36,623 research outputs found
Quantum wires in magnetic field: A comparative study of the Hartree-Fock and the spin density functional approaches
We present a detailed comparison of the self-consistent calculations based on
the Hartree-Fock and the spin density functional theory for a spit-gate quantum
wire in the IQH regime. We demonstrate that both approaches provide
qualitatively (and in most cases quantitatively) similar results for the
spin-resolved electron density, spin polarization, spatial spin separation at
the edges and the effective factor. The both approach give the same values
of the magnetic fields corresponding to the successive subband depopulation and
qualitatively similar evolution of the magnetosubbands. Quantitatively,
however, the HF and the DFT subbands are different (even though the
corresponding total electron densities are practically the same). In contrast
to the HF approach, the DFT calculations predict much larger spatial spin
separation near the wire edge for the low magnetic fields (when the
compressible strips for spinless electrons are not formed yet). In the opposite
limit of the large fields, the Hatree-Fock and the DFT approaches give very
similar values for the spatial spin separation.Comment: 5 pages, 3 figure
Propagation of the First Flames in Type Ia Supernovae
We consider the competition of the different physical processes that can
affect the evolution of a flame bubble in a Type Ia supernovae -- burning,
turbulence and buoyancy. Even in the vigorously turbulent conditions of a
convecting white dwarf, thermonuclear burning that begins at a point near the
center (within 100 km) of the star is dominated by the spherical laminar
expansion of the flame, until the burning region reaches kilometers in size.
Consequently flames that ignite in the inner ~20 km promptly burn through the
center, and flame bubbles anywhere must grow quite large--indeed, resolvable by
large-scale simulations of the global system--for significant motion or
deformation occur. As a result, any hot-spot that successfully ignites into a
flame can burn a significant amount of white dwarf material. This potentially
increases the stochastic nature of the explosion compared to a scenario where a
simmering progenitor can have small early hot-spots float harmlessly away.
Further, the size where the laminar flame speed dominates other relevant
velocities sets a characteristic scale for fragmentation of larger flame
structures, as nothing--by definition--can easily break the burning region into
smaller volumes. This makes possible the development of semi-analytic
descriptions of the earliest phase of the propagation of burning in a Type Ia
supernovae, which we present here. Our analysis is supported by fully resolved
numerical simulations of flame bubbles.Comment: 33 pages, 14 figures, accepted for publication in Ap
Self-consistent equilibrium of a two-dimensional electron system with a reservoir in a quantizing magnetic field: Analytical approach
An analytical approach has been developed to describe grand canonical
equilibrium between a three dimensional (3D) electron system and a two
dimensional (2D) one, an energy of which is determined self-consistently with
an electron concentration. Main attention is paid to a Landau level (LL)
pinning effect. Pinning means a fixation of the LL on a common Fermi level of
the 2D and the 3D systems in a finite range of the magnetic field due to an
electron transfer from the 2D to the 3D system. A condition and a start of LL
pinning has been found for homogeneously broadened LLs. The electronic transfer
from the 3D to the 2D system controls an extremely sharp magnetic dependency of
an energy of the upper filled LL at integer filling of the LLs. This can cause
a significant increase of inhomogeneous broadening of the upper LL that was
observed in recent local probe experiments.Comment: 12 pages, 2 figures, revtex
Magnetoconductance switching in an array of oval quantum dots
Employing oval shaped quantum billiards connected by quantum wires as the
building blocks of a linear quantum dot array, we calculate the ballistic
magnetoconductance in the linear response regime. Optimizing the geometry of
the billiards, we aim at a maximal finite- over zero-field ratio of the
magnetoconductance. This switching effect arises from a relative phase change
of scattering states in the oval quantum dot through the applied magnetic
field, which lifts a suppression of the transmission characteristic for a
certain range of geometry parameters. It is shown that a sustainable switching
ratio is reached for a very low field strength, which is multiplied by
connecting only a second dot to the single one. The impact of disorder is
addressed in the form of remote impurity scattering, which poses a temperature
dependent lower bound for the switching ratio, showing that this effect should
