15,725 research outputs found
Byzantine Vector Consensus in Complete Graphs
Consider a network of n processes each of which has a d-dimensional vector of
reals as its input. Each process can communicate directly with all the
processes in the system; thus the communication network is a complete graph.
All the communication channels are reliable and FIFO (first-in-first-out). The
problem of Byzantine vector consensus (BVC) requires agreement on a
d-dimensional vector that is in the convex hull of the d-dimensional input
vectors at the non-faulty processes. We obtain the following results for
Byzantine vector consensus in complete graphs while tolerating up to f
Byzantine failures:
* We prove that in a synchronous system, n >= max(3f+1, (d+1)f+1) is
necessary and sufficient for achieving Byzantine vector consensus.
* In an asynchronous system, it is known that exact consensus is impossible
in presence of faulty processes. For an asynchronous system, we prove that n >=
(d+2)f+1 is necessary and sufficient to achieve approximate Byzantine vector
consensus.
Our sufficiency proofs are constructive. We show sufficiency by providing
explicit algorithms that solve exact BVC in synchronous systems, and
approximate BVC in asynchronous systems.
We also obtain tight bounds on the number of processes for achieving BVC
using algorithms that are restricted to a simpler communication pattern
Symmetry energy from the nuclear collective motion: constraints from dipole, quadrupole, monopole and spin-dipole resonances
The experimental and theoretical studies of Giant Resonances, or more
generally of the nuclear collective vibrations, are a well established domain
in which sophisticated techniques have been introduced and firm conclusions
reached after an effort of several decades. From it, information on the nuclear
equation of state can be extracted, albeit not far from usual nuclear
densities. In this contribution, which complements other contributions
appearing in the current volume, we survey some of the constraints that have
been extracted recently concerning the parameters of the nuclear symmetry
energy. Isovector modes, in which neutrons and protons are in opposite phase,
are a natural source of information and we illustrate the values of symmetry
energy around saturation deduced from isovector dipole and isovector quadrupole
states. The isotopic dependence of the isoscalar monopole energy has also been
suggested to provide a connection to the symmetry energy: relevant theoretical
arguments and experimental results are thoroughly discussed. Finally, we
consider the case of the charge-exchange spin-dipole excitations in which the
sum rule associated with the total strength gives in principle access to the
neutron skin and thus, indirectly, to the symmetry energy.Comment: Updated version, with small corrections based on comments/suggestions
from the referee. 12 pages, 9 figures; submitted to EPJA "Special Issue on
Symmetry Energy
Doping a correlated band insulator: A new route to half metallic behaviour
We demonstrate in a simple model the surprising result that turning on an
on-site Coulomb interaction U in a doped band insulator leads to the formation
of a half-metallic state. In the undoped system, we show that increasing U
leads to a first order transition between a paramagnetic, band insulator and an
antiferomagnetic Mott insulator at a finite value U_{AF}. Upon doping, the
system exhibits half metallic ferrimagnetism over a wide range of doping and
interaction strengths on either side of U_{AF}. Our results, based on dynamical
mean field theory, suggest a novel route to half-metallic behavior and provide
motivation for experiments on new materials for spintronics.Comment: 5 pages, 7 figure
Phase Diagram of the Half-Filled Ionic Hubbard Model
We study the phase diagram of the ionic Hubbard model (IHM) at half-filling
using dynamical mean field theory (DMFT), with two impurity solvers, namely,
iterated perturbation theory (IPT) and continuous time quantum Monte Carlo
(CTQMC). The physics of the IHM is governed by the competition between the
staggered potential and the on-site Hubbard U. In both the methods we
find that for a finite and at zero temperature, anti-ferromagnetic
(AFM) order sets in beyond a threshold via a first order phase
transition below which the system is a paramagnetic band insulator. Both the
methods show a clear evidence for a transition to a half-metal phase just after
the AFM order is turned on, followed by the formation of an AFM insulator on
further increasing U. We show that the results obtained within both the methods
have good qualitative and quantitative consistency in the intermediate to
strong coupling regime. On increasing the temperature, the AFM order is lost
via a first order phase transition at a transition temperature within both the methods, for weak to intermediate values of U/t. But
in the strongly correlated regime, where the effective low energy Hamiltonian
is the Heisenberg model, IPT is unable to capture the thermal (Neel) transition
from the AFM phase to the paramagnetic phase, but the CTQMC does. As a result,
at any finite temperature T, DMFT+CTQMC shows a second phase transition (not
seen within DMFT+IPT) on increasing U beyond . At , when
the Neel temperature for the effective Heisenberg model becomes lower
than T, the AFM order is lost via a second order transition. In the
3-dimensonal parameter space of , there is a line of
tricritical points that separates the surfaces of first and second order phase
transitions.Comment: Revised versio
Can correlations drive a band insulator metallic?
