16,991 research outputs found
Comment on "Photoluminescence ring formation in coupled quantum wells: Excitonic versus ambipolar diffusion"
This is a comment on M. Stern, V. Garmider, E. Segre, M. Rappaport, V.
Umansky, Y. Levinson, and I. Bar-Joseph, Phys. Rev. Lett. 101, 257402 (2008)
The gravitational wave spectrum of non-axisymmetric, freely precessing neutron stars
Evidence for free precession has been observed in the radio signature of
several pulsars. Freely precessing pulsars radiate gravitationally at
frequencies near the rotation rate and twice the rotation rate, which for
rotation frequencies greater than Hz is in the LIGO band. In older
work, the gravitational wave spectrum of a precessing neutron star has been
evaluated to first order in a small precession angle. Here we calculate the
contributions to second order in the wobble angle, and we find that a new
spectral line emerges. We show that for reasonable wobble angles, the
second-order line may well be observable with the proposed advanced LIGO
detector for precessing neutron stars as far away as the galactic center.
Observation of the full second-order spectrum permits a direct measurement of
the star's wobble angle, oblateness, and deviation from axisymmetry, with the
potential to significantly increase our understanding of neutron star
structure.Comment: 22 pages, 1 figure. Minor changes in the text, typos correcte
Exciton Mott transition in Si Revealed by Terahertz Spectroscopy
Exciton Mott transition in Si is investigated by using terahertz time-domain
spectroscopy. The excitonic correlation as manifested by the 1s-2p resonance is
observed above the Mott density. The scattering rate of charge carriers is
prominently enhanced at the proximity of Mott density, which is attributed to
the non-vanishing exciton correlation in the metallic electron-hole plasma.
Concomitantly, the signature of plasmon-exciton coupling is observed in the
loss function spectra.Comment: 5 pages, 3 figure
Perioperative infection prophylaxis and risk factor impact in colon surgery
Background: A prospective observational study was undertaken in 2,481 patients undergoing elective colon resection in 114 German centers to identify optimal drug and dosing modalities and risk factors for postoperative infection. Methods: Patients were pair matched using six risk factors and divided into 672 pairs (ceftriaxone vs, other cephalosporins, group A) and 400 pairs (ceftriaxone vs. penicillins, group B). End points were local and systemic postoperative infection and cost effectiveness. Results: Local infection rates were 6.0 versus 6.5% (group A) and 4.0 versus 10.5% (group B); systemic infection rates in groups A and B were 4.9 versus 6.3% and 3.3 versus 10.5%, respectively. Ceftriaxone was more effective than penicillins overall (6.8 vs. 17.8%, p < 0.001). Length of postoperative hospital stay was 16.2 versus 16.9 days (group A) and 15.8 versus 17.6 days (group B). Of the six risk factors, age and concomitant disease were significant for systemic infection, and blood loss, rectum resection and immunosuppressive therapy were significant for local infection. Penicillin was a risk factor compared to ceftriaxone (p < 0.0001). Ceftriaxone saved Q160.7 versus other cephalosporins and O416.2 versus penicillins. Conclusion: Clinical and microbiological efficacy are responsible for the cost effectiveness of ceftriaxone for perioperative prophylaxis in colorectal surgery. Copyright (C) 2000 S. Karger AG, Basel
Direct measurement of shear-induced cross-correlations of Brownian motion
Shear-induced cross-correlations of particle fluctuations perpendicular and
along stream-lines are investigated experimentally and theoretically. Direct
measurements of the Brownian motion of micron-sized beads, held by optical
tweezers in a shear-flow cell, show a strong time-asymmetry in the
cross-correlation, which is caused by the non-normal amplification of
fluctuations. Complementary measurements on the single particle probability
distribution substantiate this behavior and both results are consistent with a
Langevin model. In addition, a shear-induced anti-correlation between
orthogonal random-displacements of two trapped and hydrodynamically interacting
particles is detected, having one or two extrema in time, depending on the
positions of the particles.Comment: 4 pages, 4 figure
Carrier-wave Rabi flopping signatures in high-order harmonic generation for alkali atoms
