4,719 research outputs found
Canonical quantization of non-local field equations
We consistently quantize a class of relativistic non-local field equations
characterized by a non-local kinetic term in the lagrangian. We solve the
classical non-local equations of motion for a scalar field and evaluate the
on-shell hamiltonian. The quantization is realized by imposing Heisenberg's
equation which leads to the commutator algebra obeyed by the Fourier components
of the field. We show that the field operator carries, in general, a reducible
representation of the Poincare group. We also consider the Gupta-Bleuler
quantization of a non-local gauge field and analyze the propagators and the
physical states of the theory.Comment: 18 p., LaTe
A first order Tsallis theory
We investigate first-order approximations to both i) Tsallis' entropy
and ii) the -MaxEnt solution (called q-exponential functions ). It is
shown that the functions arising from the procedure ii) are the MaxEnt
solutions to the entropy emerging from i). The present treatment is free of the
poles that, for classic quadratic Hamiltonians, appear in Tsallis' approach, as
demonstrated in [Europhysics Letters {\bf 104}, (2013), 60003]. Additionally,
we show that our treatment is compatible with extant date on the ozone layer.Comment: 4 figures adde
Exciton-phonon scattering and photo-excitation dynamics in J-aggregate microcavities
We have developed a model accounting for the photo-excitation dynamics and
the photoluminescence of strongly coupled J-aggregate microcavities. Our model
is based on a description of the J-aggregate film as a disordered Frenkel
exciton system in which relaxation occurs due to the presence of a thermal bath
of molecular vibrations. In a strongly coupled microcavity exciton-polaritons
are formed, mixing superradiant excitons and cavity photons. The calculation of
the microcavity steady-state photoluminescence, following a CW non resonant
pumping, is carried out. The experimental photoluminescence intensity ratio
between upper and lower polariton branches is accurately reproduced. In
particular both thermal activation of the photoluminescence intensity ratio and
its Rabi splitting dependence are a consequence of the bottleneck in the
relaxation, occurring at the bottom of the excitonic reservoir. The effects due
to radiative channels of decay of excitons and to the presence of a
paritticular set of discrete optical molecular vibrations active in relaxation
processes are investigared.Comment: 8 pages, 6 figure
Role of anisotropy in the F\"orster energy transfer from a semiconductor quantum well to an organic crystalline overlayer
We consider the non-radiative resonant energy transfer from a two-dimensional
Wannier exciton (donor) to a Frenkel exciton of a molecular crystal overlayer
(acceptor). We characterize the effect of the optical anisotropy of the organic
subsystem on this process. Using realistic values of material parameters, we
show that it is possible to change the transfer rate within typically a factor
of two depending on the orientation of the crystalline overlayer. The resonant
matching of donor and acceptor energies is also partly tunable via the organic
crystal orientation.Comment: 6 pages, 8 figure
Measurement of the current-phase relation of superconducting atomic contacts
We have probed the current-phase relation of an atomic contact placed with a
tunnel junction in a small superconducting loop. The measurements are in
quantitative agreement with the predictions of a resistively shunted SQUID
model in which the Josephson coupling of the contact is calculated using the
independently determined transmissions of its conduction channels.Comment: to be published in Physical Review Letter
Detection of brain functional-connectivity difference in post-stroke patients using group-level covariance modeling
Functional brain connectivity, as revealed through distant correlations in
the signals measured by functional Magnetic Resonance Imaging (fMRI), is a
promising source of biomarkers of brain pathologies. However, establishing and
using diagnostic markers requires probabilistic inter-subject comparisons.
Principled comparison of functional-connectivity structures is still a
challenging issue. We give a new matrix-variate probabilistic model suitable
for inter-subject comparison of functional connectivity matrices on the
manifold of Symmetric Positive Definite (SPD) matrices. We show that this model
leads to a new algorithm for principled comparison of connectivity coefficients
between pairs of regions. We apply this model to comparing separately
post-stroke patients to a group of healthy controls. We find
neurologically-relevant connection differences and show that our model is more
sensitive that the standard procedure. To the best of our knowledge, these
results are the first report of functional connectivity differences between a
single-patient and a group and thus establish an important step toward using
functional connectivity as a diagnostic tool
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