15,839 research outputs found
Ferromagnetism in two band metals: Combined effect of Coulomb correlation, hybridization and band widths
We study the possibility of ferromagnetism in metals. The metal is described
by two hybridized bands one of which includes Hubbard correlation whereas the
other is uncorrelated. We parametrize the ratio of the band widths and their
centers as well. The original Hamiltonian is transformed in an effective and
simpler one. Only one site retains the full correlation (U) while in the others
acts as an internal field, the self-energy, in the framework of an alloy
analogy approximation. This field, in turn, is self-consistently determined by
imposing the translational invariance of the problem. For several total
electronic occupation numbers (n_{total}) we compare the spin dependent free
energies with the corresponding paramagnetic ones. We present several results
pointing out the mechanism by which the self-consistency introduces a sort of
constraints, for given values of band width and band shift .Comment: 6 pages, 7 figure
Shapes of Polynomial Julia Sets
Any Jordan curve in the complex plane can be approximated arbitrarily well in
the Hausdorff topology by Julia sets of polynomials. Finite collections of
disjoint Jordan domains can be approximated by the basins of attraction of
rational maps.Comment: 13 pages, 3 figure
Possible Solutions to the Radius Anomalies of Transiting Giant Planets
We calculate the theoretical evolution of the radii of all fourteen of the
known transiting extrasolar giant planets (EGPs) for a variety of assumptions
concerning atmospheric opacity, dense inner core masses, and possible internal
power sources. We incorporate the effects of stellar irradiation and customize
such effects for each EGP and star. Looking collectively at the family as a
whole, we find that there are in fact two radius anomalies to be explained. Not
only are the radii of a subset of the known transiting EGPs larger than
expected from previous theory, but many of the other objects are smaller than
the default theory would allow. We suggest that the larger EGPs can be
explained by invoking enhanced atmospheric opacities that naturally retain
internal heat. This explanation might obviate the necessity for an extra
internal power source. We explain the smaller radii by the presence in perhaps
all the known transiting EGPs of dense cores, such as have been inferred for
Saturn and Jupiter. Importantly, we derive a rough correlation between the
masses of our "best-fit" cores and the stellar metallicity that seems to
buttress the core-accretion model of their formation. Though many caveats and
uncertainties remain, the resulting comprehensive theory that incorporates
enhanced-opacity atmospheres and dense cores is in reasonable accord with all
the current structural data for the known transiting giant planets.Comment: 22 pages in emulateapj format, including 10 figures (mostly in
color), accepted to the Astrophysical Journal (February 9, 2007); to appear
in volume 661, June 200
An Investigation Into the Electrical Activity of Tender, Resting Paraspinal Muscles Using Surface Electromyography: A Pilot Study
Abnormal resting paraspinal muscle activity has been claimed to be responsible for changes in spinal tissue texture which are detectible by manual palpation. This pilot study investigated whether there was significant electrical activity in paraspinal musculature that was tender and that appeared to have altered tissue texture on palpation. Sixteen healthy volunteers between 18 and 35 years of age had their thoracic erector spinae mass palpated bilaterally from spinal levels T3 to T10 to identify paraspinal regions exhibiting altered tissue texture relative to the contralateral muscle mass. Surface electromyography (sEMG) was used to measure electrical activity in the muscle mass at the selected levels. No significant differences in electrical activity were observed between the tender and non-tender muscle masses, although a large difference existed in the one symptomatic subject. All muscle sites displayed EMG activity at rest, although the source of activity is not clear. A number of methodological problems with the EMG recording were encountered and are discussed. Future research is recommended using symptomatic participants
Comparative Evolution of Jupiter and Saturn
We present evolutionary sequences for Jupiter and Saturn, based on new
nongray model atmospheres, which take into account the evolution of the solar
luminosity and partitioning of dense components to deeper layers. The results
are used to set limits on the extent to which possible interior phase
separation of hydrogen and helium may have progressed in the two planets. When
combined with static models constrained by the gravity field, our evolutionary
calculations constrain the helium mass fraction in Jupiter to be between 0.20
and 0.27, relative to total hydrogen and helium. This is in agreement with the
Galileo determination. The helium mass fraction in Saturn's atmosphere lies
between 0.11 and 0.25, higher than the Voyager determination. Based on the
discrepancy between the Galileo and Voyager results for Jupiter, and our
models, we predict that Cassini measurements will yield a higher atmospheric
helium mass fraction for Saturn relative to the Voyager value.Comment: 18 pages, LaTeX, 4 figures. submitted to ``Planetary and Space
Science.'
