2,927 research outputs found
Performance of distributed mechanisms for flow admission in wireless adhoc networks
Given a wireless network where some pairs of communication links interfere
with each other, we study sufficient conditions for determining whether a given
set of minimum bandwidth quality-of-service (QoS) requirements can be
satisfied. We are especially interested in algorithms which have low
communication overhead and low processing complexity. The interference in the
network is modeled using a conflict graph whose vertices correspond to the
communication links in the network. Two links are adjacent in this graph if and
only if they interfere with each other due to being in the same vicinity and
hence cannot be simultaneously active. The problem of scheduling the
transmission of the various links is then essentially a fractional, weighted
vertex coloring problem, for which upper bounds on the fractional chromatic
number are sought using only localized information. We recall some distributed
algorithms for this problem, and then assess their worst-case performance. Our
results on this fundamental problem imply that for some well known classes of
networks and interference models, the performance of these distributed
algorithms is within a bounded factor away from that of an optimal, centralized
algorithm. The performance bounds are simple expressions in terms of graph
invariants. It is seen that the induced star number of a network plays an
important role in the design and performance of such networks.Comment: 21 pages, submitted. Journal version of arXiv:0906.378
Role of many-body entanglement in decoherence processes
A pure state decoheres into a mixed state as it entangles with an
environment. When an entangled two-mode system is embedded in a thermal
environment, however, each mode may not be entangled with its environment by
their simple linear interaction. We consider an exactly solvable model to study
the dynamics of a total system, which is composed of an entangled two-mode
system and a thermal environment, and also an array of infinite beam splitters.
It is shown that many-body entanglement of the system and the environment plays
a crucial role in the process of disentangling the system.Comment: 4 pages, 1 figur
Magnetic monopoles from gauge theory phase transitions
Thermal fluctuations of the gauge field lead to monopole formation at the
grand unified phase transition in the early Universe, even if the transition is
merely a smooth crossover. The dependence of the produced monopole density on
various parameters is qualitatively different from theories with global
symmetries, and the monopoles have a positive correlation at short distances.
The number density of monopoles may be suppressed if the grand unified symmetry
is only restored for a short time by, for instance, nonthermal symmetry
restoration after preheating.Comment: 5 pages, updated to match the version published in PRD
(http://link.aps.org/abstract/PRD/v68/e021301) on 11 July 200
Information-Geometric Indicators of Chaos in Gaussian Models on Statistical Manifolds of Negative Ricci Curvature
A new information-geometric approach to chaotic dynamics on curved
statistical manifolds based on Entropic Dynamics (ED) is proposed. It is shown
that the hyperbolicity of a non-maximally symmetric 6N-dimensional statistical
manifold M_{s} underlying an ED Gaussian model describing an arbitrary system
of 3N degrees of freedom leads to linear information-geometric entropy growth
and to exponential divergence of the Jacobi vector field intensity, quantum and
classical features of chaos respectively.Comment: 8 pages, final version accepted for publicatio
Two-Hole and Four-Hole Bound States in a t-J Ladder at half-filling
The two-hole excitation spectrum of the t-J ladder at half-filling is studied
using linked-cluster series expansion methods. A rich spectrum of bound states
emerges, particularly at small . Their dispersion relations and coherence
lengths are computed, along with the threshold behaviour as the bound states
merge into the continuum. A class of 4-hole bound states is also studied,
leading to the conclusion that phase separation occurs for ,
in agreement with other studies.Comment: revtex
Raman scattering studies of spin, charge, and lattice dynamics in Ca_{2-x}Sr_{x}RuO_{4} (0 =< x < 0.2)
We use Raman scattering to study spin, charge, and lattice dynamics in
various phases of Ca_{2-x}Sr_{x}RuO_{4}. With increasing substitution of Ca by
Sr in the range 0 =< x < 0.2, we observe (1) evidence for an increase of the
electron-phonon interaction strength, (2) an increased temperature-dependence
of the two-magnon energy and linewidth in the antiferromagnetic insulating
phase, and (3) evidence for charge gap development, and hysteresis associated
with the structural phase change, both of which are indicative of a first-order
metal-insulator transition (T_{MI}) and a coexistence of metallic and
insulating components for T < T_{MI}
Relativistic Quantum Mechanics - Particle Production and Cluster Properties
This paper constructs relativistic quantum mechanical models of particles
satisfying cluster properties and the spectral condition which do not conserve
particle number. The treatment of particle production is limited to systems
with a bounded number of bare-particle degrees of freedom. The focus of this
paper is about the realization of cluster properties in these theories.Comment: 36 pages, Late
Revisiting vertical structure of neutrino-dominated accretion disks: Bernoulli parameter, neutrino trapping and other distributions
We revisit the vertical structure of neutrino dominated accretion flows
(NDAFs) in spherical coordinates with a new boundary condition based on the
mechanical equilibrium. The solutions show that NDAF is significantly thick.
The Bernoulli parameter and neutrino trapping are determined by the mass
accretion rate and the viscosity parameter. According to the distribution of
the Bernoulli parameter, the possible outflow may appear in the outer region of
the disk. The neutrino trapping can essentially affect the neutrino radiation
luminosity. The vertical structure of NDAF is like a "sandwich", and the
multilayer accretion may account for the flares in gamma-ray bursts.Comment: 7 pages, 2 figures, Accepted for publication in Astrophysics & Space
Scienc
Quantum Characterization of a Werner-like Mixture
We introduce a Werner-like mixture [R. F. Werner, Phys. Rev. A {\bf 40}, 4277
(1989)] by considering two correlated but different degrees of freedom, one
with discrete variables and the other with continuous variables. We evaluate
the mixedness of this state, and its degree of entanglement establishing its
usefulness for quantum information processing like quantum teleportation. Then,
we provide its tomographic characterization. Finally, we show how such a
mixture can be generated and measured in a trapped system like one electron in
a Penning trap.Comment: 8 pages ReVTeX, 8 eps figure
The interplay between shell effects and electron correlations in quantum dots
We use the Path Integral Monte Carlo method to investigate the interplay
between shell effects and electron correlations in single quantum dots with up
to 12 electrons. By use of an energy estimator based on the hypervirial theorem
of Hirschfelder we study the energy contributions of different interaction
terms in detail. We discuss under which conditions the total spin of the
electrons is given by Hund's rule, and the temperature dependence of the
crystallization effects.Comment: 6 pages, 4 figure
- …