2,285 research outputs found
Correlated defects, metal-insulator transition, and magnetic order in ferromagnetic semiconductors
The effect of disorder on transport and magnetization in ferromagnetic III-V
semiconductors, in particular (Ga,Mn)As, is studied theoretically. We show that
Coulomb-induced correlations of the defect positions are crucial for the
transport and magnetic properties of these highly compensated materials. We
employ Monte Carlo simulations to obtain the correlated defect distributions.
Exact diagonalization gives reasonable results for the spectrum of valence-band
holes and the metal-insulator transition only for correlated disorder. Finally,
we show that the mean-field magnetization also depends crucially on defect
correlations.Comment: 4 pages RevTeX4, 5 figures include
The Pacific Drought Knowledge Exchange: A Co-Production Approach to Deliver Climate Resources to User Groups
Drought is a growing threat to hydrological, ecological, agricultural, and socio-cultural systems of the tropics, especially tropical islands of the Pacific where severe droughts can compromise food and water security. Overcoming barriers to knowledge sharing between land managers and researchers is a critical cross-sector strategy for engaging and mitigating or adapting to drought. Here we describe the establishment and functioning of the Pacific Drought Knowledge Exchange (PDKE), which provides users with easier access to: (1) sector- and geography-specific climate information; (2) better and more comprehensive information; (3) improved technical assistance; and (4) a more collaborative information-transfer environment through participation in knowledge co-production. We focus on our collaborative work with managers of important tropical dryland ecosystems from three distinct geographies to pilot the collaborative development of climate change, climate variability, and drought “portfolios” featuring site-specific historical and forecasted future information. This information was then used to collaboratively produce factsheets that partners used to: (i) better understand past and projected climate for specific management units; (ii) integrate new climate knowledge into management planning; and (iii) support climate-focused educational and outreach efforts. This pilot effort demonstrates the successful application of climate-focused co-production in dry tropical landscapes
Constraints on Kerr-Newman black holes from merger-ringdown gravitational-wave observations
We construct a template to model the post-merger phase of a binary black hole
coalescence in the presence of a remnant charge. We include the
quasi-normal modes typically dominant during a binary black hole coalescence,
and also present analytical fits for the
quasinormal mode frequencies of a Kerr-Newman black hole in terms of its spin
and charge, here also including the mode. Aside from astrophysical
electric charge, our template can accommodate extensions of the Standard Model,
such as a dark photon. Applying the model to LIGO-Virgo detections, we find
that we are unable to distinguish between the charged and uncharged hypotheses
from a purely post-merger analysis of the current events. However, restricting
the mass and spin to values compatible with the analysis of the full signal, we
obtain a 90th percentile bound on the black hole
charge-to-mass ratio, for the most favorable case of GW150914. Under similar
assumptions, by simulating a typical loud signal observed by the LIGO-Virgo
network at its design sensitivity, we assess that this model can provide a
robust measurement of the charge-to-mass ratio only for values ; here we also assume that the mode amplitudes are similar to the uncharged
case in creating our simulated signal. Lower values, down to , could instead be detected when evaluating the consistency of the
pre-merger and post-merger emission.Comment: 21 pages, 11 figures, 4 tables. Matches published versio
Transcription-associated breaks in Xeroderma Pigmentosum group D cells from patients with combined features of Xeroderma Pigmentosum and Cockayne Syndrome
Defects in the XPD gene can result in several clinical phenotypes, including xeroderma pigmentosum (XP), trichothiodystrophy, and, less frequently, the combined phenotype of XP and Cockayne syndrome (XP-D/CS). We previously showed that in cells from two XP-D/CS patients, breaks were introduced into cellular DNA on exposure to UV damage, but these breaks were not at the sites of the damage. In the present work, we show that three further XP-D/CS patients show the same peculiar breakage phenomenon. We show that these breaks can be visualized inside the cells by immunofluorescence using antibodies to either gamma-H2AX or poly-ADP-ribose and that they can be generated by the introduction of plasmids harboring methylation or oxidative damage as well as by UV photoproducts. Inhibition of RNA polymerase II transcription by four different inhibitors dramatically reduced the number of UV-induced breaks. Furthermore, the breaks were dependent on the nucleotide excision repair (NER) machinery. These data are consistent with our hypothesis that the NER machinery introduces the breaks at sites of transcription initiation. During transcription in UV-irradiated XP-D/CS cells, phosphorylation of the carboxy-terminal domain of RNA polymerase II occurred normally, but the elongating form of the polymerase remained blocked at lesions and was eventually degraded
