3,964 research outputs found
Electronic Structure of gated graphene and graphene ribbons
We study the electronic structure of gated graphene sheets. We consider both
infinite graphene and finite width ribbons. The effect of Coulomb interactions
between the electrically injected carriers and the coupling to the external
gate are computed self-consistently in the Hartree approximation. We compute
the average density of extra carriers, , the number of occupied
subbands and the density profiles as a function of the gate potential . We
discuss quantum corrections to the classical capacitance and we calculate the
threshold above which semiconducting armchair ribbons conduct. We find
that the ideal conductance of perfectly transmitting wide ribbons is
proportional to the square root of the gate voltage.Comment: 8 pages, 6 figure
Evolution of small-scale magnetic elements in the vicinity of granular-size swirl convective motions
Advances in solar instrumentation have led to a widespread usage of time
series to study the dynamics of solar features, specially at small spatial
scales and at very fast cadences. Physical processes at such scales are
determinant as building blocks for many others occurring from the lower to the
upper layers of the solar atmosphere and beyond, ultimately for understanding
the bigger picture of solar activity. Ground-based (SST) and space-borne
(Hinode) high-resolution solar data are analyzed in a quiet Sun region
displaying negative polarity small-scale magnetic concentrations and a cluster
of bright points observed in G-band and Ca II H images. The studied region is
characterized by the presence of two small-scale convective vortex-type plasma
motions, one of which appears to be affecting the dynamics of both, magnetic
features and bright points in its vicinity and therefore the main target of our
investigations. We followed the evolution of bright points, intensity
variations at different atmospheric heights and magnetic evolution for a set of
interesting selected regions. A description of the evolution of the
photospheric plasma motions in the region nearby the convective vortex is
shown, as well as some plausible cases for convective collapse detected in
Stokes profiles.Comment: 9 figure
Metastability and paramagnetism in superconducting mesoscopic disks
A projected order parameter is used to calculate, not only local minima of
the Ginzburg-Landau energy functional, but also saddle points or energy
barriers responsible for the metastabilities observed in superconducting
mesoscopic disks (Geim et al. Nature {\bf 396}, 144 (1998)). We calculate the
local minima magnetization and find the energetic instability points between
vortex configurations with different vorticity. We also find that, for any
vorticity, the supercurrent can reverse its flow direction on decreasing the
magnetic field before one vortex can escape.Comment: Modified version as to appear in Phys. Rev. Let
Simulation Application for the LHCb Experiment
We describe the LHCb detector simulation application (Gauss) based on the
Geant4 toolkit. The application is built using the Gaudi software framework,
which is used for all event-processing applications in the LHCb experiment. The
existence of an underlying framework allows several common basic services such
as persistency, interactivity, as well as detector geometry description or
particle data to be shared between simulation, reconstruction and analysis
applications. The main benefits of such common services are coherence between
different event-processing stages as well as reduced development effort. The
interfacing to Geant4 toolkit is realized through a facade (GiGa) which
minimizes the coupling to the simulation engine and provides a set of abstract
interfaces for configuration and event-by-event communication. The Gauss
application is composed of three main blocks, i.e. event generation, detector
response simulation and digitization which reflect the different stages
performed during the simulation job. We describe the overall design as well as
the details of Gauss application with a special emphasis on the configuration
and control of the underlying simulation engine. We also briefly mention the
validation strategy and the planing for the LHCb experiment simulation.Comment: Talk from the 2003 Computing in High Energy and Nuclear Physics
(CHEP03), La Jolla, Ca, USA, March 2003, 6 pages, LaTeX, 9 eps figures. PSN
TUMT00
Vortex matter in superconducting mesoscopic disks: Structure, magnetization, and phase transitions
The dense vortex matter structure and associated magnetization are calculated
for type-II superconducting mesoscopic disks. The magnetization exhibits
generically first-order phase transitions as the number of vortices changes by
one and presents two well-defined regimes: A non-monotonous evolution of the
magnitude of the magnetization jumps signals the presence of a vortex glass
structure which is separated by a second-order phase transition at
from a condensed state of vortices (giant vortex) where the magnitude of the
jumps changes monotonously. We compare our results with Hall magnetometry
measurements by Geim et al. (Nature 390, 259 (1997)) and claim that the
magnetization exhibits clear traces of the presence of these vortex glass
states.Comment: 4 pages, 3 figure
Vortex structure of thin mesoscopic disks in the presence of an inhomogeneous magnetic field
The vortex states in a thin mesoscopic disk are investigated within the
phenomenological Ginzburg-Landau theory in the presence of different ''model''
magnetic field profiles with zero average field which may result from a
ferromagnetic disk or circulating currents in a loop near the superconductor.
We calculated the dependences of both the ground and metastable states on the
magnitude and shape of the magnetic field profile for different values of the
order parameter angular moment, i.e. the vorticity. The regions of existence of
the multi-vortex state and the giant vortex state are found. We analysed the
phase transitions between these states and studied the contribution from
ring-shaped vortices. A new transition between different multi-vortex
configurations as the ground state is found. Furthermore, we found a vortex
state consisting of a central giant vortex surrounded by a collection of
anti-vortices which are located in a ring around this giant vortex. The limit
to a disk with an infinite radius, i.e. a film, will also be discussed. We also
extended our results to ''real'' magnetic field profiles and to the case in
which an external homogeneous magnetic field is present.Comment: 17 pages, 23 figures. Submitted to PR
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