We analyze the effects of the on-site Coulomb repulsion U on a band insulator
using dynamical mean field theory (DMFT). We find the surprising result that
the gap is suppressed to zero at a critical Uc1 and remains zero within a
metallic phase. At a larger Uc2 there is a second transition from the metal to
a Mott insulator, in which the gap increases with increasing U. These results
are qualitatively different from Hartree-Fock theory which gives a
monotonically decreasing but non-zero insulating gap for all finite U.Comment: 4 pages, 5 figure
Hygrothermal effects on mechanical behavior of graphite/epoxy laminates beyond initial failure
An investigation was conducted to determine the critical load levels and associated cracking beyond which a multidirectional laminate can be considered as structurally failed. Graphite/epoxy laminates were loaded to different strain levels up to ultimate failure. Transverse matrix cracking was monitored by acoustic and optical methods. Residual stiffness and strength that were parallel and perpendicular to the cracks were determined and related to the environmental/loading history. Results indicate that cracking density in the transverse layers has no major effect on laminate residual properties as long as the angle ply layers retain their structural integrity. Exposure to hot water revealed that cracking had only a small effect on absorption and reduced swelling when these specimens were compared with uncracked specimens. Cracked, moist specimens showed a moderate reduction in strength when compared with their uncracked counterparts. Within the range of environmental/loading conditions of the present study, it is concluded that the transverse cracking process is not crucial in its effect on the structural performance of multidirectional composite laminates
Quantum tunneling of magnetization in dipolar spin-1 condensates under external fields
We study the macroscopic quantum tunneling of magnetization of the F=1 spinor
condensate interacting through dipole-dipole interaction with an external
magnetic field applied along the longitudinal or transverse direction. We show
that the ground state energy and the effective magnetic moment of the system
exhibit an interesting macroscopic quantum oscillation phenomenon originating
from the oscillating dependence of thermodynamic properties of the system on
the vacuum angle. Tunneling between two degenerate minima are analyzed by means
of an effective potential method and the periodic instanton method.Comment: 2 figures, accepted PR
Effect of pairing correlations on incompressibility and symmetry energy in nuclear matter and finite nuclei
The role of superfluidity in the incompressibility and in the symmetry energy
is studied in nuclear matter and finite nuclei. Several pairing interactions
are used: surface, mixed and isovector dependent. Pairing has a small effect on
the nuclear matter incompressibility at saturation density, but the effects are
significant at lower densities. The pairing effect on the centroid energy of
the isoscalar Giant Monopole Resonance (GMR) is also evaluated for Pb and Sn
isotopes by using a microscopic constrained-HFB approach, and found to change
at most by 10% the nucleus incompressibility . It is shown by using the
Local Density Approximation (LDA) that most of the pairing effect on the GMR
centroid come from the low-density nuclear surface.Comment: 9 pages, 6 figure
Quantum Nucleation in a Ferromagnetic Film Placed in a Magnetic Field at an Arbitrary Angle
We study the quantum nucleation in a thin ferromagnetic film placed in a
magnetic field at an arbitrary angle. The dependence of the quantum nucleation
and the temperature of the crossover from thermal to quantum regime on the
direction and the strength of the applied field are presented. It is found that
the maximal value of the rate and that of the crossover temperature are
obtained at a some angle with the magnetic field, not in the direction of the
applied field opposite to the initial easy axis.Comment: 15 pages, RevTex, 3 PostScript figures. To appear in Phys. Rev.
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