6,178 research outputs found
Universality in Blow-Up for Nonlinear Heat Equations
We consider the classical problem of the blowing-up of solutions of the
nonlinear heat equation. We show that there exist infinitely many profiles
around the blow-up point, and for each integer , we construct a set of
codimension in the space of initial data giving rise to solutions that
blow-up according to the given profile.Comment: 38 page
Propagating, evanescent, and localized states in carbon nanotube-graphene junctions
We study the electronic structure of the junctions between a single graphene
layer and carbon nanotubes, using a tight-binding model and the continuum
theory based on Dirac fermion fields. The latter provides a unified description
of different lattice structures with curvature, which is always localized at
six heptagonal carbon rings around each junction. When these are evenly spaced,
we find that it is possible to curve the planar lattice into armchair (6n,6n)
as well as zig-zag (6n,0) nanotubes. We show that the junctions fall into two
different classes, regarding the low-energy electronic behavior. One of them,
constituted by the junctions made of the armchair nanotubes and the zig-zag
(6n,0) geometries when n is a multiple of 3, is characterized by the presence
of two quasi-bound states at the Fermi level, which are absent for the rest of
the zig-zag nanotubes. These states, localized at the junction, are shown to
arise from the effective gauge flux induced by the heptagonal carbon rings,
which has a direct reflection in the local density of states around the
junction. Furthermore, we also analyze the band structure of the arrays of
junctions, finding out that they can also be classified into two different
groups according to the low-energy behavior. In this regard, the arrays made of
armchair and (6n,0) nanotubes with n equal to a multiple of 3 are characterized
by the presence of a series of flat bands, whose number grows with the length
of the nanotubes. We show that such flat bands have their origin in the
formation of states confined to the nanotubes in the array. This is explained
in the continuum theory from the possibility of forming standing waves in the
mentioned nanotube geometries, as a superposition of modes with opposite
momenta and the same quantum numbers under the C_6v symmetry of the junction.Comment: 13 pages, 8 figure
Molar volume of solid isotopic helium mixtures
Solid isotopic helium mixtures have been studied by path-integral Monte Carlo
simulations in the isothermal-isobaric ensemble. This method allowed us to
study the molar volume as a function of temperature, pressure, and isotopic
composition. At 25 K and 0.2 GPa, the relative difference between molar volumes
of isotopically-pure crystals of 3He and 4He is found to be about 3%. This
difference decreases under pressure, and for 12 GPa it is smaller than 1%. For
isotopically-mixed crystals, a linear relation between lattice parameters and
concentrations of helium isotopes is found, in agreement with Vegard's law. The
virtual crystal approximation, valid for isotopic mixtures of heavier atoms,
does not give reliable results for solid solutions of helium isotopes.Comment: 7 pages, 5 figure
Solid helium at high pressure: A path-integral Monte Carlo simulation
Solid helium (3He and 4He) in the hcp and fcc phases has been studied by
path-integral Monte Carlo. Simulations were carried out in the
isothermal-isobaric (NPT) ensemble at pressures up to 52 GPa. This allows one
to study the temperature and pressure dependences of isotopic effects on the
crystal volume and vibrational energy in a wide parameter range. The obtained
equation of state at room temperature agrees with available experimental data.
