1,346 research outputs found
Selfconsistent hybridization expansions for static properties of the Anderson impurity model
By means of a projector-operator formalism we derive an approximation based
on a self consistent hybridization expansion to study the ground state
properties of the Anderson Impurity model. We applied the approximation to the
general case of finite Coulomb repulsion , extending previous work with the
same formalism in the infinite- case. The treatment provides a very accurate
calculation of the ground state energy and their related zero temperature
properties in the case in which is large enough, but still finite, as
compared with the rest of energy scales involved in the model. The results for
the valence of the impurity are compared with exact results that we obtain from
equations derived using the Bethe ansatz and with a perturbative approach. The
magnetization and magnetic susceptibility is also compared with Bethe ansatz
results. In order to do this comparison, we also show how to regularize the
Bethe ansatz integral equations necessary to calculate the impurity valence,
for arbitrary values of the parameters.Comment: 8 pages, 5 figure
Modeling power grids
We present a method to construct random model power grids that closely match
statistical properties of a real power grid. The model grids are more difficult
to partition than a real grid.Comment: 9 pages, 5 figure
Spin polaron in the J1-J2 Heisenberg model
We have studied the validity of the spin polaron picture in the frustrated
J1-J2 Heisenberg model. For this purpose, we have computed the hole spectral
functions for the Neel, collinear, and disordered phases of this model, by
means of the self-consistent Born approximation and Lanczos exact
diagonalization on finite-size clusters. We have found that the spin polaron
quasiparticle excitation is always well defined for the magnetically ordered
Neel and collinear phases, even in the vicinity of the magnetic quantum
critical points, where the local magnetization vanishes. As a general feature,
the effect of frustration is to increase the amplitude of the multimagnon
states that build up the spin polaron wave function, leading to the reduction
of the quasiparticle coherence. Based on Lanczos results, we discuss the
validity of the spin polaron picture in the disordered phase.Comment: 9 pages, 12 figure
Soil resources and potential for agricultural development in Bahr El Jebel in southern Sudan, Jonglei Canal project area
The author has identified the following significant results. Fourteen LANDSAT scenes were used to produce mosaics of the 167, 474 sq km study area. These were black and white MSS 7 images and false color composite images. Five major soil-landscape units were delineated on the mosaics, and these were subdivided into a total of 40 soil mapping units. Aerial reconnaissance was useful in defining boundaries between mapping units and in estimating the proportion of the various soils which composed each mapping unit. Ground surveying permitted first-hand observation of major soils and sampling for quantitative laboratory analysis. Soil interpretations were made, including properties, potentials, and limitations
X-ray harmonic comb from relativistic electron spikes
X-ray devices are far superior to optical ones for providing nanometre
spatial and attosecond temporal resolutions. Such resolution is indispensable
in biology, medicine, physics, material sciences, and their applications. A
bright ultrafast coherent X-ray source is highly desirable, for example, for
the diffractive imaging of individual large molecules, viruses, or cells. Here
we demonstrate experimentally a new compact X-ray source involving high-order
harmonics produced by a relativistic-irradiance femtosecond laser in a gas
target. In our first implementation using a 9 Terawatt laser, coherent soft
X-rays are emitted with a comb-like spectrum reaching the 'water window' range.
The generation mechanism is robust being based on phenomena inherent in
relativistic laser plasmas: self-focusing, nonlinear wave generation
accompanied by electron density singularities, and collective radiation by a
compact electric charge. The formation of singularities (electron density
spikes) is described by the elegant mathematical catastrophe theory, which
explains sudden changes in various complex systems, from physics to social
sciences. The new X-ray source has advantageous scalings, as the maximum
harmonic order is proportional to the cube of the laser amplitude enhanced by
relativistic self-focusing in plasma. This allows straightforward extension of
the coherent X-ray generation to the keV and tens of keV spectral regions. The
implemented X-ray source is remarkably easily accessible: the requirements for
the laser can be met in a university-scale laboratory, the gas jet is a
replenishable debris-free target, and the harmonics emanate directly from the
gas jet without additional devices. Our results open the way to a compact
coherent ultrashort brilliant X-ray source with single shot and high-repetition
rate capabilities, suitable for numerous applications and diagnostics in many
research fields
Quasi-energy-independent solar neutrino transitions
Current solar, atmospheric, and reactor neutrino data still allow oscillation
scenarios where the squared mass differences are all close to 10^-3 eV^2,
rather than being hierarchically separated. For solar neutrinos, this situation
(realized in the upper part of the so-called large-mixing angle solution)
implies adiabatic transitions which depend weakly on the neutrino energy and on
the matter density, as well as on the ``atmospheric'' squared mass difference.
