1,349 research outputs found
Electronic Excitations and Correlation Effects in Metals
Theoretical descriptions of the spectrum of electronic excitations in real
metals have not yet reached a fully predictive, "first-principles" stage. In
this paper we begin by presenting brief highlights of recent progress made in
the evaluation of dynamical electronic response in metals. A comparison between
calculated and measured spectra - we use the loss spectra of Al and Cs as test
cases - leads us to the conclusion that, even in "weakly-correlated" metals,
correlation effects beyond mean-field theory play an important role.
Furthermore, the effects of the underlying band structure turn out to be
significant. Calculations which incorporate the effects of both dynamical
correlations and band structure from first principles are not yet available. As
a first step towards such goal, we outline a numerical algorithm for the
self-consistent solution of the Dyson equation for the one-particle Green's
function. The self-energy is evaluated within the shielded-interaction
approximation of Baym and Kadanoff. Our method, which is fully conserving, is a
finite-temperature scheme which determines the Green's function and the
self-energy at the Matsubara frequencies on the imaginary axis. The analytical
continuation to real frequencies is performed via Pade approximants. We present
results for the homogeneous electron gas which exemplify the importance of
many-body self-consistency.Comment: 32 pages, 6 figures; "Fifty Years of the Correlation Problem",
invited paper, to be published in Mol.Phy
Evolution of Cooperative Networks and the Emergence of Leadership
A generic property of biological, social and economical networks is their ability to evolve in time, creating or supressing links. We model this situation with an adaptive network of agents playing a Prisoner's Dilemma game. Each agent plays with its local neighbors, collects an aggregate payoff and imitates the strategy of its best neighbor. Furthermore we allow the agents adapt their local neighborhood according to their satisfaction level and the strategy played. Therefore each agent will have diverse environments that induces an interesting dynamics in the cooperation fraction of the whole network. In the absence of noise, a steady state is always reached, where the strategies and the neighborhoods remain stationary, and where for a wide range of parameter values, an almost full cooperative outcome is obtained. The topology of the network in these states reveals that cooperators with a large number of connections emerges. These "leaders" are shown to be very important in understanding the global stability of the final steady state. If the "leaders" are perturbated, then global cascades arise and the system oscillates between the nearly full defection network and the fully cooperative outcome, before settling again in a nearly fully cooperative outcome.Cooperation -- Evolutionary Game Theory -- Stochastic Networks -- Prisoner Dilemma
Absorbing and Shattered Fragmentation Transitions in Multilayer Coevolution
We introduce a coevolution voter model in a multilayer, by coupling a
fraction of nodes across two network layers and allowing each layer to evolve
according to its own topological temporal scale. When these time scales are the
same the dynamics preserve the absorbing-fragmentation transition observed in a
monolayer network at a critical value of the temporal scale that depends on
interlayer connectivity. The time evolution equations obtained by pair
approximation can be mapped to a coevolution voter model in a single layer with
an effective average degree. When the two layers have different topological
time scales we find an anomalous transition, named shattered fragmentation, in
which the network in one layer splits into two large components in opposite
states and a multiplicity of isolated nodes. We identify the growth of the
number of components as a signature of this anomalous transition. We also find
a critical level of interlayer coupling needed to prevent the fragmentation in
a layer connected to a layer that does not fragment.Comment: 7 pages, 6 figures, last figure caption includes link to animation
Noise in Coevolving Networks
Coupling dynamics of the states of the nodes of a network to the dynamics of
the network topology leads to generic absorbing and fragmentation transitions.
The coevolving voter model is a typical system that exhibits such transitions
at some critical rewiring. We study the robustness of these transitions under
two distinct ways of introducing noise. Noise affecting all the nodes destroys
the absorbing-fragmentation transition, giving rise in finite-size systems to
two regimes: bimodal magnetisation and dynamic fragmentation. Noise Targeting a
fraction of nodes preserves the transitions but introduces shattered
fragmentation with its characteristic fraction of isolated nodes and one or two
giant components. Both the lack of absorbing state for homogenous noise and the
shift in the absorbing transition to higher rewiring for targeted noise are
supported by analytical approximations.Comment: 20 page
Data-driven modeling of systemic delay propagation under severe meteorological conditions
The upsetting consequences of weather conditions are well known to any person
involved in air transportation. Still the quantification of how these
disturbances affect delay propagation and the effectiveness of managers and
pilots interventions to prevent possible large-scale system failures needs
further attention. In this work, we employ an agent-based data-driven model
developed using real flight performance registers for the entire US airport
network and focus on the events occurring on October 27 2010 in the United
States. A major storm complex that was later called the 2010 Superstorm took
place that day. Our model correctly reproduces the evolution of the
delay-spreading dynamics. By considering different intervention measures, we
can even improve the model predictions getting closer to the real delay data.
Our model can thus be of help to managers as a tool to assess different
intervention measures in order to diminish the impact of disruptive conditions
in the air transport system.Comment: 9 pages, 5 figures. Tenth USA/Europe Air Traffic Management Research
and Development Seminar (ATM2013
The Band-Gap Problem in Semiconductors Revisited: Effects of Core States and Many-Body Self-Consistency
A novel picture of the quasiparticle (QP) gap in prototype semiconductors Si
and Ge emerges from an analysis based on all-electron, self-consistent, GW
calculations. The deep-core electrons are shown to play a key role via the
exchange diagram --if this effect is neglected, Si becomes a semimetal.
Contrary to current lore, the Ge 3d semicore states (e.g., their polarization)
have no impact on the GW gap. Self-consistency improves the calculated gaps --a
first clear-cut success story for the Baym-Kadanoff method in the study of
real-materials spectroscopy; it also has a significant impact on the QP
lifetimes. Our results embody a new paradigm for ab initio QP theory
- …