1,879 research outputs found
Spin effects in a confined 2DEG: Enhancement of the g-factor, spin-inversion states and their far-infrared absorption
We investigate several spin-related phenomena in a confined two-dimensional
electron gas (2DEG) using the Hartree-Fock approximation for the mutual Coulomb
interaction of the electrons. The exchange term of the interaction causes a
large splitting of the spin levels whenever the chemical potential lies within
a Landau band (LB). This splitting can be reinterpreted as an enhancement of an
effective g-factor, g*. The increase of g* when a LB is half filled can be
accompanied by a spontaneous formation of a static spin-inversion state (SIS)
whose details depend on the system sision state (SIS) whose details depend on
the system size. The coupling of the states of higher LB's into the lowest band
by the Coulomb interaction of the 2DEG is essential for the SIS to occur. The
far-infrared absorption of the system, relatively insensitive to the spin
splitting, develops clear signs of the SIS.Comment: 7 figure
Modelling ground pickup for microwave telescopes
Microwave telescopes require an ever-increasing control of experimental
systematics in their quest to measure the Cosmic Microwave Background (CMB) to
exquisite levels of precision. One important systematic for ground and
balloon-borne experiments is ground pickup, where beam sidelobes detect the
thermal emission of the much warmer ground while the main beam is scanning the
sky. This generates scan-synchronous noise in experiment timestreams, which is
difficult to filter out without also deleting some of the signal from the sky.
Therefore, efficient modelling of pickup can help guide the design of
experiments and of analysis pipelines. In this work, we present an extension to
the beamconv algorithm that enables us to generate time-ordered data (TOD) from
beam-convolved sky and ground maps simultaneously. We simulate ground pickup
for both a ground-based experiment and a telescope attached to a stratospheric
balloon. Ground templates for the balloon experiment are obtained by
re-projecting satellite maps of the Earth's microwave emission.Comment: 10 pages, 6 figures, paper 12190-165 for the "SPIE AT+I 2022 mm,
sub-mmm, and Far-Infrared Detectors and Instrumentation for Astronomy XI"
conferenc
Lens magnification by CL0024+1654 in the U and R band
[ABRIDGED] We estimate the total mass distribution of the galaxy cluster
CL0024+1654 from the measured source depletion due to lens magnification in the
R band. Within a radius of 0.54Mpc/h, a total projected mass of
(8.1+/-3.2)*10^14 M_sol/h (EdS) is measured, which corresponds to a mass-
to-light ratio of M/L(B)=470+/-180. We compute the luminosity function of
CL0024+1654 in order to estimate contamination of the background source counts
from cluster galaxies. Three different magnification-based reconstruction
methods are employed using both local and non-local techniques. We have
modified the standard single power-law slope number count theory to incorporate
a break and applied this to our observations. Fitting analytical magnification
profiles of different cluster models to the observed number counts, we find
that the cluster is best described either by a NFW model with scale radius
r_s=334+/-191 kpc/h and normalisation kappa_s=0.23+/-0.08 or a power-law
profile with slope xi=0.61+/-0.11, central surface mass density
kappa_0=1.52+/-0.20 and assuming a core radius of r_core=35 kpc/h. The NFW
model predicts that the cumulative projected mass contained within a radius R
scales as M(<R)=2.9*10^14*(R/1')^[1.3-0.5lg (R/1')] M_sol/h. Finally, we have
exploited the fact that flux magnification effectively enables us to probe
deeper than the physical limiting magnitude of our observations in searching
for a change of slope in the U band number counts. We rule out both a total
flattening of the counts with a break up to U_AB<=26.6 and a change of slope,
reported by some studies, from dlog N/dm=0.4->0.15 up to U_AB<=26.4 with 95%
confidence.Comment: 19 pages, 12 figures, submitted to A&A. New version includes more
robust U band break analysis and contamination estimates, plus new plot
Transient regime in non-linear transport through many-level quantum dots
We investigate the nonstationary electronic transport in noninteracting
nanostructures driven by a finite bias and time-dependent signals applied at
their contacts to the leads. The systems are modelled by a tight-binding
Hamiltonian and the transient currents are computed from the non-equilibrium
Green-Keldysh formalism. The numerical implementation is not restricted to weak
coupling to the leads and does not imply the wide-band limit assumption for the
spectral width of the leads. As an application of the method we study in detail
the transient behavior and the charge dynamics in single and double quantum
dots connected to leads by a step-like potential, but the method allows as well
the consideration of non-periodic potentials or short pulses. We show that when
the higher energy levels of the isolated system are located within the bias
window of the leads the transient current approaches the steady state in a
non-oscillatory smooth fashion. At moderate coupling to the leads and fixed
bias the transient acquires a step-like structure, the length of the steps
increasing with the system size. The number of levels inside a finite bias
window can be tuned by a constant gate potential. We find also that the
transient behavior depends on the specific way of coupling the leads to the
mesoscopic system.Comment: RevTeX, 12 pages, 11 include .eps figure
Manifestation of the Hofstadter butterfly in far-infrared absorption
The far-infrared absorption of a two-dimensional electron gas with a
square-lattice modulation in a perpendicular constant magnetic field is
calculated self-consistently within the Hartree approximation. For strong
modulation and short period we obtain intra- and intersubband magnetoplasmon
modes reflecting the subbands of the Hofstadter butterfly in two or more Landau
bands. The character of the absorption and the correlation of the peaks to the
number of flux quanta through each unit cell of the periodic potential depends