We present the first theoretical investigation of carrier-wave Rabi flopping
in real atoms by employing numerical simulations of high-order harmonic
generation (HHG) in alkali species. Given the short HHG cutoff, related to the
low saturation intensity, we concentrate on the features of the third harmonic
of sodium (Na) and potassium (K) atoms. For pulse areas of 2 and Na atoms,
a characteristic unique peak appears, which, after analyzing the ground state
population, we correlate with the conventional Rabi flopping. On the other
hand, for larger pulse areas, carrier-wave Rabi flopping occurs, and is
associated with a more complex structure in the third harmonic. These new
characteristics observed in K atoms indicate the breakdown of the area theorem,
as was already demonstrated under similar circumstances in narrow band gap
semiconductors
Analysis of the exciton-exciton interaction in semiconductor quantum wells
The exciton-exciton interaction is investigated for quasi-two-dimensional
quantum structures. A bosonization scheme is applied including the full spin
structure. For generating the effective interaction potentials, the
Hartree-Fock and Heitler-London approaches are improved by a full two-exciton
calculation which includes the van der Waals effect. With these potentials the
biexciton formation in bilayer systems is investigated. For coupled quantum
wells the two-body scattering matrix is calculated and employed to give a
modified relation between exciton density and blue shift. Such a relation is of
central importance for gauging exciton densities in experiments which pave the
way toward Bose-Einstein condensation of excitons
Optical exciton Aharonov-Bohm effect, persistent current, and magnetization in semiconductor nanorings of type I and II
The optical exciton Aharonov-Bohm effect, i. e. an oscillatory component in
the energy of optically active (bright) states, is investigated in nanorings.
It is shown that a small effective electron mass, strong confinement of the
electron, and high barrier for the hole, achieved e. g. by an InAs nanoring
embedded in an AlGaSb quantum well, are favorable for observing the optical
exciton Aharonov-Bohm effect. The second derivative of the exciton energy with
respect to the magnetic field is utilized to extract Aharonov-Bohm oscillations
even for the lowest bright state unambiguously. A connection between the
theories for infinitesimal narrow and finite width rings is established.
Furthermore, the magnetization is compared to the persistent current, which
oscillates periodically with the magnetic field and confirms thus the
non-trivial (connected) topology of the wave function in the nanoring.Comment: 12 pages, 11 figure
Searching for periodic sources with LIGO
We investigate the computational requirements for all-sky, all-frequency
searches for gravitational waves from spinning neutron stars, using archived
data from interferometric gravitational wave detectors such as LIGO. These
sources are expected to be weak, so the optimal strategy involves coherent
accumulaton of signal-to-noise using Fourier transforms of long stretches of
data (months to years). Earth-motion-induced Doppler shifts, and intrinsic
pulsar spindown, will reduce the narrow-band signal-to-noise by spreading power
across many frequency bins; therefore, it is necessary to correct for these
effects before performing the Fourier transform. The corrections can be
implemented by a parametrized model, in which one does a search over a discrete
set of parameter values. We define a metric on this parameter space, which can
be used to determine the optimal spacing between points in a search; the metric
is used to compute the number of independent parameter-space points Np that
must be searched, as a function of observation time T. The number Np(T) depends
on the maximum gravitational wave frequency and the minimum spindown age
tau=f/(df/dt) that the search can detect. The signal-to-noise ratio required,
in order to have 99% confidence of a detection, also depends on Np(T). We find
that for an all-sky, all-frequency search lasting T=10^7 s, this detection
threshhold is at a level of 4 to 5 times h(3/yr), where h(3/yr) is the
corresponding 99% confidence threshhold if one knows in advance the pulsar
position and spin period.Comment: 18 pages, LaTeX, 12 PostScript figures included using psfig.
Submitted to Phys. Rev.
- …