3,644 research outputs found
Exact fluctuations of nonequilibrium steady states from approximate auxiliary dynamics
We describe a framework to significantly reduce the computational effort to
evaluate large deviation functions of time integrated observables within
nonequilibrium steady states. We do this by incorporating an auxiliary dynamics
into trajectory based Monte Carlo calculations, through a transformation of the
system's propagator using an approximate guiding function. This procedure
importance samples the trajectories that most contribute to the large deviation
function, mitigating the exponentially complexity of such calculations. We
illustrate the method by studying driven diffusions and interacting lattice
models in one and two dimensions. Our work offers an avenue to calculate large
deviation functions for high dimensional systems driven far from equilibrium.Comment: Accepted in Physical Review Letters (2018). v1: Main document: 5
pages, 3 figures. Supplementary information: 5 pages, 3 figures. v2: Main
document: 5 pages, 3 figures. Supplementary information: 6 pages, 3 figures.
Fixed some typos and notational inconsistencies. Expanded continuum tilted
operator derivation in supplementary sectio
Importance sampling large deviations in nonequilibrium steady states. I
Large deviation functions contain information on the stability and response
of systems driven into nonequilibrium steady states, and in such a way are
similar to free energies for systems at equilibrium. As with equilibrium free
energies, evaluating large deviation functions numerically for all but the
simplest systems is difficult, because by construction they depend on
exponentially rare events. In this first paper of a series, we evaluate
different trajectory-based sampling methods capable of computing large
deviation functions of time integrated observables within nonequilibrium steady
states. We illustrate some convergence criteria and best practices using a
number of different models, including a biased Brownian walker, a driven
lattice gas, and a model of self-assembly. We show how two popular methods for
sampling trajectory ensembles, transition path sampling and diffusion Monte
Carlo, suffer from exponentially diverging correlations in trajectory space as
a function of the bias parameter when estimating large deviation functions.
Improving the efficiencies of these algorithms requires introducing guiding
functions for the trajectories.Comment: Published in JC
Transformation Optics scheme for two-dimensional materials
Two dimensional optical materials, such as graphene can be characterized by a
surface conductivity. So far, the transformation optics schemes have focused on
three dimensional properties such as permittivity and permeability
. In this paper, we use a scheme for transforming surface currents to
highlight that the surface conductivity transforms in a way different from
and . We use this surface conductivity transformation to
demonstrate an example problem of reducing scattering of plasmon mode from
sharp protrusions in graphene
Embedding theory for excited states with inclusion of self-consistent environment screening
We present a general embedding theory of electronic excitations of a
relatively small, localized system in contact with an extended, chemically
complex environment. We demonstrate how to include the screening response of
the environment into highly accurate electronic structure calculation of the
localized system by means of an effective interaction between the electrons,
which contains only screening processes occurring in the environment. For the
common case of a localized system which constitutes an inhomogeneity in an
otherwise homogeneous system, such as a defect in a crystal, we show how matrix
elements of the environment-screened interaction can be calculated from
density-functional calculations of the homogeneous system only. We apply our
embedding theory to the calculation of excitation energies in crystalline
ethylene
Photon Emission Rate Engineering using Graphene Nanodisc Cavities
In this work, we present a systematic study of the plasmon modes in a system
of vertically stacked pair of graphene discs. Quasistatic approximation is used
to model the eigenmodes of the system. Eigen-response theory is employed to
explain the spatial dependence of the coupling between the plasmon modes and a
quantum emitter. These results show a good match between the semi-analytical
calculation and full-wave simulations. Secondly, we have shown that it is
possible to engineer the decay rates of a quantum emitter placed inside and
near this cavity, using Fermi level tuning, via gate voltages and variation of
emitter location and polarization. We highlighted that by coupling to the
bright plasmon mode, the radiative efficiency of the emitter can be enhanced
compared to the single graphene disc case, whereas the dark plasmon mode
suppresses the radiative efficiency
Negative Optical Torque
Maxwell noted that light carries angular momentum, and as such it can exert
torques on material objects. This was subsequently proved by Beth in 1936.
Applications of these opto-mechanical effects were limited initially due to
their smallness in magnitude, but later enabled by the invention of laser.
Novel and practical approaches for harvesting light for particle rotation have
been demonstrated, where the structure is subjected to a positive optical
torque along a certain axis21 if the incident angular momentum has a positive
projection on the same axis. We report here a counter-intuitive phenomenon of
negative optical torque, meaning that incoming photons carrying angular
momentum rotate an object in the opposite sense. Surprisingly this can be
realized quite straightforwardly in simple planar structures. Field retardation
is a necessary condition. The optimal conditions are explored and explained.Comment: 15 pages, 3 figure
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