46,091 research outputs found
Heisenberg exchange in magnetic monoxides
The superexchange intertacion in transition-metal oxides, proposed initially
by Anderson in 1950, is treated using contemporary tight-binding theory and
existing parameters. We find also a direct exchange for nearest-neighbor metal
ions, larger by a factor of order five than the superexchange. This direct
exchange arises from Vddm coupling, rather than overlap of atomic charge
densities, a small overlap exchange contribution which we also estimate. For
FeO and CoO there is also an important negative contribution, related to Stoner
ferromagnetism, from the partially filled minority-spin band which broadens
when ionic spins are aligned. The corresponding J1 and J2 parameters are
calculated for MnO, FeO, CoO, and NiO. They give good accounts of the Neel and
the Curie-Weiss temperatures, show appropriate trends, and give a reasonable
account of their volume dependences. For MnO the predicted value for the
magnetic susceptibility at the Neel temperature and the crystal distortion
arising from the antiferromagnetic transition were reasonably well given.
Application to CuO2 planes in the cuprates gives J=1220oK, compared to an
experimental 1500oK, and for LiCrO2 gives J1=4 50oK compared to an experimental
230oK.Comment: 21 pages, 1 figure, submitted to Phys. Rev. B 1/19/07. Realized
J=4V^2/U applies generally, as opposed to J=2V^2/U from one-electron theory
(1/28 revision
Gamma-Ray Bursts and the Cosmic Star Formation Rate
We have tested several models of GRB luminosity and redshift distribution
functions for compatibility with the BATSE 4B number versus peak flux relation.
Our results disagree with recent claims that current GRB observations can be
used to strongly constrain the cosmic star formation history. Instead, we find
that relaxing the assumption that GRBs are standard candles renders a very
broad range of models consistent with the BATSE number-flux relation. We
explicitly construct two sample distributions, one tracing the star formation
history and one with a constant comoving density. We show that both
distributions are compatible with the observed fluxes and redshifts of the
bursts GRB970508, GRB971214, and GRB980703, and we discuss the measurements
required to distinguish the two models.Comment: 12 pages, 2 postscript figures, uses AAS LaTex macros v4.0. To be
published in Astrophysical Journal Letters, accepted August 20, 1998. Revised
for publicatio
Development of a Miniature Electrostatic Accelerometer /MESA/ for low g applications Summary report
Design, fabrication, and testing of miniature digital electrostatic accelerometer for low gravity measurements in spac
Learning versus stealing : how Important are market-share -- reallocations to India's productivity growth?
Recent trade theory emphasizes the role of market-share reallocations across firms ("stealing") in driving productivity growth, while the older literature focused on average productivity improvements ("learning"). The authors use comprehensive, firm-level data from India's organized manufacturing sector to show that market-share reallocations did play an important role in aggregate productivity gains immediately following the start of India's trade reforms in 1991. However, aggregate productivity gains during the overall period from 1985 to 2004 were driven largely by improvements in average productivity, which can be attributed to India's trade liberalization and FDI reforms.Economic Theory&Research,Industrial Management,E-Business,Labor Policies,Debt Markets
Effect of strain on the orbital and magnetic ordering of manganite thin films and their interface with an insulator
We study the effect of uniform uniaxial strain on the ground state electronic
configuration of a thin film manganite. Our model Hamiltonian includes the
double-exchange, the Jahn-Teller electron-lattice coupling, and the
antiferromagnetic superexchange. The strain arises due to the lattice mismatch
between an insulating substrate and a manganite which produces a tetragonal
distortion. This is included in the model via a modification of the hopping
amplitude and the introduction of an energy splitting between the Mn e_g
levels. We analyze the bulk properties of half-doped manganites and the
electronic reconstruction at the interface between a ferromagnetic and metallic
manganite and the insulating substrate. The strain drives an orbital selection
modifying the electronic properties and the magnetic ordering of manganites and
their interfaces.Comment: 8 pages, 8 figure
Spatial Relationship between Solar Flares and Coronal Mass Ejections
We report on the spatial relationship between solar flares and coronal mass
ejections (CMEs) observed during 1996-2005 inclusive. We identified 496
flare-CME pairs considering limb flares (distance from central meridian > 45
deg) with soft X-ray flare size > C3 level. The CMEs were detected by the Large
Angle and Spectrometric Coronagraph (LASCO) on board the Solar and Heliospheric
Observatory (SOHO). We investigated the flare positions with respect to the CME
span for the events with X-class, M-class, and C-class flares separately. It is
found that the most frequent flare site is at the center of the CME span for
all the three classes, but that frequency is different for the different
classes. Many X-class flares often lie at the center of the associated CME,
while C-class flares widely spread to the outside of the CME span. The former
is different from previous studies, which concluded that no preferred flare
site exists. We compared our result with the previous studies and conclude that
the long-term LASCO observation enabled us to obtain the detailed spatial
relation between flares and CMEs. Our finding calls for a closer flare-CME
relationship and supports eruption models typified by the CSHKP magnetic
reconnection model.Comment: 7 pages; 4 figures; Accepted by the Astrophysical Journa
The NuSTAR View of the Seyfert 2 Galaxy NGC 4388
We present analysis of NuSTAR X-ray observations in the 3-79 keV energy band
of the Seyfert 2 galaxy NGC 4388, taken in 2013. The broadband sensitivity of
NuSTAR, covering the Fe K line and Compton reflection hump, enables
tight constraints to be placed on reflection features in AGN X-ray spectra,
thereby providing insight into the geometry of the circumnuclear material. In
this observation, we found the X-ray spectrum of NGC 4388 to be well described
by a moderately absorbed power law with non-relativistic reflection. We fit the
spectrum with phenomenological reflection models and a physical torus model,
and find the source to be absorbed by Compton-thin material (N cm) with a very weak Compton reflection hump
(R 0.09) and an exceptionally large Fe K line (EW eV) for a source with weak or no reflection. Calculations
using a thin-shell approximation for the expected Fe K EW indicate that
an Fe K line originating from Compton-thin material presents a possible
explanation.Comment: 5 pages, 2 figures. Accepted for publication in Ap
Conjugative transfer frequencies of mef(A)-containing Tn1207.3 to macrolide-susceptible Streptococcus pyogenes belonging to different emm types
The aim of this study was to examine the gene transfer potential of mef(A)-containing Tn120.3 to macrolide-susceptible Streptococcus pyogenes belonging to different emm types. Using the filter mating technique, Tn1207.3 was transferred by conjugation to 23 macrolide-susceptible recipients representing 11 emm types. PCR analysis confirmed the presence of the mef(A) gene and the comEC junction regions of the Tn1207.3 insertion in resultant transconjugants. Significant variation was found in the transfer frequency of Tn1207.3 to different Strep. pyogenes strains, and this phenomenon may contribute to the differences in mef(A) frequency observed among clinical isolates. Significance and Impact of the Study: The spread of antimicrobial resistance among pathogenic bacteria is an important problem, but the mechanisms of horizontal transfer between strains and species are often poorly understood. For instance, little is known on how macrolide resistance spreads between strains of the human pathogen Strep. pyogenes and why certain strains more commonly display resistance than others. Here, we show that Strep. pyogenes strains vary greatly in their ability to acquire a transposon encoding macrolide resistance by horizontal gene transfer in vitro. These data provide a novel insight into the transfer of antibiotic resistance between bacterial strains and offer an explanation for the differences in the frequency of resistance determinates and resistance seen among clinical isolates. © 2014 The Authors Letters in Applied Microbiology
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