16,268 research outputs found
Symbolic calculus on the time-frequency half-plane
The study concerns a special symbolic calculus of interest for signal
analysis. This calculus associates functions on the time-frequency half-plane
f>0 with linear operators defined on the positive-frequency signals. Full
attention is given to its construction which is entirely based on the study of
the affine group in a simple and direct way. The correspondence rule is
detailed and the associated Wigner function is given. Formulas expressing the
basic operation (star-bracket) of the Lie algebra of symbols, which is
isomorphic to that of the operators, are obtained. In addition, it is shown
that the resulting calculus is covariant under a three-parameter group which
contains the affine group as subgroup. This observation is the starting point
of an investigation leading to a whole class of symbolic calculi which can be
considered as modifications of the original one.Comment: 25 pages, Latex, minor changes and more references; to be published
in the "Journal of Mathematical Physics" (special issue on "Wavelet and
Time-Frequency Analysis"
Why Christians Should Not Be Kaneans about Freedom
Abstract: In this paper we argue that Robert Kane’s theory of free will cannot accommodate the
possibility of a sinless individual who faces morally significant choices because a sinless agent
cannot voluntarily accord value to an immoral desire, and we argue that Kane’s theory requires this. Since the Jesus of the historic Christian tradition is held to be sinless, we think Christians should reject Kane’s theory because it seems irreconcilable with historic Christian Christology. We consider two objections to our argument and argue that both fail
Conductance beyond the Landauer limit and charge pumping in quantum wires
Periodically driven systems, which can be described by Floquet theory, have
been proposed to show characteristic behavior that is distinct from static
Hamiltonians. Floquet theory proposes to describe such periodically driven
systems in terms of states that are indexed by a photon number in addition to
the usual Hilbert space of the system. We propose a way to measure directly
this additional Floquet degree of freedom by the measurement of the DC
conductance of a single channel quantum point contact. Specifically, we show
that a single channel wire augmented with a grating structure when irradiated
with microwave radiation can show a DC conductance above the limit of one
conductance quantum set by the Landauer formula. Another interesting feature of
the proposed system is that being non-adiabatic in character, it can be used to
pump a strong gate-voltage dependent photo-current even with linearly polarized
radiation.Comment: 9 pages; 3 figures: Final published version; includes minor revisions
from the last versio
Inelastic Neutron and X-ray Scattering from Incommensurate Magnetic Systems
Neutrons and X-rays are powerful probes for studying magnetic and lattice
excitations in strongly correlated materials over very wide ranges of momentum
and energy transfers. In the focus of the present work are the incommensurate
magnetic systems MnSi and Cr. Under application of a magnetic field, helically
ordered MnSi transforms into a weak itinerant ferromagnet. Using polarized
neutrons we demonstrate that the Stoner excitations are spin flip excitations.
The amplitude (longitudinal) fluctuations associated with the magnon modes are
already strong far away from T_C. Interestingly, even the non spin flip
excitations associated with the Stoner modes are observable. In Cr, we have
observed Kohn anomalies in the phonon spectrum at those incommensurate
positions in reciprocal space, where the spin density wave is observed. The
corresponding phonon and magnon modes are not coupled. In addition, an
anomalous softening of a transverse phonon branch along the N-H zone boundary
line is observed that is caused by strong electron phonon coupling. High
resolution neutron scattering indicate that the low energy Fincher-Burke
excitations may rather correspond to localized modes in momentum and energy and
not to propagating collective modes. Finally, we demonstrate that in the near
future it may become feasible to investigate excitations in very small samples
thus allowing to measure the dynamics of strongly correlated materials under
extreme conditions and in the vicinity of quantum phase transitions
Concept for controlled transverse emittance transfer within a linac ion beam
For injection of beams into circular machines with different horizontal and
vertical emittance acceptance, the injection efficiency can be increased if
these beams are flat, i.e. if they feature unequal transverse emittances.
Generation of flat electron beams is well known and has been demonstrated
already in beam experiments. It was proposed also for ion beams that were
generated in an Electron Cyclotron-Resonance (ECR) source. We introduce an
extension of the method to beams that underwent charge state stripping without
requiring their generation inside an ECR source. Results from multi-particle
simulations are presented to demonstrate the validity of the method.Comment: 23 pages (preprint style), 14 Figures, submitted to PRST-A
Carbon Nanotube Based Bearing for Rotational Motions
We report the fabrication of a nanoelectromechanical system consisting of a
plate rotating around a multiwalled nanotube bearing. The motion is possible
thanks to the low intershell friction. Indeed, the nanotube has been engineered
so that the sliding happens between different shells. The plate rotation is
activated electrostatically with stator electrodes. The static friction force
is estimated at N/\AA.Comment: 4 pages, 3 figure
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