1,964 research outputs found
Anderson localisation in tight-binding models with flat bands
We consider the effect of weak disorder on eigenstates in a special class of
tight-binding models. Models in this class have short-range hopping on periodic
lattices; their defining feature is that the clean systems have some energy
bands that are dispersionless throughout the Brillouin zone. We show that
states derived from these flat bands are generically critical in the presence
of weak disorder, being neither Anderson localised nor spatially extended.
Further, we establish a mapping between this localisation problem and the one
of resonances in random impedance networks, which previous work has suggested
are also critical. Our conclusions are illustrated using numerical results for
a two-dimensional lattice, known as the square lattice with crossings or the
planar pyrochlore lattice.Comment: 5 pages, 3 figures, as published (this version includes minor
corrections
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Measles vaccination and antibody response in autism spectrum disorder
OBJECTIVE: To test the hypothesis that measles vaccination was involved in the pathogenesis of autism spectrum disorders (ASD) as evidenced by signs of a persistent measles infection or abnormally persistent immune response shown by circulating measles virus or raised antibody titres in children with ASD who had been vaccinated against measles, mumps and rubella (MMR) compared with controls. DESIGN: Case-control study, community based. METHODS: A community sample of vaccinated children aged 10-12 years in the UK with ASD (n = 98) and two control groups of similar age, one with special educational needs but no ASD (n = 52) and one typically developing group (n = 90), were tested for measles virus and antibody response to measles in the serum. RESULTS: No difference was found between cases and controls for measles antibody response. There was no dose-response relationship between autism symptoms and antibody concentrations. Measles virus nucleic acid was amplified by reverse transcriptase-PCR in peripheral blood mononuclear cells from one patient with autism and two typically developing children. There was no evidence of a differential response to measles virus or the measles component of the MMR in children with ASD, with or without regression, and controls who had either one or two doses of MMR. Only one child from the control group had clinical symptoms of possible enterocolitis. CONCLUSION: No association between measles vaccination and ASD was shown
The strong coupling constant from lattice QCD with N_f=2 dynamical quarks
We compute for two flavors of light dynamical quarks
using non-perturbatively improved Wilson fermions. We improve on a
recent calculation by employing Pad\'e-improved two-loop and three-loop
perturbation theory to convert the lattice numbers to the scheme.Comment: Contribution to Lattice 2001 (matrix elements), typo correcte
Deployment status of the Las Cumbres Observatory Global Telescope
Our global network of telescopes is designed to provide maximally available optical monitoring of time variable sources, from solar system to extra-galactic objects, and ranging in brightness from about 7-20m. We are providing a distributed network with varied apertures but homogeneous instrumentation: optical imaging, with spectroscopic capabilities. A key component is a single centralized process that accepts (in real time) and schedules TAC approved observing requests across the network; then continuously updates schedules based on status, weather and other availability criteria. Requests range from occasional to continuous monitoring, at slow to high-speed cadences (imaging and fast photometry), and includes rapid response to targets of opportunity. Each node of the network must be fully autonomous, with software agents to control and monitor all functions, to provide auto-recovery as necessary, and to announce their status and capabilities up the control structure. Real-time monitoring or interaction by humans should be infrequent. Equipment is designed to be reliable over long periods to minimize hands-on maintenance, by local or LCOGT staff. Our first 1m deployment was to McDonald Obs. in April 2012. Eight more 1m telescopes are close to deployment to complete the Southern ring, scheduled by end-2012
On the secondary star of the cataclysmic variable 1RXS J094432.1+035738
We present V and Rc band photometry and optical near-infrared spectroscopy of
the cataclysmic variable 1RXS J094432.1+035738. We detected features of a cool
secondary star, which can be modeled with a red dwarf of spectral type M2 (+0.5
-1.0) V at a distance of 433 +- 100 pc.Comment: Accepted for publication in Astronomy and Astrophysic
Fermion zero modes in N=2 supervortices
We study the fermionic zero modes of BPS semilocal magnetic vortices in N=2
supersymmetric QED with a Fayet-Iliopoulos term and two matter hypermultiplets
of opposite charge. There is a one-parameter family of vortices with
arbitrarily wide magnetic cores. Contrary to the situation in pure
Nielsen-Olesen vortices, new zero modes are found which get their masses from
Yukawa couplings to scalar fields that do not wind and are non-zero at the
core. We clarify the relation between fermion mass and zero modes. The new zero
modes have opposite chiralities and therefore do not affect the net counting
(left minus right) of zero modes coming from index theorems but manage to evade
other index theorems in the literature that count the total number (left plus
right) of zero modes in simpler systems.Comment: 9 pages, 1 figure. Uses Revtex4. Revised version includes discussion
about the back-reaction of the fermions on the background vortex. Version to
be published in Phys. Rev.
SDSS Absolute Magnitudes for Thin Disc Stars based on Trigonometric Parallaxes
We present a new luminosity-colour relation based on trigonometric parallaxes
for thin disc main-sequence stars in SDSS photometry. We matched stars from the
newly reduced Hipparcos catalogue with the ones taken from 2MASS All-Sky
Catalogue of Point Sources, and applied a series of constraints, i.e. relative
parallax errors (), metallicity
( dex), age ( Gyr) and surface gravity
(), and obtained a sample of thin disc main-sequence stars. Then, we
used our previous transformation equations (Bilir et al. 2008a) between SDSS
and 2MASS photometries and calibrated the absolute magnitudes to the
and colours. The transformation formulae between 2MASS
and SDSS photometries along with the absolute magnitude calibration provide
space densities for bright stars which saturate the SDSS magnitudes.Comment: 7 pages, including 7 figures and 2 tables, accepted for publication
in MNRA
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