be readily observable in experiments.Comment: 11 pages, 8 figure
Origin of the 0.25-anomaly in the nonlinear conductance of a quantum point contact
We calculate the non-linear conductance of a quantum point contact using the
non-equilibrium Greens function technique within the Hartree approximation of
spinless electrons. We quantitative reproduce the 0.25-anomaly in the
differential conductance (i.e. the lowest plateau at 0.25-0.3*2e^2/h) as well
as an upward bending of higher conductance half-integer plateaus seen in the
experiments, and relate these features to the non-linear screening and pinning
effects.Comment: 6 pages, 4 figure
A semantic web service-based architecture for the interoperability of e-government services
We propose a semantically-enhanced architecture to address the issues of interoperability and service integration in e-government web information systems. An architecture for a life event portal based on Semantic Web Services (SWS) is described. The architecture includes loosely-coupled modules organized in three distinct layers: User Interaction, Middleware and Web Services. The Middleware provides the semantic infrastructure for ontologies and SWS. In particular a conceptual model for integrating domain knowledge (Life Event Ontology), application knowledge (E-government Ontology) and service description (Service Ontology) is defined. The model has been applied to a use case scenario in e-government and the results of a system prototype have been reported to demonstrate some relevant features of the proposed approach
Exploring the Design of Pay-Per-Use Objects in the Construction Domain
Equipment used in the construction domain is often hired in order to reduce cost and maintenance overhead. The cost of hire is dependent on the time period involved and does not take into account the actual use equipment has received. This paper presents our initial investigation into how physical objects augmented with sensing and communication technologies can measure use in order to enable new pay-per-use payment models for equipment hire. We also explore user interaction with pay-per-use objects via mobile devices. The user interactions that take place within our prototype scenario range from simple information access to transactions involving multiple users. This paper presents the design, implementation and evaluation of a prototype pay-per-use system motivated by a real world equipment hire scenario. We also provide insights into the various challenges introduced by supporting a pay-per-use model, including data storage and data security in addition to user interaction issues
The Propeller Regime of Disk Accretion to a Rapidly Rotating Magnetized Star
The propeller regime of disk accretion to a rapidly rotating magnetized star
is investigated here for the first time by axisymmetric 2.5D
magnetohydrodynamic simulations. An expanded, closed magnetosphere forms in
which the magnetic field is predominantly toroidal. A smaller fraction of the
star's poloidal magnetic flux inflates vertically, forming a magnetically
dominated tower. Matter accumulates in the equatorial region outside
magnetosphere and accretes to the star quasi-periodically through elongated
funnel streams which cause the magnetic field to reconnect. The star spins-down
owing to the interaction of the closed magnetosphere with the disk. For the
considered conditions, the spin-down torque varies with the angular velocity of
the star omega* as omega*^1.3 for fixed mass accretion rate. The propeller
stage may be important in the evolution of X-ray pulsars, cataclysmic variables
and young stars. In particular, it may explain the present slow rotation of the
classical T Tauri stars.Comment: 5 pages with 4 figures, LaTeX, macros: emulapj.sty, avi movies are
available at http://www.astro.cornell.edu/us-russia/disk_prop.ht
Microwave-induced resistance oscillations and zero-resistance states in 2D electron systems with two occupied subbands
We report on theoretical studies of recently discovered microwave-induced
resistance oscillations and zero resistance states in Hall bars with two
occupied subbands. In the same results, resistance presents a peculiar shape
which appears to have a built-in interference effect not observed before. We
apply the microwave-driven electron orbit model, which implies a
radiation-driven oscillation of the two-dimensional electron system. Thus, we
calculate different intra and inter-subband electron scattering rates and times
that are revealing as different microwave-driven oscillations frequencies for
the two electronic subbands. Through scattering, these subband-dependent
oscillation motions interfere giving rise to a striking resistance profile. We
also study the dependence of irradiated magnetoresistance with power and
temperature. Calculated results are in good agreement with experiments.Comment: 7 pages, 6 figure
- …