strongly on the location of the chemical potential with respect to the
subbands, or what is the same, on the density of electrons in the system.Comment: RevTeX file + 4 postscript figures, to be published Phys. Rev. B
Rapid Com
Butterfly-like spectra and collective modes of antidot superlattices in magnetic fields
We calculate the energy band structure for electrons in an external periodic
potential combined with a perpendicular magnetic field. Electron-electron
interactions are included within a Hartree approximation. The calculated energy
spectra display a considerable degree of self-similarity, just as the
``Hofstadter butterfly.'' However, screening affects the butterfly, most
importantly the bandwidths oscillate with magnetic field in a characteristic
way. We also investigate the dynamic response of the electron system in the
far-infrared (FIR) regime. Some of the peaks in the FIR absorption spectra can
be interpreted mainly in semiclassical terms, while others originate from
inter(sub)band transitions.Comment: 4 pages with 2 embeded eps figures. Uses revtex, multicol and
graphicx styles. Accepted for publication in PRB Brief Report
Thomas-Fermi Calculations of Atoms and Matter in Magnetic Neutron Stars II: Finite Temperature Effects
We present numerical calculations of the equation of state for dense matter
in high magnetic fields, using a temperature dependent Thomas-Fermi theory with
a magnetic field that takes all Landau levels into account. Free energies for
atoms and matter are also calculated as well as profiles of the electron
density as a function of distance from the atomic nucleus for representative
values of the magnetic field strength, total matter density, and temperature.
The Landau shell structure, which is so prominent in cold dense matter in high
magnetic fields, is still clearly present at finite temperature as long as it
is less than approximately one tenth of the cyclotron energy. This structure is
reflected in an oscillatory behaviour of the equation of state and other
thermodynamic properties of dense matter and hence also in profiles of the
density and pressure as functions of depth in the surface layers of magnetic
neutron stars. These oscillations are completely smoothed out by thermal
effects at temperatures of the order of the cyclotron energy or higher.Comment: 37 pages, 17 figures included, submitted to Ap
A brief exposure to rightward prismatic adaptation changes resting-state network characteristics of the ventral attentional system.
A brief session of rightward prismatic adaptation (R-PA) has been shown to alleviate neglect symptoms in patients with right hemispheric damage, very likely by switching hemispheric dominance of the ventral attentional network (VAN) from the right to the left and by changing task-related activity within the dorsal attentional network (DAN). We have investigated this very rapid change in functional organisation with a network approach by comparing resting-state connectivity before and after a brief exposure i) to R-PA (14 normal subjects; experimental condition) or ii) to plain glasses (12 normal subjects; control condition). A whole brain analysis (comprising 129 regions of interest) highlighted R-PA-induced changes within a bilateral, fronto-temporal network, which consisted of 13 nodes and 11 edges; all edges involved one of 4 frontal nodes, which were part of VAN. The analysis of network characteristics within VAN and DAN revealed a R-PA-induced decrease in connectivity strength between nodes and a decrease in local efficiency within VAN but not within DAN. These results indicate that the resting-state connectivity configuration of VAN is modulated by R-PA, possibly by decreasing its modularity
Quantification of differences between occupancy and total monitoring periods for better assessment of exposure to particles in indoor environments
For the assessment of personal exposure, information about the concentration of pollutants when people are in given indoor environments (occupancy time) are of prime importance. However this kind of data frequently is not reported. The aim of this study was to assess differences in particle characteristics between occupancy time and the total monitoring period, with the latter being the most frequently used averaging time in the published data. Seven indoor environments were selected in Sweden and Finland: an apartment, two houses, two schools, a supermarket, and a restaurant. They were assessed for particle number and mass concentrations and number size distributions. The measurements using a Scanning Mobility Particle Sizer and two photometers were conducted for seven consecutive days during winter in each location. Particle concentrations in residences and schools were, as expected, the highest during occupancy time. In the apartment average and median PM2.5 mass concentrations during the occupancy time were 29% and 17% higher, respectively compared to total monitoring period. In both schools, the average and medium values of the PM2.5 mass concentrations were on average higher during teaching hours compared to the total monitoring period by 16% and 32%, respectively. When it comes to particle number concentrations (PNC), in the apartment during occupancy, the average and median values were 33% and 58% higher, respectively than during the total monitoring period. In both houses and schools the average and median PNC were similar for the occupancy and total monitoring periods. General conclusions on the basis of measurements in the limited number of indoor environments cannot be drawn. However the results confirm a strong dependence on type and frequency of indoor activities that generate particles and site specificity. The results also indicate that the exclusion of data series during non-occupancy periods can improve the estimates of particle concentrations and characteristics suitable for exposure assessment, which is crucial for estimating health effects in epidemiological and toxicological studies. (C) 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/3.0/).Peer reviewe
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