6,142 research outputs found
Dynamics of molecular nanomagnets in time-dependent external magnetic fields: Beyond the Landau-Zener-St\"{u}ckelberg model
The time evolution of the magnetization of a magnetic molecular crystal is
obtained in an external time-dependent magnetic field, with sweep rates in the
kT/s range. We present the 'exact numerical' solution of the time dependent
Schr\"{o}dinger equation, and show that the steps in the hysteresis curve can
be described as a sequence of two-level transitions between adiabatic states.
The multilevel nature of the problem causes the transition probabilities to
deviate significantly from the predictions of the Landau-Zener-St\"{u}ckelberg
model. These calculations allow the introduction of an efficient approximation
method that accurately reproduces the exact results. When including phase
relaxation by means of an appropriate master equation, we observe an interplay
between coherent dynamics and decoherence. This decreases the size of the
magnetization steps at the transitions, but does not modify qualitatively the
physical picture obtained without relaxation.Comment: 8 pages, 7 figure
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.
Herschel PACS and SPIRE spectroscopy of the Photodissociation Regions associated with S 106 and IRAS 23133+6050
Photodissociation regions (PDRs) contain a large fraction of all of the
interstellar matter in galaxies. Classical examples include the boundaries
between ionized regions and molecular clouds in regions of massive star
formation, marking the point where all of the photons energetic enough to
ionize hydrogen have been absorbed. In this paper we determine the physical
properties of the PDRs associated with the star forming regions IRAS 23133+6050
and S 106 and present them in the context of other Galactic PDRs associated
with massive star forming regions. We employ Herschel PACS and SPIRE
spectroscopic observations to construct a full 55-650 {\mu}m spectrum of each
object from which we measure the PDR cooling lines, other fine- structure
lines, CO lines and the total far-infrared flux. These measurements are then
compared to standard PDR models. Subsequently detailed numerical PDR models are
compared to these predictions, yielding additional insights into the dominant
thermal processes in the PDRs and their structures. We find that the PDRs of
each object are very similar, and can be characterized by a two-phase PDR model
with a very dense, highly UV irradiated phase (n 10^6 cm^(-3), G
10^5) interspersed within a lower density, weaker radiation field phase
(n 10^4 cm^(-3), G 10^4). We employed two different numerical
models to investigate the data, firstly we used RADEX models to fit the peak of
the CO ladder, which in conjunction with the properties derived yielded
a temperature of around 300 K. Subsequent numerical modeling with a full PDR
model revealed that the dense phase has a filling factor of around 0.6 in both
objects. The shape of the CO ladder was consistent with these components
with heating dominated by grain photoelectric heating. An extra excitation
component for the highest J lines (J > 20) is required for S 106.Comment: 20 pages, 10 figures, A&A Accepte
Snake orbits and related magnetic edge states
We study the electron motion near magnetic field steps at which the strength
and/or sign of the magnetic field changes. The energy spectrum for such systems
is found and the electron states (bound and scattered) are compared with their
corresponding classical paths. Several classical properties as the velocity
parallel to the edge, the oscillation frequency perpendicular to the edge and
the extent of the states are compared with their quantum mechanical
counterpart. A class of magnetic edge states is found which do not have a
classical counterpart.Comment: 8 pages, 10 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
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
Precession-torque-driven domain-wall motion in out-of-plane materials
Domain-wall (DW) motion in magnetic nanostrips is intensively studied, in
particular because of the possible applications in data storage. In this work,
we will investigate a novel method of DW motion using magnetic field pulses,
with the precession torque as the driving mechanism. We use a one dimensional
(1D) model to show that it is possible to drive DWs in out-of-plane materials
using the precession torque, and we identify the key parameters that influence
this motion. Because the DW moves back to its initial position at the end of
the field pulse, thereby severely complicating direct detection of the DW
motion, depinning experiments are used to indirectly observe the effect of the
precession torque. The 1D model is extended to include an energy landscape in
order to predict the influence of the precession torque in the depinning
experiments. Although preliminary experiments did not yet show an effect of the
precession torque, our calculations indicate that depinning experiments can be
used to demonstrate this novel method of DW motion in out-of-plane materials,
which even allows for coherent motion of multiple domains when the
Dzyaloshinskii-Moriya interaction is taken into account
Topological confinement in graphene bilayer quantum rings
We demonstrate the existence of localized electron and hole states in a
ring-shaped potential kink in biased bilayer graphene. Within the continuum
description, we show that for sharp potential steps the Dirac equation
describing carrier states close to the K (or K') point of the first Brillouin
zone can be solved analytically for a circular kink/anti-kink dot. The
solutions exhibit interfacial states which exhibit Aharonov-Bohm oscillations
as functions of the height of the potential step and/or the radius of the ring
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