1,032 research outputs found
Terahertz dynamics of a topologically protected state: quantum Hall effect plateaus near cyclotron resonance in a GaAs/AlGaAs heterojunction
We measure the Hall conductivity of a two-dimensional electron gas formed at
a GaAs/AlGaAs heterojunction in the terahertz regime close to the cyclotron
resonance frequency by employing a highly sensitive Faraday rotation method
coupled with electrical gating of the sample to change the electron density. We
observe clear plateau-and step-like features in the Faraday rotation angle vs.
electron density and magnetic field (Landau-level filling factor), which are
the high frequency manifestation of quantum Hall plateaus - a signature of
topologically protected edge states. The results are compared to a recent
dynamical scaling theory.Comment: 18 pages, 3 figure
Submillimeter wavelength survey of the galactic plane from l = -5 deg to l = +62 deg: Structure and energetics of the inner disk
Results from a large scale survey of the first quadrant of the Milky Way galactic plane at wavelengths of 150, 250, and 300 microns with a 10x10 arcmin beam are presented. The emission detected in the survey arises from compact sources, most of which are identified with known peaks of 5 GHz and/or CO emission, and from an underlying diffuse background with a typical angular width of approximately 0.9 deg (FWHM) which accounts for most of the emission. A total of 80 prominent discrete sources were identified and characterized, of which about half were not previously reported at far infrared wavelengths. The total infrared luminosity within the solar circle is approximately 1 to 2x10 to the 10th power L sub 0, and is probably emitted by dust that resides in molecular clouds
Revealing exciton masses and dielectric properties of monolayer semiconductors with high magnetic fields
In semiconductor physics, many essential optoelectronic material parameters
can be experimentally revealed via optical spectroscopy in sufficiently large
magnetic fields. For monolayer transition-metal dichalcogenide semiconductors,
this field scale is substantial --tens of teslas or more-- due to heavy carrier
masses and huge exciton binding energies. Here we report absorption
spectroscopy of monolayer MoS, MoSe, MoTe, and WS in very high
magnetic fields to 91~T. We follow the diamagnetic shifts and valley Zeeman
splittings of not only the exciton's ground state but also its excited
, , ..., Rydberg states. This provides a direct experimental
measure of the effective (reduced) exciton masses and dielectric properties.
Exciton binding energies, exciton radii, and free-particle bandgaps are also
determined. The measured exciton masses are heavier than theoretically
predicted, especially for Mo-based monolayers. These results provide essential
and quantitative parameters for the rational design of opto-electronic van der
Waals heterostructures incorporating 2D semiconductors.Comment: updated; now also including data on MoTe2. Accepted & in press,
Nature Commu
Eight-band calculations of strained InAs/GaAs quantum dots compared with one, four, and six-band approximations
The electronic structure of pyramidal shaped InAs/GaAs quantum dots is
calculated using an eight-band strain dependent Hamiltonian. The
influence of strain on band energies and the conduction-band effective mass are
examined. Single particle bound-state energies and exciton binding energies are
computed as functions of island size. The eight-band results are compared with
those for one, four and six bands, and with results from a one-band
approximation in which m(r) is determined by the local value of the strain. The
eight-band model predicts a lower ground state energy and a larger number of
excited states than the other approximations.Comment: 8 pages, 7 figures, revtex, eps
Anomalous quantum confined Stark effects in stacked InAs/GaAs self-assembled quantum dots
Vertically stacked and coupled InAs/GaAs self-assembled quantum dots (SADs)
are predicted to exhibit a strong non-parabolic dependence of the interband
transition energy on the electric field, which is not encountered in single SAD
structures nor in other types of quantum structures. Our study based on an
eight-band strain-dependent Hamiltonian indicates that
this anomalous quantum confined Stark effect is caused by the three-dimensional
strain field distribution which influences drastically the hole states in the
stacked SAD structures.Comment: 4 pages, 4 figure
Development of a SiPM Camera for a Schwarzschild-Couder Cherenkov Telescope for the Cherenkov Telescope Array
We present the development of a novel 11328 pixel silicon photomultiplier
(SiPM) camera for use with a ground-based Cherenkov telescope with
Schwarzschild-Couder optics as a possible medium-sized telescope for the
Cherenkov Telescope Array (CTA). The finely pixelated camera samples air-shower
images with more than twice the optical resolution of cameras that are used in
current Cherenkov telescopes. Advantages of the higher resolution will be a
better event reconstruction yielding improved background suppression and
angular resolution of the reconstructed gamma-ray events, which is crucial in
morphology studies of, for example, Galactic particle accelerators and the
search for gamma-ray halos around extragalactic sources. Packing such a large
number of pixels into an area of only half a square meter and having a fast
readout directly attached to the back of the sensors is a challenging task. For
the prototype camera development, SiPMs from Hamamatsu with through silicon via
(TSV) technology are used. We give a status report of the camera design and
highlight a number of technological advancements that made this development
possible.Comment: 8 pages, 5 figures, In Proceedings of the 34th International Cosmic
Ray Conference (ICRC2015), The Hague, The Netherlands. All CTA contributions
at arXiv:1508.0589
Phase diagrams of in Double Exchange Model with added antiferromagnetic and Jahn-Teller interaction
The phase diagram of the multivalent manganites , in
space of temperature and doping , is a challenge for the theoretical
physics. It is an important test for the model used to study these compounds
and the method of calculation. To obtain theoretically this diagram for
, we consider the two-band Double Exchange Model for manganites with
added Jahn-Teller coupling and antiferromagnetic Heisenberg term. In order to
calculate Curie and N\'{e}el temperatures we derive an effective Heisenberg
model for a vector which describes the local orientation of the total
magnetization of the system. The exchange constants of this model are different
for different space directions and depend on the density of electrons,
antiferromagnetic constants and the Jahn-Teller energy. To reproduce the well
known phase transitions from A-type antiferromagnetism to ferromagnetism at low
and C-type antiferromagnetism to G-type antiferromagnetism at large , we
argue that the antiferromagnetic exchange constants should depend on the
lattice direction. We show that ferromagnetic to A-type antiferromagnetic
transition results from the Jahn-Teller distortion. Accounting adequately for
the magnon-magnon interaction, Curie and N\'{e}el temperatures are calculated.
