2,899 research outputs found
Exponential Krylov time integration for modeling multi-frequency optical response with monochromatic sources
Light incident on a layer of scattering material such as a piece of sugar or
white paper forms a characteristic speckle pattern in transmission and
reflection. The information hidden in the correlations of the speckle pattern
with varying frequency, polarization and angle of the incident light can be
exploited for applications such as biomedical imaging and high-resolution
microscopy. Conventional computational models for multi-frequency optical
response involve multiple solution runs of Maxwell's equations with
monochromatic sources. Exponential Krylov subspace time solvers are promising
candidates for improving efficiency of such models, as single monochromatic
solution can be reused for the other frequencies without performing full
time-domain computations at each frequency. However, we show that the
straightforward implementation appears to have serious limitations. We further
propose alternative ways for efficient solution through Krylov subspace
methods. Our methods are based on two different splittings of the unknown
solution into different parts, each of which can be computed efficiently.
Experiments demonstrate a significant gain in computation time with respect to
the standard solvers.Comment: 22 pages, 4 figure
Causality and dispersion relations and the role of the S-matrix in the ongoing research
The adaptation of the Kramers-Kronig dispersion relations to the causal
localization structure of QFT led to an important project in particle physics,
the only one with a successful closure. The same cannot be said about the
subsequent attempts to formulate particle physics as a pure S-matrix project.
The feasibility of a pure S-matrix approach are critically analyzed and their
serious shortcomings are highlighted. Whereas the conceptual/mathematical
demands of renormalized perturbation theory are modest and misunderstandings
could easily be corrected, the correct understanding about the origin of the
crossing property requires the use of the mathematical theory of modular
localization and its relation to the thermal KMS condition. These new concepts,
which combine localization, vacuum polarization and thermal properties under
the roof of modular theory, will be explained and their potential use in a new
constructive (nonperturbative) approach to QFT will be indicated. The S-matrix
still plays a predominant role but, different from Heisenberg's and
Mandelstam's proposals, the new project is not a pure S-matrix approach. The
S-matrix plays a new role as a "relative modular invariant"..Comment: 47 pages expansion of arguments and addition of references,
corrections of misprints and bad formulation
Shot noise in non-adiabatically driven nanoscale conductors
We investigate the noise properties of pump currents through molecular wires
and coupled quantum dots. As a model we employ a two level system that is
connected to electron reservoirs and is non-adiabatically driven. Concerning
the electron-electron interaction, we focus on two limits: non-interacting
electrons and strong Coulomb repulsion. While the former case is treated within
a Floquet scattering formalism, we derive for the latter case a master equation
formalism for the computation of the current and the zero-frequency noise. For
a pump operated close to internal resonances, the differences between the
non-interacting and the strongly interacting limit turn out to be surprisingly
small.Comment: 17 pages, 2 figure
Thermoelectric efficiency has three Degrees of Freedom
Thermal energy can be directly converted to electrical energy as a result of
thermoelectric effects. Because this conversion realises clean energy
technology, such as waste heat recovery and energy harvesting, substantial
efforts have been made to search for thermoelectric materials. Under the belief
that the material figure of merit represents the energy conversion
efficiencies of thermoelectric devices, various high peak- materials have
been explored for half a century. However, thermoelectric properties vary
greatly with temperature , so the single value does not represent
device efficiency accurately. Here we show that the efficiency of
thermoelectric conversion is completely determined by \emph{three} parameters
, , and , which we call the \emph{thermoelectric
degrees of freedom}. The , which is an average of material
properties, is a generalisation of the traditional figure of merit. The
and , which reflect the gradients of the material properties, are
proportional to escaped heat caused by the Thomson effect and asymmetric Joule
heat, respectively. Our finding proposes new directions for achieving high
thermoelectric efficiency; increasing one of the thermoelectric degrees of
freedom results in higher efficiency. For example, thermoelectric efficiency
can be enhanced up to 176\% by tuning the thermoelectric degrees of freedom in
segmented legs, compared to the best efficiency of single-material legs.Comment: main articles with 9 pages, 4 figures, supplementary information with
35 pages, 9 figures, 6 table
Full Counting Statistics and Field Theory
We review the relations between the full counting statistics and the field
theory of electric circuits. We demonstrate that for large conductances the
counting statistics is determined by non-trivial saddle-point of the field.
Coulomb effects in this limit are presented as quantum corrections that can
stongly renormalize the action at low energies.Comment: microreview, 15 pages, accepted to Ann. Phys. (Leipzig
Effect of resonant magnetic perturbations on low collisionality discharges in MAST and a comparison with ASDEX Upgrade
Sustained ELM mitigation has been achieved on MAST and AUG using RMPs with a
range of toroidal mode numbers over a wide region of low to medium
collisionality discharges. The ELM energy loss and peak heat loads at the
divertor targets have been reduced. The ELM mitigation phase is typically
associated with a drop in plasma density and overall stored energy. In one
particular scenario on MAST, by carefully adjusting the fuelling it has been
possible to counteract the drop in density and to produce plasmas with
mitigated ELMs, reduced peak divertor heat flux and with minimal degradation in
pedestal height and confined energy. While the applied resonant magnetic
perturbation field can be a good indicator for the onset of ELM mitigation on
MAST and AUG there are some cases where this is not the case and which clearly
emphasise the need to take into account the plasma response to the applied
perturbations. The plasma response calculations show that the increase in ELM
frequency is correlated with the size of the edge peeling-tearing like response
of the plasma and the distortions of the plasma boundary in the X-point region.Comment: 31 pages, 28 figures. This is an author-created, un-copyedited
version of an article submitted for publication in Nuclear Fusion. IoP
Publishing Ltd is not responsible for any errors or omissions in this version
of the manuscript or any version derived from i
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