306 research outputs found
The Mott Metal-Insulator transition in the half-filled Hubbard model on the Triangular Lattice
We investigate the metal-insulator transition in the half-filled Hubbard
model on a two-dimensional triangular lattice using both the
Kotliar-Ruckenstein slave-boson technique, and exact numerical diagonalization
of finite clusters. Contrary to the case of the square lattice, where the
perfect nesting of the Fermi surface leads to a metal-insulator transition at
arbitrarily small values of U, always accompanied by antiferromagnetic
ordering, on the triangular lattice, due to the lack of perfect nesting, the
transition takes place at a finite value of U, and frustration induces a
non-trivial competition among different magnetic phases. Indeed, within the
mean-field approximation in the slave-boson approach, as the interaction grows
the paramagnetic metal turns into a metallic phase with incommensurate spiral
ordering. Increasing further the interaction, a linear spin-density-wave is
stabilized, and finally for strong coupling the latter phase undergoes a
first-order transition towards an antiferromagnetic insulator. No trace of the
intermediate phases is instead seen in the exact diagonalization results,
indicating a transition between a paramagnetic metal and an antiferromagnetic
insulator.Comment: 5 pages, 4 figure
Traffic Management Applications for Stateful SDN Data Plane
The successful OpenFlow approach to Software Defined Networking (SDN) allows
network programmability through a central controller able to orchestrate a set
of dumb switches. However, the simple match/action abstraction of OpenFlow
switches constrains the evolution of the forwarding rules to be fully managed
by the controller. This can be particularly limiting for a number of
applications that are affected by the delay of the slow control path, like
traffic management applications. Some recent proposals are pushing toward an
evolution of the OpenFlow abstraction to enable the evolution of forwarding
policies directly in the data plane based on state machines and local events.
In this paper, we present two traffic management applications that exploit a
stateful data plane and their prototype implementation based on OpenState, an
OpenFlow evolution that we recently proposed.Comment: 6 pages, 9 figure
SPIDER: Fault Resilient SDN Pipeline with Recovery Delay Guarantees
When dealing with node or link failures in Software Defined Networking (SDN),
the network capability to establish an alternative path depends on controller
reachability and on the round trip times (RTTs) between controller and involved
switches. Moreover, current SDN data plane abstractions for failure detection
(e.g. OpenFlow "Fast-failover") do not allow programmers to tweak switches'
detection mechanism, thus leaving SDN operators still relying on proprietary
management interfaces (when available) to achieve guaranteed detection and
recovery delays. We propose SPIDER, an OpenFlow-like pipeline design that
provides i) a detection mechanism based on switches' periodic link probing and
ii) fast reroute of traffic flows even in case of distant failures, regardless
of controller availability. SPIDER can be implemented using stateful data plane
abstractions such as OpenState or Open vSwitch, and it offers guaranteed short
(i.e. ms) failure detection and recovery delays, with a configurable trade off
between overhead and failover responsiveness. We present here the SPIDER
pipeline design, behavioral model, and analysis on flow tables' memory impact.
We also implemented and experimentally validated SPIDER using OpenState (an
OpenFlow 1.3 extension for stateful packet processing), showing numerical
results on its performance in terms of recovery latency and packet losses.Comment: 8 page
Robust Energy Management for Green and Survivable IP Networks
Despite the growing necessity to make Internet greener, it is worth pointing
out that energy-aware strategies to minimize network energy consumption must
not undermine the normal network operation. In particular, two very important
issues that may limit the application of green networking techniques concern,
respectively, network survivability, i.e. the network capability to react to
device failures, and robustness to traffic variations. We propose novel
modelling techniques to minimize the daily energy consumption of IP networks,
while explicitly guaranteeing, in addition to typical QoS requirements, both
network survivability and robustness to traffic variations. The impact of such
limitations on final network consumption is exhaustively investigated. Daily
traffic variations are modelled by dividing a single day into multiple time
intervals (multi-period problem), and network consumption is reduced by putting
to sleep idle line cards and chassis. To preserve network resiliency we
consider two different protection schemes, i.e. dedicated and shared
protection, according to which a backup path is assigned to each demand and a
certain amount of spare capacity has to be available on each link. Robustness
to traffic variations is provided by means of a specific modelling framework
that allows to tune the conservatism degree of the solutions and to take into
account load variations of different magnitude. Furthermore, we impose some
inter-period constraints necessary to guarantee network stability and preserve
the device lifetime. Both exact and heuristic methods are proposed.