Effect of Particle-Hole Asymmetry on the Mott-Hubbard Metal-Insulator Transition
The Mott-Hubbard metal-insulator transition is one of the most important
problems in correlated electron systems. In the past decade, much progress has
been made on examining a particle-hole symmetric form of the transition in the
Hubbard model with dynamical mean field theory where it was found that the
electronic self energy develops a pole at the transition. We examine the
particle-hole asymmetric metal-insulator transition in the Falicov-Kimball
model, and find that a number of features change when the noninteracting
density of states has a finite bandwidth. Since, generically particle-hole
symmetry is broken in real materials, our results have an impact on
understanding the metal-insulator transition in real materials.Comment: 5 pages, 3 figure
Sequence composition and environment effects on residue fluctuations in protein structures
The spectrum and scale of fluctuations in protein structures affect the range
of cell phenomena, including stability of protein structures or their
fragments, allosteric transitions and energy transfer. The study presents a
statistical-thermodynamic analysis of relationship between the sequence
composition and the distribution of residue fluctuations in protein-protein
complexes. A one-node-per residue elastic network model accounting for the
nonhomogeneous protein mass distribution and the inter-atomic interactions
through the renormalized inter-residue potential is developed. Two factors, a
protein mass distribution and a residue environment, were found to determine
the scale of residue fluctuations. Surface residues undergo larger fluctuations
than core residues, showing agreement with experimental observations. Ranking
residues over the normalized scale of fluctuations yields a distinct
classification of amino acids into three groups. The structural instability in
proteins possibly relates to the high content of the highly fluctuating
residues and a deficiency of the weakly fluctuating residues in irregular
secondary structure elements (loops), chameleon sequences and disordered
proteins. Strong correlation between residue fluctuations and the sequence
composition of protein loops supports this hypothesis. Comparing fluctuations
of binding site residues (interface residues) with other surface residues shows
that, on average, the interface is more rigid than the rest of the protein
surface and Gly, Ala, Ser, Cys, Leu and Trp have a propensity to form more
stable docking patches on the interface. The findings have broad implications
for understanding mechanisms of protein association and stability of protein
structures.Comment: 8 pages, 4 figure
Cumulant expansion of the periodic Anderson model in infinite dimension
The diagrammatic cumulant expansion for the periodic Anderson model with
infinite Coulomb repulsion () is considered here for an hypercubic
lattice of infinite dimension (). The same type of simplifications
obtained by Metzner for the cumulant expansion of the Hubbard model in the
limit of , are shown to be also valid for the periodic Anderson
model.Comment: 13 pages, 7 figures.ps. To be published in J. Phys. A: Mathematical
and General (1997
A noise study of the A-6 airplane and techniques for reducing its aural detection distance
A study was undertaken to determine the noise reduction potential of the A-6 airplane in order to reduce its aural detection distance. Static and flyby noise measurements were taken to document the basic airplane signature. The low-frequency noise which is generally most critical for aural detection was found to be broad-band in nature from this airplane, and its source is the turbojet engine exhaust. High-frequency compressor noise, which is characteristic of turbojet powerplants, and which is prominent at close range for this airplane, has no measurable effect on aural detection distance. The use of fluted-engine exhaust nozzles to change the far-field noise spectra is suggested as a possible means for reducing the aural detection distances. Detection distances associated with eight-lobe and four-lobe nozzles are estimated for a 1,000-foot altitude and grassy terrain to decrease from 4 miles to about 3 miles, and from 3 miles to about 2 miles for a 300-foot altitude and grassy terrain
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