23,414 research outputs found
Wave packet approach to transport in mesoscopic systems
Wave packets provide a well established and versatile tool for studying
time-dependent effects in molecular physics. Here, we demonstrate the
application of wave packets to mesoscopic nanodevices at low temperatures. The
electronic transport in the devices is expressed in terms of scattering and
transmission coefficients, which are efficiently obtained by solving an initial
value problem (IVP) using the time-dependent Schroedinger equation. The
formulation as an IVP makes non-trivial device topologies accessible and by
tuning the wave packet parameters one can extract the scattering properties for
a large range of energies.Comment: 12 pages, 4 figure
Revivals of quantum wave-packets in graphene
We investigate the propagation of wave-packets on graphene in a perpendicular
magnetic field and the appearance of collapses and revivals in the
time-evolution of an initially localised wave-packet. The wave-packet evolution
in graphene differs drastically from the one in an electron gas and shows a
rich revival structure similar to the dynamics of highly excited Rydberg
states.
We present a novel numerical wave-packet propagation scheme in order to solve
the effective single-particle Dirac-Hamiltonian of graphene and show how the
collapse and revival dynamics is affected by the presence of disorder. Our
effective numerical method is of general interest for the solution of the Dirac
equation in the presence of potentials and magnetic fields.Comment: 22 pages, 10 figures, 3 movies, to appear in New Journal of Physic
On some geometric features of the Kramer interior solution for a rotating perfect fluid
Geometric features (including convexity properties) of an exact interior
gravitational field due to a self-gravitating axisymmetric body of perfect
fluid in stationary, rigid rotation are studied. In spite of the seemingly
non-Newtonian features of the bounding surface for some rotation rates, we
show, by means of a detailed analysis of the three-dimensional spatial
geodesics, that the standard Newtonian convexity properties do hold. A central
role is played by a family of geodesics that are introduced here, and provide a
generalization of the Newtonian straight lines parallel to the axis of
rotation.Comment: LaTeX, 15 pages with 4 Poscript figures. To be published in Classical
and Quantum Gravit
Negative differential conductance induced by spin-charge separation
Spin-charge states of correlated electrons in a one-dimensional quantum dot
attached to interacting leads are studied in the non-linear transport regime.
With non-symmetric tunnel barriers, regions of negative differential
conductance induced by spin-charge separation are found. They are due to a
correlation-induced trapping of higher-spin states without magnetic field, and
associated with a strong increase in the fluctuations of the electron spin.Comment: REVTEX, 4 pages including 3 figures; Accepted for publication on
Physical Review Letter
Electron propagation in crossed magnetic and electric fields
Laser-atom interaction can be an efficient mechanism for the production of
coherent electrons. We analyze the dynamics of monoenergetic electrons in the
presence of uniform, perpendicular magnetic and electric fields. The Green
function technique is used to derive analytic results for the field--induced
quantum mechanical drift motion of i) single electrons and ii) a dilute Fermi
gas of electrons. The method yields the drift current and, at the same time it
allows us to quantitatively establish the broadening of the (magnetic) Landau
levels due to the electric field: Level number k is split into k+1 sublevels
that render the th oscillator eigenstate in energy space. Adjacent Landau
levels will overlap if the electric field exceeds a critical strength. Our
observations are relevant for quantum Hall configurations whenever electric
field effects should be taken into account.Comment: 11 pages, 2 figures, submitte
Context-awareness to increase inclusion of people with DS in society
Assistive technologies have the potential to enhance the quality of life of citizens. Most especially of interest are those cases where a person is affected by some physical or cognitive impairment. Whilst most work in this area have been focused on assisting people indoors to support their independence, the POSEIDON project is focused on empowering citizens with Down’s Syndrome to support their independence outdoors. This paper explains the POSEIDON module which we are in the process of developing to make the system context-aware,reactive and adaptive
Interference in interacting quantum dots with spin
We study spectral and transport properties of interacting quantum dots with
spin. Two particular model systems are investigated: Lateral multilevel and two
parallel quantum dots. In both cases different paths through the system can
give rise to interference. We demonstrate that this strengthens the multilevel
Kondo effect for which a simple two-stage mechanism is proposed. In parallel
dots we show under which conditions the peak of an interference-induced orbital
Kondo effect can be split.Comment: 8 pages, 8 figure
Apparatus for determining thermophysical properties of test specimens
Apparatus is described for directly measuring the quantity square root of pck of a test specimen such as a wind tunnel model where p is density, c is the specific heat and k is the thermal conductivity of the specimen. The test specimen and a reference specimen are simultaneously subjected to the heat from a heat source. A thermocouple is attached to the reference specimen for producing a first electrical analog signal proportional to the heat rate Q that the test specimen is subjected to and an infrared radiometer that is aimed at the test specimen produces a second electrical analog signal proportional to the surface temperature T of the test specimen. An analog-to-digital converter converts the first and second electrical analog signals to digital signals. These digital signals are applied to a computer for determining the quantity
Axisymmetric Stationary Solutions as Harmonic Maps
We present a method for generating exact solutions of Einstein equations in
vacuum using harmonic maps, when the spacetime possesses two commutating
Killing vectors. This method consists in writing the axisymmetric stationry
Einstein equations in vacuum as a harmonic map which belongs to the group
SL(2,R), and decomposing it in its harmonic "submaps". This method provides a
natural classification of the solutions in classes (Weil's class, Lewis' class
etc).Comment: 17 TeX pages, one table,( CINVESTAV- preprint 12/93
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