Experimentations carried out with realistic networks operated with flow-based
routing protocols (i.e. MPLS) show that significant savings, up to 30%, can be
achieved also when both survivability and robustness are fully guaranteed
Energy management in communication networks: a journey through modelling and optimization glasses
The widespread proliferation of Internet and wireless applications has
produced a significant increase of ICT energy footprint. As a response, in the
last five years, significant efforts have been undertaken to include
energy-awareness into network management. Several green networking frameworks
have been proposed by carefully managing the network routing and the power
state of network devices.
Even though approaches proposed differ based on network technologies and
sleep modes of nodes and interfaces, they all aim at tailoring the active
network resources to the varying traffic needs in order to minimize energy
consumption. From a modeling point of view, this has several commonalities with
classical network design and routing problems, even if with different
objectives and in a dynamic context.
With most researchers focused on addressing the complex and crucial
technological aspects of green networking schemes, there has been so far little
attention on understanding the modeling similarities and differences of
proposed solutions. This paper fills the gap surveying the literature with
optimization modeling glasses, following a tutorial approach that guides
through the different components of the models with a unified symbolism. A
detailed classification of the previous work based on the modeling issues
included is also proposed
Antiferromagnetism and the gap of a Mott insulator: Results from analytic continuation of the self-energy
Direct analytic continuation of the self energy is used to determine the
effect of antiferromagnetic ordering on the spectral function and optical
conductivity of a Mott insulator. Comparison of several methods shows that the
most robust estimation of the gap value is obtained by use of the real part of
the continued self energy in the quasiparticle equation within the single-site
dynamical mean field theory of the two dimensional square lattice Hubbard
model, where for U slightly greater than the Mott critical value,
antiferromagnetism increases the gap by about 80%.Comment: 8 pages, 9 figures. An error in normalization of optical conductivity
(Fig. 9) corrected. to appear in Phys. Rev.
Effect of anisotropy on the onset of convection in rotating bi-disperse Brinkman porous media
AbstractThermal convection in a horizontally isotropic bi-disperse porous medium (BDPM) uniformly heated from below is analysed. The combined effects of uniform vertical rotation and Brinkman law on the stability of the steady state of the momentum equations in a BDPM are investigated. Linear and nonlinear stability analysis of the conduction solution is performed, and the coincidence between linear instability and nonlinear stability thresholds in theL2-norm is obtained
Accelerating cosmology in Rastall's theory
In an attempt to look for a viable mechanism leading to a present-day
accelerated expansion, we investigate the possibility that the observed cosmic
speed up may be recovered in the framework of the Rastall's theory, relying on
the non-conservativity of the stress-energy tensor, i.e. . We derive the modified Friedmann equations and show that they
correspond to Cardassian-like equations. We also show that, under suitable
assumptions on the equation of state of the matter term sourcing the
gravitational field, it is indeed possible to get an accelerated expansion, in
agreement with the Hubble diagram of both Type Ia Supernovae (SNeIa) and Gamma
Ray Bursts (GRBs). Unfortunately, to achieve such a result one has to postulate
a matter density parameter much larger than the typical
value inferred from cluster gas mass fraction data.Comment: 8 pages, 1 eps figure; revised to match the version accepted for
publication in Il Nuovo Cimento
Analysis of a model for waterborne diseases with Allee effect on bacteria
A limitation of current modeling studies in waterborne diseases (one of the leading causes of death worldwide) is that the intrinsic dynamics of the pathogens is poorly addressed, leading to incomplete, and often, inadequate understanding of the pathogen evolution and its impact on disease transmission and spread. To overcome these limitations, in this paper, we consider an ODEs model with bacterial growth inducing Allee effect. We adopt an adequate functional response to significantly express the shape of indirect transmission. The existence and stability of biologically meaningful equilibria is investigated through a detailed discussion of both backward and Hopf bifurcations. The sensitivity analysis of the basic reproduction number is performed. Numerical simulations confirming the obtained results in two different scenarios are shown
Orbital-Selective Mott transition out of band degeneracy lifting
We outline a general mechanism for Orbital-selective Mott transition (OSMT),
the coexistence of both itinerant and localized conduction electrons, and show
how it can take place in a wide range of realistic situations, even for bands
of identical width and correlation, provided a crystal field splits the energy
levels in manifolds with different degeneracies and the exchange coupling is
large enough to reduce orbital fluctuations. The mechanism relies on the
different kinetic energy in manifolds with different degeneracy. This phase has
Curie-Weiss susceptibility and non Fermi-liquid behavior, which disappear at a
critical doping, all of which is reminiscent of the physics of the pnictides.Comment: Published versio
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