Performance of a BGO PET/CT with Higher Resolution PET Detectors
A new PET detector block has been designed to replace the standard detector of the Discovery ST PET/CT system. The new detector block is the same size as the original, but consists of an 8/spl times/6 (tangential× axial) matrix of crystals rather than the original 6/spl times/6. The new crystal dimensions are 4.7× 6.3× 30 mm/sup 3/ (tangential× axial× radial). Full PET/CT systems have been built with these detectors (Discovery STE). Most other aspects of the system are identical to the standard Discovery ST, with differences including the low energy threshold for 3D imaging (now 425 keV) and front-end electronics. Initial performance evaluation has been done, including NEMA NU2-2001 tests and imaging of the 3D Hoffman brain phantom and a neck phantom with small lesions. The system sensitivity was 1.90 counts/s/kBq in 2D, and 9.35 counts/s/kBq in 3D. Scatter fractions measured for 2D and 3D, respectively, were 18.6% and 34.5%. In 2D, the peak NEC of 89.9 kcps occurred at 47.0 kBq/cc. In 3D, the peak NEC of 74.3 kcps occurred at 8.5 kBq/cc. Spatial resolution (all expressed in mm FWHM) measured in 2D for 1 cm off-axis source 5.06 transaxial, 5.14 axial and for 10 cm source 5.45 radial, 5.86 tangential, and 6.23 axial. In 3D for 1 cm off-axis source 5.13 transaxial, 5.74 axial, and for 10 cm source 5.92 radial, 5.54 tangential, and 6.16 axial. Images of the brain and neck phantom demonstrate some improvement, compared to measurements on a standard Discovery ST
Identification of Genes Associated with Water Restriction Expressed in the Renal Cortex and Hypothalamus in Cattle
The short-term objective of this study is to discover genes associated with water restriction expressed in the renal cortex and hypothalamus in beef calves. The long-term goal is to understand genes and pathways important for thirst response in cattle. This knowledge may lead to discovery of genetic variants associated with water intake. Identification of animals with genetic potential to grow and thrive under drought conditions would be an asset to beef producers and communities which rely on beef production for a large part of their livelihoods. Both selected tissue types are known to be involved in response to hypertonicity (e.g., water restriction or dehydration)
Interactions of Single-Nozzle Sonic Propulsive Deceleration Jets on Mars Entry Aeroshells
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/83612/1/AIAA-2010-4888-277.pd
Observation of isotonic symmetry for enhanced quadrupole collectivity in neutron-rich 62,64,66Fe isotopes at N=40
The transition rates for the 2_{1}^{+} states in 62,64,66Fe were studied
using the Recoil Distance Doppler-Shift technique applied to projectile Coulomb
excitation reactions. The deduced E2 strengths illustrate the enhanced
collectivity of the neutron-rich Fe isotopes up to N=40. The results are
interpreted by the generalized concept of valence proton symmetry which
describes the evolution of nuclear structure around N=40 as governed by the
number of valence protons with respect to Z~30. The deformation suggested by
the experimental data is reproduced by state-of-the-art shell calculations with
a new effective interaction developed for the fpgd valence space.Comment: 4 pages, 2 figure
Towards Spectral Classification of L and T Dwarfs: Infrared and Optical Spectroscopy and Analysis
We present 0.6-2.5um, R~400 spectra of twenty-seven cool, low luminosity
stars and substellar objects. Based on these and previously published spectra
we develop a preliminary spectral classification system for L and T dwarfs. For
late L and T types the classification system is based entirely on four spectral
indices in the 1-2.5um interval. Two of these indices are derived from water
absorption bands at 1.15um and 1.4um, the latter of which shows a smooth
increase in depth through the L and T sequences and can be used to classify
both spectral types. The other two indices make use of methane absorption
features in the H and K bands, with the K band index also applicable to mid to
late L dwarfs. Continuum indices shortward of 1um used by previous authors to
classify L dwarfs are found to be useful only through mid L subclasses. We
employ the 1.5um water index and the 2.2um methane index to complete the L
classification through L9.5 and to link the new system with a modified version
of the 2MASS ``Color-d'' index. By correlating the depths of the methane and
water absorption features, we establish a T spectral sequence from types T0 to
T8, based on all four indices, which is a smooth continuation of the L
sequence. We reclassify two 2MASS L8 dwarfs as L9 and L9.5 and identify one
SDSS object as L9. In the proposed system methane absorption appears in the K
band approximately at L8, two subclasses earlier than its appearance in the H
band. The L and T spectral classes are distinguished by the absence and
presence, respectively, of H band methane absorption.Comment: 40 pages, 14 figures, to be published in Ap.J., Jan 1, 200
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