The kinetic energy, E_k, of solid helium is found to be larger than the
vibrational potential energy, E_p. The ratio E_k/E_p amounts to about 1.4 at
low pressures, and decreases as the applied pressure is raised, converging to
1, as in a harmonic solid. Results of these simulations have been compared with
those yielded by previous path integral simulations in the NVT ensemble. The
validity range of earlier approximations is discussed.Comment: 7 pages, 5 figure
Improved determination of the 1(0)-0(0) rotational frequency of NH3D+ from the high resolution spectrum of the v4 infrared band
The high resolution spectrum of the v4 band of NH3D+ has been measured by
difference frequency IR laser spectroscopy in a multipass hollow cathode
discharge cell. From the set of molecular constants obtained from the analysis
of the spectrum, a value of 262817(6) MHz (3sigma) has been derived for the
frequency of the 1(0)-0(0) rotational transition. This value supports the
assignment to NH3D+ of lines at 262816.7 MHz recorded in radio astronomy
observations in Orion-IRc2 and the cold prestellar core B1-bS.Comment: Accepted for publication in the Astrophysical Journal Letters 04 June
201
New accurate measurement of 36ArH+ and 38ArH+ ro-vibrational transitions by high resolution IR absorption spectroscopy
The protonated Argon ion, ArH, has been identified recently in
the Crab Nebula (Barlow et al. 2013) from Herschel spectra. Given the
atmospheric opacity at the frequency of its =1-0 and =2-1 rotational
transitions (617.5 and 1234.6 GHz, respectively), and the current lack of
appropriate space observatories after the recent end of the Herschel mission,
future studies on this molecule will rely on mid-infrared observations. We
report on accurate wavenumber measurements of ArH and
ArH rotation-vibration transitions in the =1-0 band in the
range 4.1-3.7 m (2450-2715 cm). The wavenumbers of the (0)
transitions of the =1-0 band are 2612.501350.00033 and
2610.701770.00042 cm () for ArH and
ArH, respectively. The calculated opacity for a gas thermalized at
a temperature of 100 K and a linewidth of 1 km.s of the (0) line is
(ArH). For column densities of
ArH larger than cm, significant absorption
by the (0) line can be expected against bright mid-IR sources
The OPTX Project I: The Flux and Redshift Catalogs for the CLANS, CLASXS, and CDF-N fields
We present the redshift catalogs for the X-ray sources detected in the
Chandra Deep Field North (CDF-N), the Chandra Large Area Synoptic X-ray Survey
(CLASXS), and the Chandra Lockman Area North Survey (CLANS). The catalogs for
the CDF-N and CLASXS fields include redshifts from previous work, while the
redshifts for the CLANS field are all new. For fluxes above 10^-14 ergs cm^-2
s^-1 (2-8 keV) we have redshifts for 76% of the sources. We extend the redshift
information for the full sample using photometric redshifts. The goal of the
OPTX Project is to use these three surveys, which are among the most
spectroscopically complete surveys to date, to analyze the effect of spectral
type on the shape and evolution of the X-ray luminosity functions and to
compare the optical spectral types with the X-ray spectral properties.
We also present the CLANS X-ray catalog. The nine ACIS-I fields cover a solid
angle of ~0.6 square degrees and reach fluxes of 7x10^-16 ergs cm^-2 s^-1
(0.5-2 keV) and 3.5x10^-15 ergs cm^-2 s^-1 (2-8 keV). We find a total of 761
X-ray point sources. Additionally, we present the optical and infrared
photometric catalog for the CLANS X-ray sources, as well as updated optical and
infrared photometric catalogs for the X-ray sources in the CLASXS and CDF-N
fields.
The CLANS and CLASXS surveys bridge the gap between the ultradeep pencil-beam
surveys, such as the CDFs, and the shallower, very large-area surveys. As a
result, they probe the X-ray sources that contribute the bulk of the 2-8 keV
X-ray background and cover the flux range of the observed break in the
logN-logS distribution. We construct differential number counts for each
individual field and for the full sample.Comment: Published in The Astrophysical Journal Supplement. 18 pages, 16
figures, 14 table
Preoperative digital three-dimensional planning for rhinoplasty
BACKGROUND: This report describes preoperative digital planning for rhinoplasty using a new three-dimensional (3D) radiologic viewer that allows both patients and surgeons to visualize on a common monitor the 3D real aspect of the nose in its inner and outer sides.
METHODS: In the period 2002 to 2008, 210 patients underwent rhinoplasty procedures in the authors' clinic. The patients were randomly divided into three groups according to the type of preoperative planning used: photos only, a simulated result by Adobe Photoshop, or the 3D radiologic viewer. The parameters evaluated included the number of patients that underwent surgery after the first consultation, the number of patients who asked for a reintervention, patient satisfaction (according to a test given to the patients 12 months postoperatively), the surgical time required for a functional intervention, and the improvement in nasal function by postoperative rhinomanometry and subjective evaluation.
RESULTS: Computer-aided technologies led to a higher number of patients deciding to undergo a rhinoplasty. Simulation of the postoperative results was not as useful in the postoperative period due to the higher number of reintervention requests.
CONCLUSION: The patients undergoing rhinoplasties preferred new technologies in the preoperative period. The advantages of using the 3D radiologic viewer included improved preoperative planning, reduction in intraoperative stress, a higher number of patients undergoing surgery, reduction in postoperative surgical corrections, reduction in surgical time for the functional intervention, a higher rate of improvement in nasal function, a higher percentage of postoperative satisfaction, and reduced costs
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