3,141 research outputs found
Alloying effects on the optical properties of GeSi nanocrystals from TDDFT and comparison with effective-medium theory
We present the optical spectra of GeSi alloy nanocrystals
calculated with time-dependent density-functional theory in the adiabatic
local-density ap proximation (TDLDA). The spectra change smoothly as a function
of the compositio n . On the Ge side of the composition range, the lowest
excitations at the ab sorption edge are almost pure Kohn-Sham
independent-particle HOMO-LUMO transitio ns, while for higher Si contents
strong mixing of transitions is found. Within T DLDA the first peak is slightly
higher in energy than in earlier independent-par ticle calculations. However,
the absorption onset and in particular its composit ion dependence is similar
to independent-particle results. Moreover, classical depolarization effects are
responsible for a very strong suppression of the abs orption intensity. We show
that they can be taken into account in a simpler way using Maxwell-Garnett
classical effective-medium theory. Emission spectra are in vestigated by
calculating the absorption of excited nanocrystals at their relaxe d geometry.
The structural contribution to the Stokes shift is about 0.5 eV. Th e
decomposition of the emission spectra in terms of independent-particle transit
ions is similar to what is found for absorption. For the emission, very weak
tra nsitions are found in Ge-rich clusters well below the strong absorption
onset.Comment: submitted to Phys. Rev.
Certification of Compiler Optimizations using Kleene Algebra with Tests
We use Kleene algebra with tests to verify a wide assortment of common compiler optimizations, including dead code elimination, common subexpression elimination, copy propagation, loop hoisting, induction variable elimination, instruction scheduling, algebraic simplification, loop unrolling, elimination of redundant instructions, array bounds check elimination, and introduction of sentinels. In each of these cases, we give a formal equational proof of the correctness of the optimizing transformation
Radiative Models of Sagittarius A* and M87 from Relativistic MHD Simulations
Ongoing millimeter VLBI observations with the Event Horizon Telescope allow
unprecedented study of the innermost portion of black hole accretion flows.
Interpreting the observations requires relativistic, time-dependent physical
modeling. We discuss the comparison of radiative transfer calculations from
general relativistic MHD simulations of Sagittarius A* and M87 with current and
future mm-VLBI observations. This comparison allows estimates of the viewing
geometry and physical conditions of the Sgr A* accretion flow. The viewing
geometry for M87 is already constrained from observations of its large-scale
jet, but, unlike Sgr A*, there is no consensus for its millimeter emission
geometry or electron population. Despite this uncertainty, as long as the
emission region is compact, robust predictions for the size of its jet
launching region can be made. For both sources, the black hole shadow may be
detected with future observations including ALMA and/or the LMT, which would
constitute the first direct evidence for a black hole event horizon.Comment: 8 pages, 2 figures, submitted to the proceedings of AHAR 2011: The
Central Kiloparse
Malignant myelomonocytic cells after in vitro infection of marrow cells with Friend leukaemia virus.
Infection of long-term BDF1 marrow cultures with Friend leukaemia virus complex (FLV) induced transformed cells with myelomonocytic characteristics, which were isolated only 14 days after the viral infection. Criteria for transformation were growth in suspension cultures and high plating efficiency in agar. The lymphatic leukaemia virus (LLV) replicates in these suspension cultures, but the spleen focus-forming virus (SFFV) component of the FLV complex has not been detected. Injection of the transformed cells into syngeneic neonatal or adult mice leads to the development of leukaemia which can be demonstrated to be of donor origin by the presence of two metacentric marker chromosomes which are also seen in the cultured cells
Infrared interferometry to spatially and spectrally resolve jets in X-ray binaries
Infrared interferometry is a new frontier for precision ground based
observing, with new instrumentation achieving milliarcsecond (mas) spatial
resolutions for faint sources, along with astrometry on the order of 10
microarcseconds. This technique has already led to breakthroughs in the
observations of the supermassive black hole at the Galactic centre and its
orbiting stars, AGN, and exo-planets, and can be employed for studying X-ray
binaries (XRBs), microquasars in particular. Beyond constraining the orbital
parameters of the system using the centroid wobble and spatially resolving jet
discrete ejections on mas scales, we also propose a novel method to discern
between the various components contributing to the infrared bands: accretion
disk, jets and companion star. We demonstrate that the GRAVITY instrument on
the Very Large Telescope Interferometer (VLTI) should be able to detect a
centroid shift in a number of sources, opening a new avenue of exploration for
the myriad of transients expected to be discovered in the coming decade of
radio all-sky surveys. We also present the first proof-of-concept GRAVITY
observation of a low-mass X-ray binary transient, MAXI J1820+070, to search for
extended jets on mas scales. We place the tightest constraints yet via direct
imaging on the size of the infrared emitting region of the compact jet in a
hard state XRB.Comment: 12 Pages, 3 figures, accepted for publication in MNRA
Locating the intense interstellar scattering towards the inner Galaxy
We use VLBA+VLA observations to measure the sizes of the scatter-broadened
images of 6 of the most heavily scattered known pulsars: 3 within the Galactic
Centre (GC) and 3 elsewhere in the inner Galactic plane. By combining the
measured sizes with temporal pulse broadening data from the literature and
using the thin-screen approximation, we locate the scattering medium along the
line of sight to these 6 pulsars. At least two scattering screens are needed to
explain the observations of the GC sample. We show that the screen inferred by