The results are in very good agreement with the experiment and provide values
for the model parameters, which best describe the behavior of the critical
temperature for .Comment: 13 pages, 5 figure
Tight-Binding model for semiconductor nanostructures
An empirical tight-binding (TB) model is applied to the
investigation of electronic states in semiconductor quantum dots. A basis set
of three -orbitals at the anions and one -orbital at the cations is
chosen. Matrix elements up to the second nearest neighbors and the spin-orbit
coupling are included in our TB-model. The parametrization is chosen so that
the effective masses, the spin-orbit-splitting and the gap energy of the bulk
CdSe and ZnSe are reproduced. Within this reduced TB-basis the
valence (p-) bands are excellently reproduced and the conduction (s-) band is
well reproduced close to the -point, i.e. near to the band gap. In
terms of this model much larger systems can be described than within a (more
realistic) -basis. The quantum dot is modelled by using the (bulk)
TB-parameters for the particular material at those sites occupied by atoms of
this material. Within this TB-model we study pyramidal-shaped CdSe quantum dots
embedded in a ZnSe matrix and free spherical CdSe quantum dots (nanocrystals).
Strain-effects are included by using an appropriate model strain field. Within
the TB-model, the strain-effects can be artifically switched off to investigate
the infuence of strain on the bound electronic states and, in particular, their
spatial orientation. The theoretical results for spherical nanocrystals are
compared with data from tunneling spectroscopy and optical experiments.
Furthermore the influence of the spin-orbit coupling is investigated
The effect of harmonized emissions on aerosol properties in global models – an AeroCom experiment
The effects of unified aerosol sources on global aerosol fields simulated by different models are examined in this paper. We compare results from two AeroCom experiments, one with different (ExpA) and one with unified emissions, injection heights, and particle sizes at the source (ExpB). Surprisingly, harmonization of aerosol sources has only a small impact on the simulated diversity for aerosol burden, and consequently optical properties, as the results are largely controlled by model-specific transport, removal, chemistry (leading to the formation of secondary aerosols) and parameterizations of aerosol microphysics (e.g. the split between deposition pathways) and to a lesser extent on the spatial and temporal distributions of the (precursor) emissions.
The burdens of black carbon and especially sea salt become more coherent in ExpB only, because the large ExpA diversity for these two species was caused by few outliers. The experiment also indicated that despite prescribing emission fluxes and size distributions, ambiguities in the implementation in individual models can lead to substantial differences.
These results indicate the need for a better understanding of aerosol life cycles at process level (including spatial dispersal and interaction with meteorological parameters) in order to obtain more reliable results from global aerosol simulations. This is particularly important as such model results are used to assess the consequences of specific air pollution abatement strategies
IL-33 promotes the egress of group 2 innate lymphoid cells from the bone marrow
Group 2 innate lymphoid cells (ILC2s) are effector cells within the mucosa and key participants in type 2 immune responses in the context of allergic inflammation and infection. ILC2s develop in the bone marrow from common lymphoid progenitor cells, but little is known about how ILC2s egress from the bone marrow for hematogenous trafficking. In this study, we identified a critical role for IL-33, a hallmark peripheral ILC2-activating cytokine, in promoting the egress of ILC2 lineage cells from the bone marrow. Mice lacking IL-33 signaling had normal development of ILC2s but retained significantly more ILC2 progenitors in the bone marrow via augmented expression of CXCR4. Intravenous injection of IL-33 or pulmonary fungal allergen challenge mobilized ILC2 progenitors to exit the bone marrow. Finally, IL-33 enhanced ILC2 trafficking to the lungs in a parabiosis mouse model of tissue disruption and repopulation. Collectively, these data demonstrate that IL-33 plays a critical role in promoting ILC2 egress from the bone marrow
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