3,628 research outputs found
Dynamical Mean-Field Study of the Ferromagnetic Transition Temperature of a Two-Band Model for Colossal Magnetoresistance Materials
The ferromagnetic (FM) transition temperature (Tc) of a two-band
Double-Exchange (DE) model for colossal magnetoresistance (CMR) materials is
studied using dynamical mean-field theory (DMFT), in wide ranges of coupling
constants, hopping parameters, and carrier densities. The results are shown to
be in excellent agreement with Monte Carlo simulations. When the bands overlap,
the value of Tc is found to be much larger than in the one-band case, for all
values of the chemical potential within the energy overlap interval. A nonzero
interband hopping produces an additional substantial increase of Tc, showing
the importance of these nondiagonal terms, and the concomitant use of multiband
models, to boost up the critical temperatures in DE-based theories.Comment: 4 pages, 4 eps figure
Strong normalization for System F by HOAS on top of FOAS
We present a point of view concerning HOAS (Higher-Order Abstract Syntax) and an extensive exercise in HOAS along this point of view. The point of view is that HOAS can be soundly and fruitfully regarded as a definitional extension on top of FOAS (First-Order Abstract Syntax). As such, HOAS is not only an encoding technique, but also a higher-order view of a first-order reality. A rich collection of concepts and proof principles is developed inside the standard mathematical universe to give technical life to this point of view. The exercise consists of a new proof of Strong Normalization for System F. The concepts and results presented here have been formalized in the theorem prover Isabelle/HOL
Performance estimation of interior permanent-magnet brushless motors using the voltage-driven flux-MMF diagram
The flux-magnetomotive force (flux-MMF) diagram, or "energy conversion loop," is a powerful tool for computing the parameters of saturated interior permanent-magnet brushless motors, especially when the assumptions underlying classical dq theory are not valid, as is often the case in modern practice. Efficient finite-element computation of the flux-MMF diagram is possible when the motor current is known a priori, but in high-speed operation the current regulator can lose control of the current waveform and the computation becomes "voltage-driven" rather than "current-driven." This paper describes an efficient method for estimating the motor performance-average torque, inductances-by solving the voltage-driven problem. It presents experimental validation for a two-pole brushless interior permanent-magnet motor. The paper also discusses the general conditions under which this method is appropriate, and compares the method with alternative approaches
Dust effects on the derived Sersic indexes of disks and bulges in spiral galaxies
We present a theoretical study that quantifies the effect of dust on the
derived Sersic indexes of disks and bulges. The changes in the derived
parameters from their intrinsic values (as seen in the absence of dust) were
obtained by fitting Sersic distributions on simulated images of disks and
bulges produced using radiative transfer calculations and the model of Popescu
et al. 2011. We found that dust has the effect of lowering the measured Sersic
index in most cases, with stronger effects for disks and bulges seen through
more optically thick lines of sight.Comment: 3 pages, 2 figures, to appear in the Proceedings of the IAU Symposium
No.284, "The Spectral Energy Distribution of Galaxies", 5-9 sept. 2011,
editors Richard J. Tuffs and Cristina C. Popesc
The Crossover from Impurity to Valence Band in Diluted Magnetic Semiconductors: The Role of the Coulomb Attraction by Acceptor
The crossover between an impurity band (IB) and a valence band (VB) regime as
a function of the magnetic impurity concentration in models for diluted
magnetic semiconductors (DMS) is studied systematically by taking into
consideration the Coulomb attraction between the carriers and the magnetic
impurities. The density of states and the ferromagnetic transition temperature
of a Spin-Fermion model applied to DMS are evaluated using Dynamical Mean-Field
Theory (DMFT) and Monte Carlo (MC) calculations. It is shown that the addition
of a square-well-like attractive potential can generate an IB at small enough
Mn doping for values of the exchange that are not strong enough
to generate one by themselves. We observe that the IB merges with the VB when
where is a function of and the Coulomb attraction strength
. Using MC calculations, we demonstrate that the range of the Coulomb
attraction plays an important role. While the on-site attraction, that has been
used in previous numerical simulations, effectively renormalizes for all
values of , an unphysical result, a nearest-neighbor range attraction
renormalizes only at very low dopings, i.e., until the bound holes wave
functions start to overlap. Thus, our results indicate that the Coulomb
attraction can be neglected to study Mn doped GaSb, GaAs, and GaP in the
relevant doping regimes, but it should be included in the case of Mn doped GaN
that is expected to be in the IB regime.Comment: 8 pages, 4 Postscript figures, RevTex
Scalar and tensorial topological matter coupled to (2+1)-dimensional gravity:A.Classical theory and global charges
We consider the coupling of scalar topological matter to (2+1)-dimensional
gravity. The matter fields consist of a 0-form scalar field and a 2-form tensor
field. We carry out a canonical analysis of the classical theory, investigating
its sectors and solutions. We show that the model admits both BTZ-like
black-hole solutions and homogeneous/inhomogeneous FRW cosmological
solutions.We also investigate the global charges associated with the model and
show that the algebra of charges is the extension of the Kac-Moody algebra for
the field-rigid gauge charges, and the Virasoro algebrafor the diffeomorphism
charges. Finally, we show that the model can be written as a generalized
Chern-Simons theory, opening the perspective for its formulation as a
generalized higher gauge theory.Comment: 40 page
Comorbidities and side effects of the imunomodulatory treatment in multiple sclerosis
Multiple sclerosis is a demyelinating, neurodegenerative disease of the central nervous system, that affects the young adult and is associated with a high degree of disability. Physical and mental comorbidity and adverse health behaviors are common in patients with MS. Comorbidities and health behaviors are associated with adverse outcomes in MS and should be considered in the assessment and management of patients with MS. We have studied a group of 150 MS patients treated with immunomodulators inside the Romanian national MS treatment programme. The patients we analysed corresponded to the available literature in matters or number, age and sex. We searched for associated pathology and side effects of the treatment. After selecting and statistically analyzing the data we concluded that dyslipidemia was the most common comorbidity, followed by hyperglicemia and vertebral hernia. The presence or not of a comorbidity before the treatment does not relate to the EDSS score (
Can Virialization Shocks be Detected Around Galaxy Clusters Through the Sunyaev-Zel'dovich Effect?
In cosmological structure formation models, massive non-linear objects in the
process of formation, such as galaxy clusters, are surrounded by large-scale
shocks at or around the expected virial radius. Direct observational evidence
for such virial shocks is currently lacking, but we show here that their
presence can be inferred from future, high resolution, high-sensitivity
observations of the Sunyaev-Zel'dovich (SZ) effect in galaxy clusters. We study
the detectability of virial shocks in mock SZ maps, using simple models of
cluster structure (gas density and temperature distributions) and noise
(background and foreground galaxy clusters projected along the line of sight,
as well as the cosmic microwave background anisotropies). We find that at an
angular resolution of 2'' and sensitivity of 10 micro K, expected to be reached
at ~ 100 GHz frequencies in a ~ 20 hr integration with the forthcoming ALMA
instrument, virial shocks associated with massive M ~ 10^15 M_Sun clusters will
stand out from the noise, and can be detected at high significance. More
generally, our results imply that the projected SZ surface brightness profile
in future, high-resolution experiments will provide sensitive constraints on
the density profile of cluster gas.Comment: 15 pages, submitted to Ap
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