8,377 research outputs found
Positively charged magneto-excitons in a semiconductor quantum well
A variational calculation of the lower singlet and triplet states of
positively charged excitons (trions) confined to a single quantum well and in
the presence of a perpendicular magnetic field is presented. We study the
dependence of the energy levels and of the binding energy on the well width and
on the magnetic field strength. Our results are compared with the available
experimental data and show a good qualitative and quantitative agreement. A
singlet-triplet crossing is found which for a 200 \AA wide GaAs is predicted to
occur for B = 15 T.Comment: 5 figs. Submitted to PR
Dirac electrons in a Kronig-Penney potential: dispersion relation and transmission periodic in the strength of the barriers
The transmission T and conductance G through one or multiple one-dimensional,
delta-function barriers of two-dimensional fermions with a linear energy
spectrum are studied. T and G are periodic functions of the strength P of the
delta-function barrier V(x,y) / hbar v_F = P delta(x). The dispersion relation
of a Kronig-Penney (KP) model of a superlattice is also a periodic function of
P and causes collimation of an incident electron beam for P = 2 pi n and n
integer. For a KP superlattice with alternating sign of the height of the
barriers the Dirac point becomes a Dirac line for P = (n + 1/2) pi.Comment: 5 pages, 6 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
"Better Safe than Sorry" - Individual Risk-free Pension Schemes in the European Union - Macroeconomic Benefits, the Mobile Working Citizen's Perspective and Why Nots
Variations between the diverse pension systems in the member states of the European Union hamper labour market mobility, across country borders but also within the countries of the European Union. From a macroeconomic perspective, and in the light of demographic pressure, this paper argues that allowing individual instead of collective pension building would greatly improve labour market flexibility and thus enhance the functioning of the monetary union. I argue that working citizens would benefit, for three reasons, from pension saving in a risk-free savings account. First, citizens would have a clear picture of the accumulation of their own pension savings throughout their working life. Second, they would pay hardly any extra costs and, third, once retired they would not be subject to the whims of government or other pension fund managers. This paper investigates the feasibility of individual pension building under various parameter settings by calculating the pension saved during a working life and the pension dis-saved after retirement. The findings show that there are no reasons why the European Union and individual member states should not allow individual risk-free pension savings accounts. This would have macroeconomic benefits and provide a solid pension provision that can enhance mobility, instead of engaging workers in different mandatory collective pension schemes that exist around in the European Union
Polaron effects in electron channels on a helium film
Using the Feynman path-integral formalism we study the polaron effects in
quantum wires above a liquid helium film. The electron interacts with
two-dimensional (2D) surface phonons, i.e. ripplons, and is confined in one
dimension (1D) by an harmonic potential. The obtained results are valid for
arbitrary temperature (), electron-phonon coupling strength (), and
lateral confinement (). Analytical and numerical results are
obtained for limiting cases of , , and . We found the
surprising result that reducing the electron motion from 2D to quasi-1D makes
the self-trapping transition more continuous.Comment: 6 pages, 7 figures, submitted to Phys. Rev.
Beats of the Magnetocapacitance Oscillations in Lateral Semiconductor Superlattices
We present calculations on the magnetocapacitance of the two-dimensional
electron gas in a lateral semiconductor superlattice under two-dimensional weak
periodic potential modulation in the presence of a perpendicular magnetic
field. Adopting a Gaussian broadening of magnetic-field-dependent width in the
density of states, we present explicit and simple expressions for the
magnetocapacitance, valid for the relevant weak magnetic fields and modulation
strengths. As the modulation strength in both directions increase, beats of the
magnetocapacitance oscillations are observed, in the low magnetic field range
(Weiss-oscillations regime), which are absent in the one-dimensional weak
modulation case.Comment: 11 pages, 7 figures, accepted by Mod. Phys. Lett. B (March 2007
Effect of the boundary condition on the vortex patterns in mesoscopic three-dimensional superconductors - disk and sphere
The vortex state of mesoscopic three-dimensional superconductors is
determined using a minimization procedure of the Ginzburg-Landau free energy.
We obtain the vortex pattern for a mesoscopic superconducting sphere and find
that vortex lines are naturally bent and are closest to each other at the
equatorial plane. For a superconducting disk with finite height, and under an
applied magnetic field perpendicular to its major surface, we find that our
method gives results consistent with previous calculations. The matching
fields, the magnetization and , are obtained for models that differ
according to their boundary properties. A change of the Ginzburg-Landau
parameters near the surface can substantially enhance as shown here.Comment: 7 pages, 4 figures (low resolution
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
On the extrapolation to ITER of discharges in present tokamaks
An expression for the extrapolated fusion gain G = Pfusion /5 Pheat (Pfusion
being the total fusion power and Pheat the total heating power) of ITER in
terms of the confinement improvement factor (H) and the normalised beta (betaN)
is derived in this paper. It is shown that an increase in normalised beta can
be expected to have a negative or neutral influence on G depending on the
chosen confinement scaling law. Figures of merit like H betaN / q95^2 should be
used with care, since large values of this quantity do not guarantee high
values of G, and might not be attainable with the heating power installed on
ITER.Comment: 6 Pages, 3 figures, Submitted to Nuclear Fusion on the 29th of
November 200
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
- …