9,971 research outputs found
BCS - BEC crossover at T=0: A Dynamical Mean Field Theory Approach
We study the T=0 crossover from the BCS superconductivity to Bose-Einstein
condensation in the attractive Hubbard Model within dynamical mean field
theory(DMFT) in order to examine the validity of Hartree-Fock-Bogoliubov (HFB)
mean field theory, usually used to describe this crossover, and to explore
physics beyond it. Quantum fluctuations are incorporated using iterated
perturbation theory as the DMFT impurity solver. We find that these
fluctuations lead to large quantitative effects in the intermediate coupling
regime leading to a reduction of both the superconducting order parameter and
the energy gap relative to the HFB results. A qualitative change is found in
the single-electron spectral function, which now shows incoherent spectral
weight for energies larger than three times the gap, in addition to the usual
Bogoliubov quasiparticle peaks.Comment: 11 pages,12 figures, Published versio
Single-particle dynamics of the Anderson model: a local moment approach
A non-perturbative local moment approach to single-particle dynamics of the
general asymmetric Anderson impurity model is developed. The approach
encompasses all energy scales and interaction strengths. It captures thereby
strong coupling Kondo behaviour, including the resultant universal scaling
behaviour of the single-particle spectrum; as well as the mixed valent and
essentially perturbative empty orbital regimes. The underlying approach is
physically transparent and innately simple, and as such is capable of practical
extension to lattice-based models within the framework of dynamical mean-field
theory.Comment: 26 pages, 9 figure
Cloud cover typing from environmental satellite imagery. Discriminating cloud structure with Fast Fourier Transforms (FFT)
The use of two dimensional Fast Fourier Transforms (FFTs) subjected to pattern recognition technology for the identification and classification of low altitude stratus cloud structure from Geostationary Operational Environmental Satellite (GOES) imagery was examined. The development of a scene independent pattern recognition methodology, unconstrained by conventional cloud morphological classifications was emphasized. A technique for extracting cloud shape, direction, and size attributes from GOES visual imagery was developed. These attributes were combined with two statistical attributes (cloud mean brightness, cloud standard deviation), and interrogated using unsupervised clustering amd maximum likelihood classification techniques. Results indicate that: (1) the key cloud discrimination attributes are mean brightness, direction, shape, and minimum size; (2) cloud structure can be differentiated at given pixel scales; (3) cloud type may be identifiable at coarser scales; (4) there are positive indications of scene independence which would permit development of a cloud signature bank; (5) edge enhancement of GOES imagery does not appreciably improve cloud classification over the use of raw data; and (6) the GOES imagery must be apodized before generation of FFTs
Method of preparing zinc orthotitanate pigment
Zinc orthotitanate suitable for use as a pigment for spacecraft thermal control coatings is prepared by heating a slightly zinc deficient reaction mixture of precipitated oxalates of zinc and titanium. The reaction mixture can be formed by coprecipitation of zinc and titanium oxalates from chloride solution or by mixing separately precipitated oxalates. The mixture is first heated to 400 to 600 C to remove volatiles and is then rapidly heated at 900 to 1200 C. Zinc orthotitanate produced by this method exhibits the very fine particle size needed for thermal control coatings as well as stability in a space environment
Can the 126 GeV boson be a pseudoscalar?
We test the possibility that the newly-discovered 126 GeV boson is a
pseudoscalar by examining the correlations among the loop-induced pseudoscalar
decay branching fractions to , , , and
final states in a model-independent way. These four decays are controlled by
only two effective operators, so that the rates in and are
predicted now that the rates in and
have been measured. We find that the pseudoscalar possibility is disfavored but
not conclusively excluded. Experimental exclusion of the decay to
well below or conclusive observation of the
decay near the Standard Model rate would eliminate the pseudoscalar
possibility. The exclusion should be possible using existing data.
The only loophole in our argument is the possibility that the signal
comes from pseudoscalar decays to a pair of new neutral gauge bosons with mass
near the pole.Comment: 8 pages, 2 figures, v2: references added, Fig. 1 improved,v3: minor
error in numbers on last paragraph of pg5 fixe
Local quantum phase transition in the pseudogap Anderson model: scales, scaling and quantum critical dynamics
The pseudogap Anderson impurity model provides a paradigm for understanding
local quantum phase transitions, in this case between generalised fermi liquid
and degenerate local moment phases. Here we develop a non-perturbative local
moment approach to the generic asymmetric model, encompassing all energy scales
and interaction strengths and leading thereby to a rich description of the
problem. We investigate in particular underlying phase boundaries, the critical
behaviour of relevant low-energy scales, and single-particle dynamics embodied
in the local spectrum. Particular attention is given to the resultant universal
scaling behaviour of dynamics close to the transition in both the GFL and LM
phases, the scale-free physics characteristic of the quantum critical point
itself, and the relation between the two.Comment: 39 pages, 19 figure
Measuring the electrical impedance of mouse brain tissue
We report on an experimental method to measure conductivity of cortical tissue. We use a pair of 5mm diameter Ag/AgCl electrodes in a Perspex sandwich device that can be brought to a distance of 400 microns apart. The apparatus is brought to uniform temperature before use. Electrical impedance of a sample is measured across the frequency range 20 Hz-2.0 MHz with an Agilent 4980A four-point impedance monitor in a shielded room. The equipment has been used to measure the conductivity of mature mouse brain cortex in vitro. Slices 400 microns in thickness are prepared on a vibratome. Slices are bathed in artificial cerebrospinal fluid (ACSF) to keep them alive. Slices are removed from the ACSF and sections of cortical tissue approximately 2 mm times 2 mm are cut with a razor blade. The sections are photographed through a calibrated microscope to allow identification of their cross-sectional areas. Excess ACSF is removed from the sample and the sections places between the electrodes. The impedance is measured across the frequency range and electrical conductivity calculated. Results show two regions of dispersion. A low frequency region is evident below approximately 10 kHz, and a high frequency dispersion above this. Results at the higher frequencies show a good fit to the Cole-Cole model of impedance of biological tissue; this model consists of resistive and non-linear capacitive elements. Physically, these elements are likely to arise due to membrane polarization and migration of ions both intra- and extra-cellularly.http://www.iupab2014.org/assets/IUPAB/NewFolder/iupab-abstracts.pd
Have We Observed the Higgs (Imposter)?
We interpret the new particle at the Large Hadron Collider as a CP-even
scalar and investigate its electroweak quantum number. Assuming an unbroken
custodial invariance as suggested by precision electroweak measurements, only
four possibilities are allowed if the scalar decays to pairs of gauge bosons,
as exemplified by a dilaton/radion, a non-dilatonic electroweak singlet scalar,
an electroweak doublet scalar, and electroweak triplet scalars. We show that
current LHC data already strongly disfavor both the dilatonic and non-dilatonic
singlet imposters. On the other hand, a generic Higgs doublet give excellent
fits to the measured event rates of the newly observed scalar resonance, while
the Standard Model Higgs boson gives a slightly worse overall fit due to the
lack signal in the tau tau channel. The triplet imposter exhibits some tension
with the data. The global fit indicates the enhancement in the diphoton channel
could be attributed to an enhanced partial decay width, while the production
rates are consistent with the Standard Model expectations. We emphasize that
more precise measurements of the ratio of event rates in the WW over ZZ
channels, as well as the event rates in b bbar and tau tau channels, are needed
to further distinguish the Higgs doublet from the triplet imposter.Comment: 20 pages, 4 figures; v2: updated with most recent public data as of
August 7. A generic Higgs doublet now gives the best fit to data, while the
triplet imposter exhibits some tensio
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