10,907 research outputs found
Rigidity of compact Riemannian spin Manifolds with Boundary
In this article, we prove new rigidity results for compact Riemannian spin
manifolds with boundary whose scalar curvature is bounded from below by a
non-positive constant. In particular, we obtain generalizations of a result of
Hang-Wang \cite{hangwang1} based on a conjecture of Schroeder and Strake
\cite{schroeder}.Comment: English version of "G\'eom\'etrie spinorielle extrins\`eque et
rigidit\'es", Corollary 6 in Section 3 added, to appear in Letters Math. Phy
Self-organized critical neural networks
A mechanism for self-organization of the degree of connectivity in model
neural networks is studied. Network connectivity is regulated locally on the
basis of an order parameter of the global dynamics which is estimated from an
observable at the single synapse level. This principle is studied in a
two-dimensional neural network with randomly wired asymmetric weights. In this
class of networks, network connectivity is closely related to a phase
transition between ordered and disordered dynamics. A slow topology change is
imposed on the network through a local rewiring rule motivated by
activity-dependent synaptic development: Neighbor neurons whose activity is
correlated, on average develop a new connection while uncorrelated neighbors
tend to disconnect. As a result, robust self-organization of the network
towards the order disorder transition occurs. Convergence is independent of
initial conditions, robust against thermal noise, and does not require fine
tuning of parameters.Comment: 5 pages RevTeX, 7 figures PostScrip
Quantum Thermodynamic Cycles and quantum heat engines
In order to describe quantum heat engines, here we systematically study
isothermal and isochoric processes for quantum thermodynamic cycles. Based on
these results the quantum versions of both the Carnot heat engine and the Otto
heat engine are defined without ambiguities. We also study the properties of
quantum Carnot and Otto heat engines in comparison with their classical
counterparts. Relations and mappings between these two quantum heat engines are
also investigated by considering their respective quantum thermodynamic
processes. In addition, we discuss the role of Maxwell's demon in quantum
thermodynamic cycles. We find that there is no violation of the second law,
even in the existence of such a demon, when the demon is included correctly as
part of the working substance of the heat engine.Comment: 17 pages, 9 figures, 4 table
Variance of transmitted power in multichannel dissipative ergodic structures invariant under time reversal
We use random matrix theory (RMT) to study the first two moments of the wave
power transmitted in time reversal invariant systems having ergodic motion.
Dissipation is modeled by a number of loss channels of variable coupling
strength. To make a connection with ultrasonic experiments on ergodic
elastodynamic billiards, the channels injecting and collecting the waves are
assumed to be negligibly coupled to the medium, and to contribute essentially
no dissipation. Within the RMT model we calculate the quantities of interest
exactly, employing the supersymmetry technique. This approach is found to be
more accurate than another method based on simplifying naive assumptions for
the statistics of the eigenfrequencies and the eigenfunctions. The results of
the supersymmetric method are confirmed by Monte Carlo numerical simulation and
are used to reveal a possible source of the disagreement between the
predictions of the naive theory and ultrasonic measurements.Comment: 10 pages, 2 figure
Laser beam coupling with capillary discharge plasma for laser wakefield acceleration applications
One of the most robust methods, demonstrated up to date, of accelerating
electron beams by laser-plasma sources is the utilization of plasma channels
generated by the capillary discharges. These channels, i.e., plasma columns
with a minimum density along the laser pulse propagation axis, may optically
guide short laser pulses, thereby increasing the acceleration length, leading
to a more efficient electron acceleration. Although the spatial structure of
the installation is simple in principle, there may be some important effects
caused by the open ends of the capillary, by the supplying channels etc., which
require a detailed 3D modeling of the processes taking place in order to get a
detailed understanding and improve the operation. However, the discharge
plasma, being one of the most crucial components of the laser-plasma
accelerator, is not simulated with the accuracy and resolution required to
advance this promising technology. In the present work, such simulations are
performed using the code MARPLE. First, the process of the capillary filling
with a cold hydrogen before the discharge is fired, through the side supply
channels is simulated. The main goal of this simulation is to get a spatial
distribution of the filling gas in the region near the open ends of the
capillary. A realistic geometry is used for this and the next stage
simulations, including the insulators, the supplying channels as well as the
electrodes. Second, the simulation of the capillary discharge is performed with
the goal to obtain a time-dependent spatial distribution of the electron
density near the open ends of the capillary as well as inside the capillary.
