13,177 research outputs found
Shielding and localization in presence of long range hopping
We investigate a paradigmatic model for quantum transport with both
nearest-neighbor and infinite range hopping coupling (independent of the
position). Due to long range homogeneous hopping, a gap between the ground
state and the excited states can be induced, which is mathematically equivalent
to the superconducting gap. In the gapped regime, the dynamics within the
excited states subspace is shielded from long range hopping, namely it occurs
as if long range hopping would be absent. This is a cooperative phenomenon
since shielding is effective over a time scale which diverges with the system
size. We named this effect {\it Cooperative Shielding}. We also discuss the
consequences of our findings on Anderson localization. Long range hopping is
usually thought to destroy localization due to the fact that it induces an
infinite number of resonances. Contrary to this common lore we show that the
excited states display strong localized features when shielding is effective
even in the regime of strong long range coupling. A brief discussion on the
extension of our results to generic power-law decaying long range hopping is
also given. Our preliminary results confirms that the effects found for the
infinite range case are generic.Comment: 7 pages, 9 figur
Multimodal imaging of human brain activity: rational, biophysical aspects and modes of integration
Until relatively recently the vast majority of imaging and electrophysiological studies of human brain activity have relied on single-modality measurements usually correlated with readily observable or experimentally modified behavioural or brain state patterns. Multi-modal imaging is the concept of bringing together observations or measurements from different instruments. We discuss the aims of multi-modal imaging and the ways in which it can be accomplished using representative applications. Given the importance of haemodynamic and electrophysiological signals in current multi-modal imaging applications, we also review some of the basic physiology relevant to understanding their relationship
Electromagnetic inertia, reactive energy, and energy flow velocity
In a recent paper titled "Coherent electromagnetic wavelets and their
twisting null congruences," I defined the local inertia density (I), reactive
energy density (R), and energy flow velocity (v) of an electromagnetic field.
These are the field equivalents of the mass, rest energy, and velocity of a
relativistic particle. Thus R and I are Lorentz-invariant and |v|<=c, with
equality if and only if R=0. The exceptional fields with |v|=c were called
"coherent" because their energy moves in complete harmony with the field,
leaving no inertia or reactive energy behind. Generic electromagnetic fields
become coherent only in the far zone. Elsewhere, their energy flows at speeds
|v|<c. The purpose of this paper is to confirm and clarify this statement by
studying the local energy flow in several common systems: a time-harmonic
electric dipole field, a time-dependent electric dipole field, and a standing
plane wave. For these fields, the energy current (Poynting vector) is too weak
to carry away all of the energy, thus leaving reactive energy in its wake. For
the time-dependent dipole field, we find that the energy can flow both
transversally and inwards, back to the source. Neither of these phenomena show
up in the usual computation of the energy transport velocity which considers
only averages over one period in the time-harmonic case.Comment: 20 pages, 7 figure
Helicity, polarization, and Riemann-Silberstein vortices
Riemann-Silberstein (RS) vortices have been defined as surfaces in spacetime
where the complex form of a free electromagnetic field given by F=E+iB is null
(F.F=0), and they can indeed be interpreted as the collective history swept out
by moving vortex lines of the field. Formally, the nullity condition is similar
to the definition of "C-lines" associated with a monochromatic electric or
magnetic field, which are curves in space where the polarization ellipses
degenerate to circles. However, it was noted that RS vortices of monochromatic
fields generally oscillate at optical frequencies and are therefore
unobservable while electric and magnetic C-lines are steady. Here I show that
under the additional assumption of having definite helicity, RS vortices are
not only steady but they coincide with both sets of C-lines, electric and
magnetic. The two concepts therefore become one for waves of definite frequency
and helicity. Since the definition of RS vortices is relativistically invariant
while that of C-lines is not, it may be useful to regard the vortices as a
wideband generalization of C-lines for waves of definite helicity.Comment: 5 pages, no figures. Submitted to J of Optics A, special issue on
Singular Optics; minor changes from v.
