17,341 research outputs found
Floquet band structure of a semi-Dirac system
In this work we use Floquet-Bloch theory to study the influence of circularly
and linearly polarized light on two-dimensional band structures with semi-Dirac
band touching points, taking the anisotropic nearest neighbor hopping model on
the honeycomb lattice as an example. We find circularly polarized light opens a
gap and induces a band inversion to create a finite Chern number in the
two-band model. By contrast, linearly polarized light can either open up a gap
(polarized in the quadratically dispersing direction) or split the semi-Dirac
band touching point into two Dirac points (polarized in the linearly dispersing
direction) by an amount that depends on the amplitude of the light. Motivated
by recent pump-probe experiments, we investigated the non-equilibrium spectral
properties and momentum-dependent spin-texture of our model in the Floquet
state following a quench in absence of phonons, and in the presence of phonon
dissipation that leads to a steady-state independent of the pump protocol.
Finally, we make connections to optical measurements by computing the frequency
dependence of the longitudinal and transverse optical conductivity for this
two-band model. We analyze the various contributions from inter-band
transitions and different Floquet modes. Our results suggest strategies for
optically controlling band structures and experimentally measuring topological
Floquet systems.Comment: 17 pages, 8 figure
Designing Network Protocols for Good Equilibria
Designing and deploying a network protocol determines the rules by which end users interact with each other and with the network. We consider the problem of designing a protocol to optimize the equilibrium behavior of a network with selfish users. We consider network cost-sharing games, where the set of Nash equilibria depends fundamentally on the choice of an edge cost-sharing protocol. Previous research focused on the Shapley protocol, in which the cost of each edge is shared equally among its users. We systematically study the design of optimal cost-sharing protocols for undirected and directed graphs, single-sink and multicommodity networks, and different measures of the inefficiency of equilibria. Our primary technical tool is a precise characterization of the cost-sharing protocols that induce only network games with pure-strategy Nash equilibria. We use this characterization to prove, among other results, that the Shapley protocol is optimal in directed graphs and that simple priority protocols are essentially optimal in undirected graphs
Authentication with Distortion Criteria
In a variety of applications, there is a need to authenticate content that
has experienced legitimate editing in addition to potential tampering attacks.
We develop one formulation of this problem based on a strict notion of
security, and characterize and interpret the associated information-theoretic
performance limits. The results can be viewed as a natural generalization of
classical approaches to traditional authentication. Additional insights into
the structure of such systems and their behavior are obtained by further
specializing the results to Bernoulli and Gaussian cases. The associated
systems are shown to be substantially better in terms of performance and/or
security than commonly advocated approaches based on data hiding and digital
watermarking. Finally, the formulation is extended to obtain efficient layered
authentication system constructions.Comment: 22 pages, 10 figure
Is the Kelvin Theorem Valid for High-Reynolds-Number Turbulence?
The Kelvin-Helmholtz theorem on conservation of circulations is supposed to
hold for ideal inviscid fluids and is believed to be play a crucial role in
turbulent phenomena, such as production of dissipation by vortex
line-stretching. However, this expectation does not take into account
singularities in turbulent velocity fields at infinite Reynolds number. We
present evidence from numerical simulations for the breakdown of the classical
Kelvin theorem in the three-dimensional turbulent energy cascade. Although
violated in individual realizations, we find that circulations are still
conserved in some average sense. For comparison, we show that Kelvin's theorem
holds for individual realizations in the two-dimensional enstrophy cascade, in
agreement with theory. The turbulent ``cascade of circulations'' is shown to be
a classical analogue of phase-slip due to quantized vortices in superfluids and
various applications in geophysics and astrophysics are outlined.Comment: 4 pages, 3 figure
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