34,850 research outputs found
The Orientation of Unsymmetrical Molecules at Interfaces
A function giving the distribution of the angles of orientation of the molecular axes from the normal to the interface can be obtained if one makes use of a number of special assumptions regarding the forces of orientation. This is compared with the distribution calculated on assuming an extreme form of Langmuir's principle of independent surface action. The importance of having such a function is pointed out
Regularization independent of the noise level: an analysis of quasi-optimality
The quasi-optimality criterion chooses the regularization parameter in
inverse problems without taking into account the noise level. This rule works
remarkably well in practice, although Bakushinskii has shown that there are
always counterexamples with very poor performance. We propose an average case
analysis of quasi-optimality for spectral cut-off estimators and we prove that
the quasi-optimality criterion determines estimators which are rate-optimal
{\em on average}. Its practical performance is illustrated with a calibration
problem from mathematical finance.Comment: 18 pages, 3 figure
A Tale of Two Theories: Quantum Griffiths Effects in Metallic Systems
We show that two apparently contradictory theories on the existence of
Griffiths-McCoy singularities in magnetic metallic systems [1,2] are in fact
mathematically equivalent. We discuss the generic phase diagram of the problem
and show that there is a non-universal crossover temperature range T* < T < W
where power law behavior (Griffiths-McCoy behavior) is expect. For T<T* power
law behavior ceases to exist due to the destruction of quantum effects
generated by the dissipation in the metallic environment. We show that T* is an
analogue of the Kondo temperature and is controlled by non-universal couplings.Comment: 4 pages, 2 figure
Shape analysis on homogeneous spaces: a generalised SRVT framework
Shape analysis is ubiquitous in problems of pattern and object recognition
and has developed considerably in the last decade. The use of shapes is natural
in applications where one wants to compare curves independently of their
parametrisation. One computationally efficient approach to shape analysis is
based on the Square Root Velocity Transform (SRVT). In this paper we propose a
generalised SRVT framework for shapes on homogeneous manifolds. The method
opens up for a variety of possibilities based on different choices of Lie group
action and giving rise to different Riemannian metrics.Comment: 28 pages; 4 figures, 30 subfigures; notes for proceedings of the Abel
Symposium 2016: "Computation and Combinatorics in Dynamics, Stochastics and
Control". v3: amended the text to improve readability and clarify some
points; updated and added some references; added pseudocode for the dynamic
programming algorithm used. The main results remain unchange
Harmonic Generation from Laser-Irradiated Clusters
The harmonic emission from cluster nanoplasmas subject to short, intense
infrared laser pulses is analyzed by means of particle-in-cell simulations. A
pronounced resonant enhancement of the low-order harmonic yields is found when
the Mie plasma frequency of the ionizing and expanding cluster resonates with
the respective harmonic frequency. We show that a strong, nonlinear resonant
coupling of the cluster electrons with the laser field inhibits coherent
electron motion, suppressing the emitted radiation and restricting the spectrum
to only low-order harmonics. A pump-probe scheme is suggested to monitor the
ionization dynamics of the expanding clusters.Comment: 4 pages, ReVTeX
Potential and limitations of nucleon transfer experiments with radioactive beams at REX-ISOLDE
As a tool for studying the structure of nuclei far off stability the
technique of gamma-ray spectroscopy after low-energy single-nucleon transfer
reactions with radioactive nuclear beams in inverse kinematics was
investigated. Modules of the MINIBALL germanium array and a thin
position-sensitive parallel plate avalanche counter (PPAC) to be employed in
future experiments at REX-ISOLDE were used in a test experiment performed with
a stable 36S beam on deuteron and 9Be targets. It is demonstrated that the
Doppler broadening of gamma lines detected by the MINIBALL modules is
considerably reduced by exploiting their segmentation, and that for beam
intensities up to 10^6 particles/s the PPAC positioned around zero degrees with
respect to the beam axis allows not only to significantly reduce the gamma
background by requiring coincidences with the transfer products but also to
control the beam and its intensity by single particle counting. The predicted
large neutron pickup cross sections of neutron-rich light nuclei on 2H and 9Be
targets at REX-ISOLDE energies of 2.2 MeV A are confirmed.Comment: 11 pages, 8 figure
Phase Transitions in a Two-Component Site-Bond Percolation Model
A method to treat a N-component percolation model as effective one component
model is presented by introducing a scaled control variable . In Monte
Carlo simulations on , , and simple cubic
lattices the percolation threshold in terms of is determined for N=2.
Phase transitions are reported in two limits for the bond existence
probabilities and . In the same limits, empirical formulas
for the percolation threshold as function of one
component-concentration, , are proposed. In the limit a new
site percolation threshold, , is reported.Comment: RevTeX, 5 pages, 5 eps-figure
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