previous observations of SGR J1745-2900 and Sgr A*, which must be located far
from the GC, falls off in strength on scales < 0.2 degree. A second scattering
component closer to (< 2 kpc) or even (tentatively) within (< 700 pc) the GC
produces most or all of the temporal broadening observed in the other GC
pulsars. Outside the GC, the scattering locations for all three pulsars are ~2
kpc from Earth, consistent with the distance of the Carina-Sagittarius or
Scutum spiral arm. For each object the 3D scattering origin coincides with a
known HII region (and in one case also a supernova remnant), suggesting that
such objects preferentially cause the intense interstellar scattering seen
towards the Galactic plane. We show that the HII regions should contribute >
25% of the total dispersion measure (DM) towards these pulsars, and calculate
reduced DM distances. Those distances for other pulsars lying behind HII
regions may be similarly overestimated.Comment: 16 pages, 10 figures, MNRAS, in pres
Coherent control of indirect excitonic qubits in optically driven quantum dot molecules
We propose an optoelectronic scheme to define and manipulate an indirect
neutral exciton qubit within a quantum dot molecule. We demonstrate coherent
dynamics of indirect excitons resilient against decoherence effects, including
direct exciton spontaneous recombination. For molecules with large interdot
separation, the exciton dressed spectrum yields an often overlooked avoided
crossing between spatially indirect exciton states. Effective two level system
Hamiltonians are extracted by Feshbach projection over the multilevel exciton
configurations. An adiabatic manipulation of the qubit states is devised using
time dependent electric field sweeps. The exciton dynamics yields the necessary
conditions for qubit initialization and near unitary rotations in the
picosecond time scale, driven by the system internal dynamics. Despite the
strong influence of laser excitation, charge tunneling, and interdot
dipole-dipole interactions, the effective relaxation time of indirect excitons
is much longer than the direct exciton spontaneous recombination time,
rendering indirect excitons as potential elemental qubits in more complex
schemes.Comment: Submitted to PRB, 11 pages and 6 figure
Mucosal biomarkers of colorectal cancer risk do not increase at 6 months following sleeve gastrectomy, unlike gastric bypass
YesObjective
The hypothesis that sleeve gastrectomy (SG) is not associated with an increase in mucosal colorectal cancer (CRC) biomarkers, unlike Roux-en-Y gastric bypass (RYGB), was tested.
Design and Methods
Rectal mucosa, blood, and urine were obtained from morbidly obese patients (n = 23) before and after (median 28 months) SG, as well as from nonobese controls (n = 20). Rectal epithelial cell mitosis and apoptosis, crypt size/fission, and pro-inflammatory gene expression were measured, as well as systemic inflammatory biomarkers, including C-reactive protein (CRP).
Results
The mean pre-operative body mass index in SG patients was 65.7 kg/m2 (24.7 kg/m2 in controls). Mean excess weight loss post-SG was 38.2%. There was a significant increase in mitosis frequency, crypt size, and crypt fission (all P < 0.01) in SG patients versus controls, as well as evidence of a chronic inflammatory state (raised CRP and mononuclear cell p65 NFκB binding), but there was no significant change in these biomarkers after SG, except CRP reduction. Macrophage migration inhibitory factor mRNA levels were increased by 39% post-SG (P = 0.038).
Conclusions
Mucosal biomarkers of CRC risk do not increase at 6 months following SG, unlike RYGB. Biomarkers of rectal crypt proliferation and systemic inflammation are increased in morbidly obese patients compared with controls
Decision Making in Mice During an Optimized Touchscreen Spatial Working Memory Task Sensitive to Medial Prefrontal Cortex Inactivation and NMDA Receptor Hypofunction
Working memory is a fundamental cognitive process for decision-making and is a hallmark impairment in a variety of neuropsychiatric and neurodegenerative diseases. Spatial working memory paradigms are a valuable tool to assess these processes in rodents and dissect the neurobiology underlying working memory. The trial unique non-match to location (TUNL) task is an automated touchscreen paradigm used to study spatial working memory and pattern separation processes in rodents. Here, animals must remember the spatial location of a stimulus presented on the screen over a delay period; and use this representation to respond to the novel location when the two are presented together. Because stimuli can be presented in a variety of spatial configurations, TUNL offers a trial-unique paradigm, which can aid in combating the development of unwanted mediating strategies. Here, we have optimized the TUNL protocol for mice to reduce training time and further reduce the potential development of mediating strategies. As a result, mice are able to accurately perform an enhanced trial-unique paradigm, where the locations of the sample and choice stimuli can be presented in any configuration on the screen during a single session. We also aimed to pharmacologically characterize this updated protocol, by assessing the roles of the medial prefrontal cortex (mPFC) and N-methyl-D-aspartate (NMDA) receptor (NMDAr) functioning during TUNL. Temporary inactivation of the medial prefrontal cortex (mPFC) was accomplished by directly infusing a mixture of GABA agonists muscimol and baclofen into the mPFC. We found that mPFC inactivation significantly impaired TUNL performance in a delay-dependent manner. In addition, mPFC inactivation significantly increased the susceptibility of mice to proactive interference. Mice were then challenged with acute systemic injections of the NMDAr antagonist ketamine, which resulted in a dose-dependent, delay-dependent working memory impairment. Together, we describe an optimized automated touchscreen task of working memory, which is dependent on the intact functioning of the mPFC and sensitive to acute NMDAr hypofunction. With the vast genetic toolbox available for modeling disease and probing neural circuit functioning in mice, the TUNL task offers a valuable paradigm to pair with these technologies to further investigate the processes underlying spatial working memory
- …