2,076 research outputs found
Effect of a skin-deep surface zone on formation of two-dimensional electron gas at a semiconductor surface
Two dimensional electron gases (2DEGs) at surfaces and interfaces of
semiconductors are described straightforwardly with a 1D self-consistent
Poisson-Schr\"{o}dinger scheme. However, their band energies have not been
modeled correctly in this way. Using angle-resolved photoelectron spectroscopy
we study the band structures of 2DEGs formed at sulfur-passivated surfaces of
InAs(001) as a model system. Electronic properties of these surfaces are tuned
by changing the S coverage, while keeping a high-quality interface, free of
defects and with a constant doping density. In contrast to earlier studies we
show that the Poisson-Schr\"{o}dinger scheme predicts the 2DEG bands energies
correctly but it is indispensable to take into account the existence of the
physical surface. The surface substantially influences the band energies beyond
simple electrostatics, by setting nontrivial boundary conditions for 2DEG
wavefunctions.Comment: 9 pages, 7 figures, 2 table
CH^+(1–0) and ^(13)CH^+(1–0) absorption lines in the direction of massive star-forming regions
We report the detection of the ground-state rotational transition of the methylidyne cation CH^+ and its isotopologue ^(13)CH^+ toward the remote
massive star-forming regions W33A, W49N, and W51 with the HIFI instrument onboard the Herschel satellite. Both lines are seen only in
absorption against the dust continuum emission of the star-forming regions. The CH^+ absorption is saturated over almost the entire velocity
ranges sampled by the lines-of-sight that include gas associated with the star-forming regions (SFR) and Galactic foreground material. The CH^+
column densities are inferred from the optically thin components. A lower limit of the isotopic ratio [^(12)CH^+]/[^(13)CH^+] > 35.5 is derived from
the absorptions of foreground material toward W49N. The column density ratio, N(CH^+)/N(HCO^+), is found to vary by at least a factor 10,
between 4 and >40, in the Galactic foreground material. Line-of-sight ^(12)CH^+ average abundances relative to total hydrogen are estimated. Their
average value, N(CH^+)/N_H > 2.6 × 10^(−8), is higher than that observed in the solar neighborhood and confirms the high abundances of CH^+ in
the Galactic interstellar medium. We compare this result to the predictions of turbulent dissipation regions (TDR) models and find that these high
abundances can be reproduced for the inner Galaxy conditions. It is remarkable that the range of predicted N(CH^+)/N(HCO^+) ratios, from 1 to
~50, is comparable to that observed
Risk-based Surveillance of Antimicrobial Residues – Identification of Potential Risk Factors
Today in Denmark, residue surveillance is conducted at random. However, if the surveillance is targeted to high-risk finisher pig herds, then the number of samples can be reduced whereby resources can be saved without jeopardizing public health. An analysis of Danish data covering 2.5 years has shown that finisher pig herds with a very high prevalence of chronic pleuritis have a higher risk of residue findings in the surveillance programme. If this is incorporated into the own control, then cost savings of 25% can be obtained. However, as shown in the Danish data there are other reasons for the presence of residues such as miscommunication and insufficient marking of treated animals. Such cases should be prevented through information campaigns addressed to the farmers and their advisors
Compact Q-balls and Q-shells in a scalar electrodynamics
We investigate spherically symmetric non topological solitons in
electrodynamics with a scalar field self interaction U ~|\psi| taken from the
complex signum-Gordon model. We find Q-balls for small absolute values of the
total electric charge Q, and Q-shells when |Q| is large enough. In both cases
the charge density exactly vanishes outside certain compact region in the three
dimensional space. The dependence of the total energy E of small Q-balls on the
total electric charge has the form E ~ |Q|^(5/6), while in the case of very
large Q-shells E ~ |Q|^(7/6).Comment: 21 pages, 7 figure
Interstellar CH absorption in the diffuse interstellar medium along the sight-lines to G10.6–0.4 (W31C), W49N, and W51
We report the detection of the ground state N, J = 1, 3/2 → 1, 1/2 doublet of the methylidyne radical CH at ~532 GHz and ~536 GHz with
the Herschel/HIFI instrument along the sight-line to the massive star-forming regions G10.6–0.4 (W31C), W49N, and W51. While the molecular
cores associated with these massive star-forming regions show emission lines, clouds in the diffuse interstellar medium are detected in absorption
against the strong submillimeter background. The combination of hyperfine structure with emission and absorption results in complex profiles,
with overlap of the different hyperfine components. The opacities of most of the CH absorption features are linearly correlated with those of CCH,
CN, and HCO^+ in the same velocity intervals. In specific narrow velocity intervals, the opacities of CN and HCO^+ deviate from the mean trends,
giving rise to more opaque absorption features. We propose that CCH can be used as another tracer of the molecular gas in the absence of better
tracers, with [CCH]/[H_2] ~3.2 ± 1.1 × 10^(−8). The observed [CN]/[CH], [CCH]/[CH] abundance ratios suggest that the bulk of the diffuse matter
along the lines of sight has gas densities n_H = n(H) + 2n(H_2) ranging between 100 and 1000 cm^(−3)
Herschel/HIFI discovery of interstellar chloronium (H_2Cl^+)
We report the first detection of chloronium, H_2Cl^+, in the interstellar medium, using the HIFI instrument aboard the Herschel Space Observatory.
