3,376 research outputs found
Spin transitions in an incompressible liquid Coulomb coupled to a quantum dot
We report on our investigation of the low-lying energy spectra and charge
density of a two-dimensional quantum Hall liquid at that is
Coulomb coupled to a quantum dot. The dot contains a hole and two/three
electrons. We found that any external perturbation (caused by the close
proximity of the quantum dot) locally changes the spin polarization of the
incompressible liquid. The effect depends crucially on the separation distance
of the quantum dot from the electron plane. Electron density distribution in
the quantum Hall layer indicates creation of a quasihole that is localized by
the close proximity of the quantum dot. Manifestation of this effect in the
photoluminescence spectroscopy is also discussed.Comment: 4 pages, 3 figure
Non-Arrhenius ionic conductivities in glasses due to a distribution of activation energies
Previously observed non-Arrhenius behavior in fast ion conducting glasses
[\textit{Phys.\ Rev.\ Lett.}\ \textbf{76}, 70 (1996)] occurs at temperatures
near the glass transition temperature, , and is attributed to changes in
the ion mobility due to ion trapping mechanisms that diminish the conductivity
and result in a decreasing conductivity with increasing temperature. It is
intuitive that disorder in glass will also result in a distribution of the
activation energies (DAE) for ion conduction, which should increase the
conductivity with increasing temperature, yet this has not been identified in
the literature. In this paper, a series of high precision ionic conductivity
measurements are reported for
glasses with compositions ranging from . The impact of the
cation site disorder on the activation energy is identified and explained using
a DAE model. The absence of the non-Arrhenius behavior in other glasses is
explained and it is predicted which glasses are expected to accentuate the DAE
effect on the ionic conductivity.Comment: 2 figure
Inelastic light scattering and the excited states of many-electron quantum dots
A consistent calculation of resonant inelastic (Raman) scattering amplitudes
for relatively large quantum dots, which takes account of valence-band mixing,
discrete character of the spectrum in intermediate and final states, and
interference effects, is presented. Raman peaks in charge and spin channels are
compared with multipole strengths and with the density of energy levels in
final states. A qualitative comparison with the available experimental results
is given.Comment: 5 pages, accepted in J. Phys.: Condens. Matte
Geometry for the accelerating universe
The Lorentzian spacetime metric is replaced by an area metric which naturally
emerges as a generalized geometry in quantum string and gauge theory. Employing
the area metric curvature scalar, the gravitational Einstein-Hilbert action is
re-interpreted as dynamics for an area metric. Without the need for dark energy
or fine-tuning, area metric cosmology explains the observed small acceleration
of the late Universe.Comment: 4 pages, 1 figur
Ammonia from cold high-mass clumps discovered in the inner Galactic disk by the ATLASGAL survey
The APEX Telescope Large Area Survey: The Galaxy (ATLASGAL) is an unbiased
continuum survey of the inner Galactic disk at 870 \mu m. It covers +/- 60 deg
in Galactic longitude and aims to find all massive clumps at various stages of
high-mass star formation in the inner Galaxy, particularly the earliest
evolutionary phases. We aim to determine properties such as the gas kinetic
temperature and dynamics of new massive cold clumps found by ATLASGAL. Most
importantly, we derived their kinematical distances from the measured line
velocities. We observed the ammonia (J,K) = (1,1) to (3,3) inversion
transitions toward 862 clumps of a flux-limited sample of submm clumps detected
by ATLASGAL and extracted 13CO (1-0) spectra from the Galactic Ring Survey
(GRS). We determined distances for a subsample located at the tangential points
(71 sources) and for 277 clumps whose near/far distance ambiguity is resolved.
Most ATLASGAL clumps are cold with rotational temperatures from 10-30 K. They
have a wide range of NH3 linewidths, which by far exceeds the thermal
linewidth, as well as a broad distribution of high column densities with an NH3
abundance in the range of 5 to 30 * 10^{-8}. We found an enhancement of clumps
at Galactocentric radii of 4.5 and 6 kpc. The high detection rate (87%)
confirms ammonia as an excellent probe of the molecular content of the massive,
cold clumps revealed by ATLASGAL. A clear trend of increasing rotational
temperatures and linewidths with evolutionary stage is seen for source samples
ranging from 24 \mu m dark clumps to clumps with embedded HII regions. The
survey provides the largest ammonia sample of high-mass star forming clumps and
thus presents an important repository for the characterization of statistical
properties of the clumps and the selection of subsamples for detailed,
high-resolution follow-up studies
Characterization of Infrared Dark Clouds -- NH Observations of an Absorption-contrast Selected IRDC Sample
Despite increasing research in massive star formation, little is known about
its earliest stages. Infrared Dark Clouds (IRDCs) are cold, dense and massive
enough to harbour the sites of future high-mass star formation. But up to now,
mainly small samples have been observed and analysed. To understand the
physical conditions during the early stages of high-mass star formation, it is
necessary to learn more about the physical conditions and stability in
relatively unevolved IRDCs. Thus, for characterising IRDCs studies of large
samples are needed. We investigate a complete sample of 218 northern hemisphere
high-contrast IRDCs using the ammonia (1,1)- and (2,2)-inversion transitions.
We detected ammonia (1,1)-inversion transition lines in 109 of our IRDC
candidates. Using the data we were able to study the physical conditions within
the star-forming regions statistically. We compared them with the conditions in
more evolved regions which have been observed in the same fashion as our sample
sources. Our results show that IRDCs have, on average, rotation temperatures of
15 K, are turbulent (with line width FWHMs around 2 km s), have ammonia
column densities on the order of cm and molecular hydrogen
column densities on the order of cm. Their virial masses are
between 100 and a few 1000 M. The comparison of bulk kinetic and
potential energies indicate that the sources are close to virial equilibrium.
IRDCs are on average cooler and less turbulent than a comparison sample of
high-mass protostellar objects, and have lower ammonia column densities. Virial
parameters indicate that the majority of IRDCs are currently stable, but are
expected to collapse in the future.Comment: 21 pages, 11 figures, 7 tables. Paper accepted for publication in
Astronomy & Astrophysic
Semiquantitative theory of electronic Raman scattering from medium-size quantum dots
A consistent semiquantitative theoretical analysis of electronic Raman
scattering from many-electron quantum dots under resonance excitation
conditions has been performed. The theory is based on
random-phase-approximation-like wave functions, with the Coulomb interactions
treated exactly, and hole valence-band mixing accounted for within the
Kohn-Luttinger Hamiltonian framework. The widths of intermediate and final
states in the scattering process, although treated phenomenologically, play a
significant role in the calculations, particularly for well above band gap
excitation. The calculated polarized and unpolarized Raman spectra reveal a
great complexity of features and details when the incident light energy is
swept from below, through, and above the quantum dot band gap. Incoming and
outgoing resonances dramatically modify the Raman intensities of the single
particle, charge density, and spin density excitations. The theoretical results
are presented in detail and discussed with regard to experimental observations.Comment: Submitted to Phys. Rev.
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