnetic response of graphene, expressed by the dielectric
function, and the spectrum of collective excitations are studied as a function
of wave vector and frequency. Our calculation is based on the full band
structure, calculated within the tight-binding approximation. As a result, we
find plasmons whose dispersion is similar to that obtained in the single-valley
approximation by Dirac fermions. In contrast to the latter, however, we find a
stronger damping of the plasmon modes due to inter-band absorption. Our
calculation also reveals effects due to deviations from the linear Dirac
spectrum as we increase the Fermi energy, indicating an anisotropic behavior
with respect to the wave vector of the external electromagnetic field
Bifurcation to Traveling Spots in Reaction-Diffusion Systems
A bifurcation leading to the onset of translational motion of localized particlelike structures (spots) in two-dimensional excitable media with long-range inhibition and global coupling is analytically and numerically investigated. Properties of slowly traveling spots and effects of collisions between these objects are studied
Multiboson effects in multiparticle production
The influence of multiboson effects on pion multiplicities, single-pion
spectra and two-pion correlation functions is discussed in terms of an
analytically solvable model. The applicability of its basic factorization
assumption is clarified. An approximate scaling of the basic observables with
the phase space density is demonstrated in the low density (gas) limit. This
scaling and also its violation at high densities due to the condensate
formation is described by approximate analytical formulae which allow, in
principle, for the identification of the multiboson effects among others. For
moderate densities indicated by the experimental data, a fast saturation of
multiboson effects with the number of contributing cumulants is obtained,
allowing for the account of these effects in realistic transport code
simulations. At high densities, the spectra are mainly determined by the
universal condensate term and the initially narrow Poisson multiplicity
distribution approaches a wide Bose-Einstein one. As a result, the intercepts
of the inclusive and fixed- correlation functions (properly normalized to 1
at large relative momenta) approach 2 and 1, respectively and their widths
logarithmically increase with the increasing phase space density. It is shown
that the neglect of energy-momentum constraints in the model is justified
except near a multipion threshold, where these constraints practically exclude
the possibility of a very cold condensate production. It is argued that
spectacular multiboson effects are likely to be observed only in the rare
events containing sufficiently high density (speckle) fluctuations.Comment: 30 pages including 10 figures, LaTex, a revised version of SUBATECH
99-04 (aps1999_mar21_001) resubmitted to Phys. Rev. C; Chapter II made
shorter, figure description made more clear, a comparison with most recent
works added in Chapter V
Nonlinear electromagnetic response of graphene: Frequency multiplication and the self-consistent-field effects
Graphene is a recently discovered carbon based material with unique physical
properties. This is a monolayer of graphite, and the two-dimensional electrons
and holes in it are described by the effective Dirac equation with a vanishing
effective mass. As a consequence, electromagnetic response of graphene is
predicted to be strongly non-linear. We develop a quasi-classical kinetic
theory of the non-linear electromagnetic response of graphene, taking into
account the self-consistent-field effects. Response of the system to both
harmonic and pulse excitation is considered. The frequency multiplication
effect, resulting from the non-linearity of the electromagnetic response, is
studied under realistic experimental conditions. The frequency up-conversion
efficiency is analysed as a function of the applied electric field and
parameters of the samples. Possible applications of graphene in terahertz
electronics are discussed.Comment: 14 pages, 7 figures, invited paper written for a special issue of
JPCM "Terahertz emitters
Geomagnetic storm effects at F1-layer heights from incoherent scatter observations
International audienceStorm effects at F1-layer heights (160?200 km) were analyzed for the first time using Millstone Hill (mid-latitudes) and EISCAT (auroral zone) incoherent scatter (IS) observations. The morphological study has shown both increases (positive effect) and decreases (negative effect) in electron concentration. Negative storm effects prevail for all seasons and show a larger magnitude than positive ones, the magnitude of the effect normally increasing with height. At Millstone Hill the summer storm effects are small compared to other seasons, but they are well detectable. At EISCAT this summer decrease takes place only with respect to the autumnal period and the autumn/spring asymmetry in the storm effects is well pronounced. Direct and significant correlation exists between deviations in electron concentration at the F1-layer heights and in the F2-layer maximum. Unlike the F2-layer the F1-region demonstrates a relatively small reaction to geomagnetic disturbances despite large perturbations in thermospheric parameters. Aeronomic parameters extracted from IS observations are used to explain the revealed morphology. A competition between atomic and molecular ion contributions to Ne variations was found to be the main physical mechanism controlling the F1-layer storm effect. The revealed morphology is shown to be related with neutral composition (O, O2, N2) seasonal and storm-time variations. The present day understanding of the F1-region formation mechanisms is sufficient to explain the observed storm effects
Self-sustained spatiotemporal oscillations induced by membrane-bulk coupling
We propose a novel mechanism leading to spatiotemporal oscillations in
extended systems that does not rely on local bulk instabilities. Instead,
oscillations arise from the interaction of two subsystems of different spatial
dimensionality. Specifically, we show that coupling a passive diffusive bulk of
dimension d with an excitable membrane of dimension d-1 produces a
self-sustained oscillatory behavior. An analytical explanation of the
phenomenon is provided for d=1. Moreover, in-phase and anti-phase
synchronization of oscillations are found numerically in one and two
dimensions. This novel dynamic instability could be used by biological systems
such as cells, where the dynamics on the cellular membrane is necessarily
different from that of the cytoplasmic bulk.Comment: Accepted for publication in Physical Review Letter
Algebraic entropy for semi-discrete equations
We extend the definition of algebraic entropy to semi-discrete
(difference-differential) equations. Calculating the entropy for a number of
integrable and non integrable systems, we show that its vanishing is a
characteristic feature of integrability for this type of equations
- …