The 2_(12)−1_(01) lines of ortho-H^(35)_2 Cl^+ and ortho-H^(37)_2 Cl^+ are detected in absorption towards NGC 6334I, and the 1_(11)−0_(00) transition of para-H^(35)_2 Cl^+ is
detected in absorption towards NGC 6334I and Sgr B2(S). The H_2Cl^+ column densities are compared to those of the chemically-related species
HCl. The derived HCl/H_2Cl^+ column density ratios, ~1–10, are within the range predicted by models of diffuse and dense photon dominated
regions (PDRs). However, the observed H_2Cl^+ column densities, in excess of 10^(13) cm^(−2), are significantly higher than the model predictions. Our
observations demonstrate the outstanding spectroscopic capabilities of HIFI for detecting new interstellar molecules and providing key constraints
for astrochemical models
Excitation and abundance of C_3 in star forming cores: Herschel/HIFI observations of the sight-lines to W31C and W49N
We present spectrally resolved observations of triatomic carbon (C_3) in several ro-vibrational transitions between the vibrational ground state and
the low-energy ν_2 bending mode at frequencies between 1654−1897 GHz along the sight-lines to the submillimeter continuum sources W31C
and W49N, using Herschel’s HIFI instrument. We detect C_3 in absorption arising from the warm envelope surrounding the hot core, as indicated
by the velocity peak position and shape of the line profile. The sensitivity does not allow to detect C_3 absorption due to diffuse foreground clouds.
From the column densities of the rotational levels in the vibrational ground state probed by the absorption we derive a rotation temperature (T_(rot))
of ~50−70 K, which is a good measure of the kinetic temperature of the absorbing gas, as radiative transitions within the vibrational ground state
are forbidden. It is also in good agreement with the dust temperatures for W31C and W49N. Applying the partition function correction based on
the derived T_(rot), we get column densities N(C_3) ~ 7−9 × 10^(14) cm^(−2) and abundance x(C_3) ~ 10^(−8) with respect to H_2. For W31C, using a radiative
transfer model including far-infrared pumping by the dust continuum and a temperature gradient within the source along the line of sight we find
that a model with x(C_3) = 10^(−8), T_(kin) = 30−50 K, N(C_3) = 1.5 × 10^(15) cm^(−2) fits the observations reasonably well and provides parameters in very
good agreement with the simple excitation analysis
Detection of interstellar oxidaniumyl: Abundant H_2O^+ towards the star-forming regions DR21, Sgr B2, and NGC6334
Aims. We identify a prominent absorption feature at 1115 GHz, detected in first HIFI spectra towards high-mass star-forming regions, and interpret its astrophysical origin.
Methods. The characteristic hyperfine pattern of the H_2O^+ ground-state rotational transition, and the lack of other known low-energy transitions in this frequency range, identifies the feature as H_2O^+ absorption against the dust continuum background and allows us to derive the velocity profile of the absorbing gas. By comparing this velocity profile with velocity profiles of other tracers in the DR21 star-forming region, we constrain the frequency of the transition and the conditions for its formation.
Results. In DR21, the velocity distribution of H_2O^+ matches that of the [C_(II)] line at 158 μm and of OH cm-wave absorption, both stemming from the hot and dense clump surfaces facing the H_(II)-region and dynamically affected by the blister outflow. Diffuse foreground gas dominates the absorption towards Sgr B2. The integrated intensity of the absorption line allows us to derive lower limits to the H_2O^+ column density of 7.2 × 10^(12) cm^(−2) in NGC 6334, 2.3 × 10^(13) cm^(−2) in DR21, and 1.1 × 10^(15) cm^(−2) in Sgr B2
- …