2,491 research outputs found
Optical Response of Grating-Coupler-Induced Intersubband Resonances: The Role of Wood's Anomalies
Grating-coupler-induced collective intersubband transitions in a
quasi-two-dimensional electron system are investigated both experimentally and
theoretically. Far-infrared transmission experiments are performed on samples
containing a quasi-two-dimensional electron gas quantum-confined in a parabolic
quantum well. For rectangular shaped grating couplers of different periods we
observe a strong dependence of the transmission line shape and peak height on
the period of the grating, i.e. on the wave vector transfer from the diffracted
beams to the collective intersubband resonance. It is shown that the line shape
transforms with increasing grating period from a Lorentzian into a strongly
asymmetric line shape. Theoretically, we treat the problem by using the
transfer-matrix method of local optics and apply the modal-expansion method to
calculate the influence of the grating. The optically uniaxial
quasi-two-dimensional electron gas is described in the long-wavelength limit of
the random-phase approximation by a local dielectric tensor, which includes
size quantization effects. Our theory reproduces excellently the experimental
line shapes. The deformation of the transmission line shapes we explain by the
occurrence of both types of Wood's anomalies.Comment: 28 pages, 7 figures. Physical Review B , in pres
Hybridization of electron subbands in a double quantum well at quantizing magnetic field
We employ magnetocapacitance and far-infrared spectroscopy techniques to
study the spectrum of the double-layer electron system in a parabolic quantum
well with a narrow tunnel barrier in the centre. For gate-bias-controlled
asymmetric electron density distributions in this soft two-subband system we
observe both individual subband gaps and double layer gaps at integer filling
factor . The bilayer gaps are shown to be either trivial common for two
subbands or caused by hybridization of electron subbands in magnetic field. We
describe the observed hybrid gaps at and within a simple model
for the modified bilayer spectrum.Comment: REVTeX, 24 pages, 9 figures included. Submitted to Phys. Rev.
A brown dwarf companion to the intermediate-mass star HR6037
In the course of an imaging survey we have detected a visual companion to the
intermediate-mass star HR 6037. In this letter, we present two epoch
observations of the binary with NACO/VLT, and near-IR spectroscopy of the
secondary with ISAAC/VLT. The NACO observations allow us to confirm HR 6037B as
a co-moving companion. Its J and H band ISAAC spectra suggest the object has an
spectral type of M9+-1, with a surface gravity intermediate between that of 10
Myr dwarfs and field dwarfs with identical spectral type. The comparison of its
Ks-band photometry with evolutionary tracks allows us to derive a mass,
effective temperature, and surface gravity of 62+-20 MJup, Teff = 2330+-200 K,
and log g = 5.1+-0.2, respectively. The small mass ratio of the binary, -0.03,
and its long orbital period, -5000 yr, makes HR 6037 a rare and uncommon binary
system.Comment: (5 pages, 4 figures, accepted for publication in A&A Letters
Do we need to fix the anterior fracture component in insufficiency fractures of the pelvis? A biomechanical comparison on an FFP type IIIc fracture in an osteoporotic pelvic bone model.
There is a growing understanding of the specific characteristics of insufficiency fractures of the pelvis and of general requirements for the treatment of affected patients with focus on early mobilization and effective pain reduction as the main goals of therapy. While there is consensus on the significance of achieving stability of the dorsal pelvic ring structures there is still an open discussion about the potential benefits of additional stabilization of an anterior fracture component. Within a biomechanical test setup, two established methods of dorsal fracture fixation were tested under axial loading (25-1200 N; 1000 test cycles) on an explicit osteoporotic bone model (n = 32) with a standardized FFP type IIIc fracture with and without additional fixation of the anterior fracture component. Dorsal fixation was performed with and long and a short 7.3 mm cannulated screw in S1 in one group (n = 16), and a trans sacral bar with an additional short 7.3 mm cannulated screw in S1 in the other group (n = 16). Half of the samples received a 7.3 mm cannulated retrograde transpubic screw for anterior fixation. The fixation with the trans sacral bar and the additional anterior screw fixation showed the highest rate of stability (p = 0.0014), followed by the double SI-screw fixation with stabilization of the anterior fracture (p = 0.0002). During testing, we observed the occurrence of new sacral fractures contralateral to the initial fracture in 22/32 samples. The results let us assume that stabilization of an additional anterior fracture component relevantly improves the stability of the entire ring construct and might prevent failure of the dorsal stabilization or further fracture progression
Magnetic-Field-Induced Hybridization of Electron Subbands in a Coupled Double Quantum Well
We employ a magnetocapacitance technique to study the spectrum of the soft
two-subband (or double-layer) electron system in a parabolic quantum well with
a narrow tunnel barrier in the centre. In this system unbalanced by gate
depletion, at temperatures T\agt 30 mK we observe two sets of quantum
oscillations: one originates from the upper electron subband in the
closer-to-the-gate part of the well and the other indicates the existence of
common gaps in the spectrum at integer fillings. For the lowest filling factors
and , both the common gap presence down to the point of one- to
two-subband transition and their non-trivial magnetic field dependences point
to magnetic-field-induced hybridization of electron subbands.Comment: Major changes, added one more figure, the latest version to be
published in JETP Let
Measurement of Electron Trapping in the CESR Storage Ring
The buildup of low-energy electrons has been shown to affect the performance
of a wide variety of particle accelerators. Of particular concern is the
persistence of the cloud between beam bunch passages, which can impose
limitations on the stability of operation at high beam current. We have
obtained measurements of long-lived electron clouds trapped in the field of a
quadrupole magnet in a positron storage ring, with lifetimes much longer than
the revolution period. Based on modeling, we estimate that about 7% of the
electrons in the cloud generated by a 20-bunch train of 5.3 GeV positrons with
16-ns spacing and population survive longer than 2.3 s in a
quadrupole field of gradient 7.4 T/m. We have observed a non-monotonic
dependence of the trapping effect on the bunch spacing. The effect of a witness
bunch on the measured signal provides direct evidence for the existence of
trapped electrons. The witness bunch is also observed to clear the cloud,
demonstrating its effectiveness as a mitigation technique.Comment: 6 pages, 9 figures, 28 citation
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