827 research outputs found
Traffic Offloading/Onloading in Multi-RAT Cellular Networks
We analyze next generation cellular networks, offering connectivity to mobile users through multiple radio access technologies (RATs), namely LTE and WiFi. We develop a framework based on the Markovian agent formalism, which can model several aspects of the system, including user traffic dynamics and radio resource allocation. In particular, through a mean-field solution, we show the ability of our framework to capture the system behavior in flash-crowd scenarios, i.e., when a burst of traffic requests takes place in some parts of the network service area. We consider a distributed strategy for the user RAT selection, which aims at ensuring high user throughput, and investigate its performance under different resource allocation scheme
Ab-initio self-energy corrections in systems with metallic screening
The calculation of self-energy corrections to the electron bands of a metal
requires the evaluation of the intraband contribution to the polarizability in
the small-q limit. When neglected, as in standard GW codes for semiconductors
and insulators, a spurious gap opens at the Fermi energy. Systematic methods to
include intraband contributions to the polarizability exist, but require a
computationally intensive Fermi-surface integration. We propose a numerically
cheap and stable method, based on a fit of the power expansion of the
polarizability in the small-q region. We test it on the homogeneous electron
gas and on real metals such as sodium and aluminum.Comment: revtex, 14 pages including 5 eps figures v2: few fixe
Condensate Fraction of a Fermi Gas in the BCS-BEC Crossover
We investigate the Bose-Einstein condensation of Fermionic pairs in a uniform
two-component Fermi gas obtaining an explicit formula for the condensate
density as a function of the chemical potential and the energy gap. We analyze
the condensate fraction in the crossover from the Bardeen-Cooper-Schrieffer
(BCS) state of weakly-interacting Cooper pairs to the Bose-Einstein Condensate
(BEC) of molecular dimers. By using the local density approximation we study
confined Fermi vapors of alkali-metal atoms for which there is experimental
evidence of condensation also on the BCS side of the Feshbach resonance. Our
theoretical results are in agreement with these experimental data and give the
behavior of the condensate on both sides of the Feshbach resonance at zero
temperature.Comment: 5 pages, 2 figure
Circular Economy: A Performance Evaluation Perspective
Circular Economy has emerged in the last years as an industrial pattern that rethinks the entire design and production cycle of products and goods, to obtain the minimal environmental impact and maximize the energy efficiency. This pattern creates product transformation cycles, that allow to re-use parts and renew energy, defining Circular Economy Ecosystems. Such cycles, are very interesting systems from a performance evaluation point of view: in this paper we give a Colored Petri Net perspective to circular economy. In particular, we focus on an example taken from the literature that considers car manufacturing, and we show how we can deal with the problem using standard performance evaluation techniques. Results from performance analysis of the case study, allow to focus on new interesting metrics and performance indicators, that might not be fully studied with conventional techniques applied by expert of the economical domain
The Phase Diagram of Correlated Electrons in a Lattice of Berry Molecules
A model for correlated electrons in a lattice with local additional spin--1
degrees of freedom inducing constrained hopping, is studied both in the low
density limit and at quarter filling. We show that in both 1D and 2D two
particles form a bound state even in presence of a repulsive U<U_c. A picture
of a dilute Bose gas, leading to off-diagonal long range order (LRO) in 2D
(quasi-LRO in 1D), is supported by quantitative calculations in 1D which allow
for a determination of the phase diagram.Comment: 7 pages + 2 ps figures, published versio
Superconductivity in a spin liquid - a one dimensional example
We study a one-dimensional model of interacting conduction electrons with a
two-fold degenerate band away from half filling. The interaction includes an
on-site Coulomb repulsion and Hund's rule coupling. We show that such
one-dimensional system has a divergent Cooper pair susceptibility at T = 0,
provided the Coulomb interaction between electrons on the same orbital and
the modulus of the Hund's exchange integral are larger than the
interorbital Coulomb interaction. It is remarkable that the superconductivity
can be achieved for {\it any} sign of . The opening of spectral gaps makes
this state stable with respect to direct electron hopping between the orbitals.
The scaling dimension of the superconducting order parameter is found to be
between 1/4 (small ) and 1/2 (large ).Comment: 11 pages, Latex, no figure
Monitoraggio ambientale e biologico dellāesposizione ad idrocarburi mono-aromatici ed a metil tert-butil etere in un gruppo di lavoratori addetti allāerogazione di carburanti
Lo studio ĆØ stato condotto per valutare
gli indicatori biologici di esposizione a vapori di benzina
in lavoratori addetti allāerogazione di carburante tramite
un approccio combinato di monitoraggio ambientale
e biologico. Lāesposizione personale a benzene, toluene,
etilbenzene e xilene (BTEX) e lāescrezione urinaria di BTEX,
metil tert-butil etere (MTBE-U), degli acidi trans,transmuconico
(t,t-MA) ed S-fenilmercapturico (S-PMA) e della
cotinina sono stati valutati con tecniche cromatografiche
accoppiate alla spettrometria di massa. I livelli di MTBE-U
erano influenzati dalla sola esposizione professionale a vapori
di benzina, mentre quelli di B-U ed S-PMA dipendevano
da abitudine tabagica ed esposizione professionale
Mean-Field vs Monte-Carlo equation of state for the expansion of a Fermi superfluid in the BCS-BEC crossover
The equation of state (EOS) of a Fermi superfluid is investigated in the
BCS-BEC crossover at zero temperature. We discuss the EOS based on Monte-Carlo
(MC) data and asymptotic expansions and the EOS derived from the extended BCS
(EBCS) mean-field theory. Then we introduce a time-dependent density
functional, based on the bulk EOS and Landau's superfluid hydrodynamics with a
von Weizs\"acker-type correction, to study the free expansion of the Fermi
superfluid. We calculate the aspect ratio and the released energy of the
expanding Fermi cloud showing that MC EOS and EBCS EOS are both compatible with
the available experimental data of Li atoms. We find that the released
energy satisfies an approximate analytical formula that is quite accurate in
the BEC regime. For an anisotropic droplet, our numerical simulations show an
initially faster reversal of anisotropy in the BCS regime, later suppressed by
the BEC fluid.Comment: 13 pages, 3 figures, presented to the 15th International Laser
Physics Workshop (Lausanne, July 24-28, 2006); to be published in Laser
Physic
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