In such a regime of ``quasi-energy-independent'' (QEI) transitions,
intermediate between the more familiar ``Mikheyev-Smirnov-Wolfenstein'' (MSW)
and energy-independent (EI) regimes, we first perform analytical calculations
of the solar nu_e survival probability at first order in the matter density,
beyond the usual hierarchical approximations. We then provide accurate,
generalized expressions for the solar neutrino mixing angles in matter, which
reduce to those valid in the MSW, QEI and EI regimes in appropriate limits.
Finally, a representative QEI scenario is discussed in some detail.Comment: Title changed; text and acronyms revised; results unchanged. To
appear in PR
Long-term effectiveness of a novel intra-oral electro-stimulator for the treatment of dry mouth in patients with Sjogren's syndrome: A randomised sham-controlled feasibility study (LEONIDAS-1)
Background
Effective treatments for dry mouth of Sjogren's syndrome are limited and hampered by adverse effects. The aim of LEONIDAS-1 was to explore the feasibility of salivary electrostimulation in individuals with primary Sjogren's syndrome, as well as parameters required to inform the design of a future phase III trial.
Methods
Multicentre, parallel-group, double-blind, randomised sham-controlled trial in two UK centres. Participants were randomised (1:1, computer-generated) to active or sham electrostimulation. The feasibility outcomes included screening/eligibility ratio, consent, and recruitment and drop-out rates. Preliminary efficacy outcome included dry mouth visual analogue scale, Xerostomia Inventory, the EULAR Sjögren's syndrome patient reported index-Q1, and unstimulated sialometry.
Results
Forty-two individuals were screened, of whom 30 (71.4%) met the eligibility criteria. All eligible individuals consented to recruitment. Out of the 30 randomised participants (active n = 15, sham n = 15), 4 dropped out and 26 (13 vs. 13) completed all study visits as per protocol. Recruitment rate was 2.73 participants/month. At 6-month post-randomisation the difference in mean reduction in visual analogue scale, xerostomia inventory and EULAR Sjögren's syndrome patient reported index-Q1 scores between groups were 0.36 (95% CI: −0.84, 1.56), 3.31 (0.43, 6.18), and 0.23 (−1.17, 1.63), respectively; unstimulated salivary flow increased by a mean of 0.98 mL/15 min, all in favour of the active group. No adverse events were reported.
Conclusion
LEONIDAS-1 results support progression to a phase III definitive randomised controlled trial of salivary electrostimulation in individuals with Sjogren's syndrome. Xerostomia inventory could be considered the primary patient-centred outcome measure and the corresponding observed treatment effect could inform the sample size of a future trial
Atlas of two-dimensional irreversible conservative lagrangian mechanical systems with a second quadratic integral
This paper aims at the most comprehensive and systematic construction and
tabulation of mechanical systems that admit a second invariant, quadratic in
velocities, other than the Hamiltonian. The configuration space is in general a
2D Riemannian or pseudo-Riemannian manifold and the determination of its
geometry is a part of the process of solution. Forces acting on the system
include a part derived from a scalar potential and a part derived from a vector
potential, associated with terms linear in velocities in the Lagrangian
function of the system. The last cause time-irreversibility of the system. We
construct 41 multi-parameter integrable systems of the type described in the
title mostly on Riemannian manifolds. They are mostly new and cover all
previously known systems as special cases, corresponding to special values of
the parameters. Those include all known cases of motion of a particle in the
plane and all known cases in the dynamics of rigid body. In the last field we
introduce a new integrable case related to Steklov's case of motion of a body
in a liquid. Several new cases of motion in the plane, on the sphere and on the
pseudo-sphere or in the hyperbolic plane are found as special cases.
Prospective applications in mathematics and physics are also pointed out.Comment: Paper to be published in "Journal of Mathematical Physics", Vol. 48,
issue 7, July 200
Effects of bone- and air-tissue inhomogeneities on the dose distributions of the Leksell Gamma Knife calculated with PENELOPE
Monte Carlo simulation with PENELOPE (v.~2003) is applied to calculate
Leksell Gamma Knife dose distributions for heterogeneous
phantoms. The usual spherical water phantom is modified with a spherical bone
shell simulating the skull and an air-filled cube simulating the frontal or
maxillary sinuses. Different simulations of the 201 source configuration of the
Gamma Knife have been carried out with a simplified model of the geometry of
the source channel of the Gamma Knife recently tested for both single source
and multisource configurations. The dose distributions determined for
heterogeneous phantoms including the bone- and/or air-tissue interfaces show
non negligible differences with respect to those calculated for a homogeneous
one, mainly when the Gamma Knife isocenter approaches the separation surfaces.
Our findings confirm an important underdosage (10%) nearby the air-tissue
interface, in accordance with previous results obtained with PENELOPE code with
a procedure different to ours. On the other hand, the presence of the spherical
shell simulating the skull produces a few percent underdosage at the isocenter
wherever it is situated.Comment: 13 pages, 8 figures, 2 table
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