6,343 research outputs found
Resistance effects due to magnetic guiding orbits
The Hall and magnetoresistance of a two dimensional electron gas subjected to
a magnetic field barrier parallel to the current direction is studied as
function of the applied perpendicular magnetic field. The recent experimental
results of Nogaret {\em et al.} [Phys. Rev. Lett. {\bf 84}, 2231 (2000)] for
the magneto- and Hall resistance are explained using a semi-classical theory
based on the Landauer-B\"{u}ttiker formula. The observed positive
magnetoresistance peak is explained as due to a competition between a decrease
of the number of conducting channels as a result of the growing magnetic field,
from the fringe field of the ferromagnetic stripe as it becomes magnetized, and
the disappearance of snake orbits and the subsequent appearance of cycloidlike
orbits.Comment: 7 pages, 7 figure
Snake states in graphene quantum dots in the presence of a p-n junction
We investigate the magnetic interface states of graphene quantum dots that
contain p-n junctions. Within a tight-binding approach, we consider rectangular
quantum dots in the presence of a perpendicular magnetic field containing p-n,
as well as p-n-p and n-p-n junctions. The results show the interplay between
the edge states associated with the zigzag terminations of the sample and the
snake states that arise at the p-n junction, due to the overlap between
electron and hole states at the potential interface. Remarkable localized
states are found at the crossing of the p-n junction with the zigzag edge
having a dumb-bell shaped electron distribution. The results are presented as
function of the junction parameters and the applied magnetic flux.Comment: 13 pages, 23 figures, to be appeared in Phys. Rev.
The split-operator technique for the study of spinorial wavepacket dynamics
The split-operator technique for wave packet propagation in quantum systems
is expanded here to the case of propagating wave functions describing
Schr\"odinger particles, namely, charge carriers in semiconductor
nanostructures within the effective mass approximation, in the presence of
Zeeman effect, as well as of Rashba and Dresselhaus spin-orbit interactions. We
also demonstrate that simple modifications to the expanded technique allow us
to calculate the time evolution of wave packets describing Dirac particles,
which are relevant for the study of transport properties in graphene.Comment: 19 pages, 4 figure
Exciton trapping in magnetic wire structures
The lateral magnetic confinement of quasi two-dimensional excitons into wire
like structures is studied. Spin effects are take into account and two
different magnetic field profiles are considered, which experimentally can be
created by the deposition of a ferromagnetic stripe on a semiconductor quantum
well with magnetization parallel or perpendicular to the grown direction of the
well. We find that it is possible to confine excitons into one-dimensional (1D)
traps. We show that the dependence of the confinement energy on the exciton
wave vector, which is related to its free direction of motion along the wire
direction, is very small. Through the application of a background magnetic
field it is possible to move the position of the trapping region towards the
edge of the ferromagnetic stripe or even underneath the stripe. The exact
position of this 1D exciton channel depends on the strength of the background
magnetic field and on the magnetic polarisation direction of the ferromagnetic
film.Comment: 10 pages, 7 figures, to be published in J. Phys: Condens. Matte
Simplified model for the energy levels of quantum rings in single layer and bilayer graphene
Within a minimal model, we present analytical expressions for the eigenstates
and eigenvalues of carriers confined in quantum rings in monolayer and bilayer
graphene. The calculations were performed in the context of the continuum
model, by solving the Dirac equation for a zero width ring geometry, i.e. by
freezing out the carrier radial motion. We include the effect of an external
magnetic field and show the appearance of Aharonov-Bohm oscillations and of a
non-zero gap in the spectrum. Our minimal model gives insight in the energy
spectrum of graphene-based quantum rings and models different aspects of finite
width rings.Comment: To appear in Phys. Rev.
Variations of the Mid-IR Aromatic Features Inside and Among Galaxies
We present the results of a systematic study of mid-IR spectra of Galactic
regions, Magellanic HII regions, and galaxies of various types (dwarf, spiral,
starburst), observed by the satellites ISO and Spitzer. We study the relative
variations of the 6.2, 7.7, 8.6 and 11.3 micron features inside spatially
resolved objects (such as M82, M51, 30 Doradus, M17 and the Orion Bar), as well
as among 90 integrated spectra of 50 objects. Our main results are that the
6.2, 7.7 and 8.6 micron bands are essentially tied together, while the ratios
between these bands and the 11.3 micron band varies by one order of magnitude.
This implies that the properties of the PAHs are remarkably universal
throughout our sample, and that the relative variations of the band ratios are
mainly controled by the fraction of ionized PAHs. In particular, we show that
we can rule out both the modification of the PAH size distribution, and the
mid-infrared extinction, as an explanation of these variations. Using a few
well-studied Galactic regions (including the spectral image of the Orion Bar),
we give an empirical relation between the I(6.2)/I(11.3) ratio and the
ionization/recombination ratio G0/ne.Tgas^0.5, therefore providing a useful
quantitative diagnostic tool of the physical conditions in the regions where
the PAH emission originates. Finally, we discuss the physical interpretation of
the I(6.2)/I(11.3) ratio, on galactic size scales.Comment: Accepted by the ApJ, 67 pages, 70 figure
Spin-orbit interaction induced singularity of the charge density relaxation propagator
The charge density relaxation propagator of a two dimensional electron
system, which is the slope of the imaginary part of the polarization function,
exhibits singularities for bosonic momenta having the order of the spin-orbit
momentum and depending on the momentum orientation. We have provided an
intuitive understanding for this non-analytic behavior in terms of the inter
chirality subband electronic transitions, induced by the combined action of
Bychkov-Rashba (BR) and Dresselhaus (D) spin-orbit coupling. It is shown that
the regular behavior of the relaxation propagator is recovered in the presence
of only one BR or D spin-orbit field or for spin-orbit interaction with equal
BR and D coupling strengths. This creates a new possibility to influence
carrier relaxation properties by means of an applied electric field.Comment: 4 figure
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