Finally, to evaluate effectiveness of the beam coupling with the channeling
plasma wave guide and electron acceleration, modeling of laser-plasma
interaction was performed with the code INF&RNOComment: 11 pages, 9 figure
Shuffling cards, factoring numbers, and the quantum baker's map
It is pointed out that an exactly solvable permutation operator, viewed as
the quantization of cyclic shifts, is useful in constructing a basis in which
to study the quantum baker's map, a paradigm system of quantum chaos. In the
basis of this operator the eigenfunctions of the quantum baker's map are
compressed by factors of around five or more. We show explicitly its connection
to an operator that is closely related to the usual quantum baker's map. This
permutation operator has interesting connections to the art of shuffling cards
as well as to the quantum factoring algorithm of Shor via the quantum order
finding one. Hence we point out that this well-known quantum algorithm makes
crucial use of a quantum chaotic operator, or at least one that is close to the
quantization of the left-shift, a closeness that we also explore
quantitatively.Comment: 12 pgs. Substantially elaborated version, including a new route to
the quantum bakers map. To appear in J. Phys.
On production and asymmetric focusing of flat electron beams using rectangular capillary discharge plasmas
A method for the asymmetric focusing of electron bunches, based on the active
plasma lensing technique is proposed. This method takes advantage of the strong
inhomogeneous magnetic field generated inside the capillary discharge plasma to
focus the ultrarelativistic electrons. The plasma and magnetic field parameters
inside the capillary discharge are described theoretically and modeled with
dissipative magnetohydrodynamic computer simulations enabling analysis of the
capillaries of rectangle cross-sections. Large aspect ratio rectangular
capillaries might be used to transport electron beams with high emittance
asymmetries, as well as assist in forming spatially flat electron bunches for
final focusing before the interaction point.Comment: 16 pages, 7 figures, 1 tabl
Effect of the frequency chirp on laser wakefield acceleration
The role of laser frequency chirps in the laser wakefield accelerator is
examined. We show that in the linear regime, the evolution of the laser pulse
length is affected by the frequency chirp, and that positive (negative) chirp
compresses (stretches) the laser pulse, thereby increasing (decreasing) the
peak vector potential and wakefield amplitude. In the blowout regime, the
frequency chirp can be used to fine tune the localized etching rates at the
front of the laser. In our simulations, chirped laser pulses can lead to 15%
higher self-trapped electrons, and 10% higher peak energies as compare to the
transform-limited pulse. Chirps may be used to control the phase velocity of
the wake, and to relax the self-guiding conditions at the front of the laser.
Our predictions are confirmed by multi-dimensional particle-in-cell simulations
with OSIRIS.Comment: Submitted in NJP, 4 figure
Using the Hadamard and related transforms for simplifying the spectrum of the quantum baker's map
We rationalize the somewhat surprising efficacy of the Hadamard transform in
simplifying the eigenstates of the quantum baker's map, a paradigmatic model of
quantum chaos. This allows us to construct closely related, but new, transforms
that do significantly better, thus nearly solving for many states of the
quantum baker's map. These new transforms, which combine the standard Fourier
and Hadamard transforms in an interesting manner, are constructed from
eigenvectors of the shift permutation operator that are also simultaneous
eigenvectors of bit-flip (parity) and possess bit-reversal (time-reversal)
symmetry.Comment: Version to appear in J. Phys. A. Added discussions; modified title;
corrected minor error
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