Radiative corrections to neutral pion-pair production
We calculate the one-photon loop radiative corrections to the neutral
pion-pair photoproduction process . At leading
order this reaction is governed by the chiral pion-pion interaction. Since the
chiral contact-vertex depends only on the final-state
invariant-mass it factors out of all photon-loop diagrams. We give analytical
expressions for the multiplicative correction factor
arising from eight classes of contributing one-photon loop diagrams. An
electromagnetic counterterm has to be included in order to cancel the
ultraviolet divergences generated by the photon-loops. Infrared finiteness of
the virtual radiative corrections is achieved (in the standard way) by
including soft photon radiation below an energy cut-off . The
radiative corrections to the total cross section vary between and
for center-of-mass energies from threshold up to . The finite part of
the electromagnetic counterterm gives an additional constant contribution of
about , however with a large uncertainty.Comment: 10 pages, 6 figures, submitted to Eur. Phys. J.
Optical pattern formation with a 2-level nonlinearity
We present an experimental and theoretical investigation of spontaneous
pattern formation in the transverse section of a single retro-reflected laser
beam passing through a cloud of cold Rubidium atoms. In contrast to previously
investigated systems, the nonlinearity at work here is that of a 2-level atom,
which realizes the paradigmatic situation considered in many theoretical
studies of optical pattern formation. In particular, we are able to observe the
disappearance of the patterns at high intensity due to the intrinsic saturable
character of 2-level atomic transitions.Comment: 5 pages, 4 figure
Magnetic Field Tomography
Neutral atoms may be trapped via the interaction of their magnetic dipole
moment with magnetic field gradients. One of the possible schemes is the
cloverleaf trap. It is often desirable to have at hand a fast and precise
technique for measuring the magnetic field distribution. We introduce a novel
diagnostic tool for instantaneous imaging the equipotential lines of a magnetic
field within a region of space (the vacuum recipient) that is not accessible to
massive probes. Our technique is based on spatially resolved observation of the
fluorescence emitted by a hot beam of sodium atoms crossing a thin slice of
resonant laser light within the magnetic field region to be investigated. The
inhomogeneous magnetic field spatially modulates the resonance condition
between the Zeeman-shifted hyperfine sublevels and the laser light and
therefore the amount of scattered photons. We demonstrate this technique by
mapping the field of our cloverleaf trap in three dimensions under various
conditions.Comment: 8 pages, 8 figure
Cluster-based feedback control of turbulent post-stall separated flows
We propose a novel model-free self-learning cluster-based control strategy
for general nonlinear feedback flow control technique, benchmarked for
high-fidelity simulations of post-stall separated flows over an airfoil. The
present approach partitions the flow trajectories (force measurements) into
clusters, which correspond to characteristic coarse-grained phases in a
low-dimensional feature space. A feedback control law is then sought for each
cluster state through iterative evaluation and downhill simplex search to
minimize power consumption in flight. Unsupervised clustering of the flow
trajectories for in-situ learning and optimization of coarse-grained control
laws are implemented in an automated manner as key enablers. Re-routing the
flow trajectories, the optimized control laws shift the cluster populations to
the aerodynamically favorable states. Utilizing limited number of sensor
measurements for both clustering and optimization, these feedback laws were
determined in only iterations. The objective of the present work is not
necessarily to suppress flow separation but to minimize the desired cost
function to achieve enhanced aerodynamic performance. The present control
approach is applied to the control of two and three-dimensional separated flows
over a NACA 0012 airfoil with large-eddy simulations at an angle of attack of
, Reynolds number and free-stream Mach number . The optimized control laws effectively minimize the flight power
consumption enabling the flows to reach a low-drag state. The present work aims
to address the challenges associated with adaptive feedback control design for
turbulent separated flows at moderate Reynolds number.Comment: 32 pages, 18 figure
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