15,644 research outputs found
Breakup of three particles within the adiabatic expansion method
General expressions for the breakup cross sections in the lab frame for
reactions are given in terms of the hyperspherical adiabatic basis. The
three-body wave function is expanded in this basis and the corresponding
hyperradial functions are obtained by solving a set of second order
differential equations. The -matrix is computed by using two recently
derived integral relations. Even though the method is shown to be well suited
to describe processes, there are nevertheless particular configurations
in the breakup channel (for example those in which two particles move away
close to each other in a relative zero-energy state) that need a huge number of
basis states. This pathology manifests itself in the extremely slow convergence
of the breakup amplitude in terms of the hyperspherical harmonic basis used to
construct the adiabatic channels. To overcome this difficulty the breakup
amplitude is extracted from an integral relation as well. For the sake of
illustration, we consider neutron-deuteron scattering. The results are compared
to the available benchmark calculations
Recombination rates from potential models close to the unitary limit
We investigate universal behavior in the recombination rate of three bosons
close to threshold. Using the He-He system as a reference, we solve the
three-body Schr\"odinger equation above the dimer threshold for different
potentials having large values of the two-body scattering length . To this
aim we use the hyperspherical adiabatic expansion and we extract the -matrix
through the integral relations recently derived. The results are compared to
the universal form, , for
different values of and selected values of the three-body parameter
. A good agreement with the universal formula is obtained after
introducing a particular type of finite-range corrections, which have been
recently proposed by two of the authors in Ref.[1]. Furthermore, we analyze the
validity of the above formula in the description of a very different system:
neutron-neutron-proton recombination. Our analysis confirms the universal
character of the process in systems of very different scales having a large
two-body scattering length
Influence of Dislocations in Thomson's Problem
We investigate Thomson's problem of charges on a sphere as an example of a
system with complex interactions. Assuming certain symmetries we can work with
a larger number of charges than before. We found that, when the number of
charges is large enough, the lowest energy states are not those with the
highest symmetry. As predicted previously by Dodgson and Moore, the complex
patterns in these states involve dislocation defects which screen the strains
of the twelve disclinations required to satisfy Euler's theorem.Comment: 9 pages, 4 figures in gif format. Original PS files can be obtained
in http://fermi.fcu.um.es/thomso
Numerical study of relaxation in electron glasses
We perform a numerical simulation of energy relaxation in three-dimensional
electron glasses in the strongly localized regime at finite temperatures. We
consider systems with no interactions, with long-range Coulomb interactions and
with short-range interactions, obtaining a power law relaxation with an
exponent of 0.15, which is independent of the parameters of the problem and of
the type of interaction. At very long times, we always find an exponential
regime whose characteristic time strongly depends on temperature, system size,
interaction type and localization radius. We extrapolate the longest relaxation
time to macroscopic sizes and, for interacting samples, obtain values much
larger than the measuring time. We finally study the number of electrons
participating in the relaxation processes of very low energy configurations.Comment: 6 eps figures. To be published in Phys. Rev.
Integral relations and the adiabatic expansion method for 1+2 reactions above the breakup threshold: Helium trimers with soft-core potentials
The integral relations formalism introduced in \cite{bar09,rom11}, and
designed to describe 1+ reactions, is extended here to collision energies
above the threshold for the target breakup. These two relations are completely
general, and in this work they are used together with the adiabatic expansion
method for the description of 1+2 reactions. The neutron-deuteron breakup, for
which benchmark calculations are available, is taken as a test of the method.
The s-wave collision between the He atom and He dimer above the
breakup threshold and the possibility of using soft-core two-body potentials
plus a short-range three-body force will be investigated. Comparisons to
previous calculations for the three-body recombination and collision
dissociation rates will be shown.Comment: To be published in Physical Review
Classical Analog of Electromagnetically Induced Transparency
We present a classical analog for Electromagnetically Induced Transparency
(EIT). In a system of just two coupled harmonic oscillators subject to a
harmonic driving force we can reproduce the phenomenology observed in EIT. We
describe a simple experiment performed with two linearly coupled RLC circuits
which can be taught in an undergraduate laboratory class.Comment: 6 pages, two-column, 6 figures, submitted to the Am. J. Phy
Probabilistic Inference from Arbitrary Uncertainty using Mixtures of Factorized Generalized Gaussians
This paper presents a general and efficient framework for probabilistic
inference and learning from arbitrary uncertain information. It exploits the
calculation properties of finite mixture models, conjugate families and
factorization. Both the joint probability density of the variables and the
likelihood function of the (objective or subjective) observation are
approximated by a special mixture model, in such a way that any desired
conditional distribution can be directly obtained without numerical
integration. We have developed an extended version of the expectation
maximization (EM) algorithm to estimate the parameters of mixture models from
uncertain training examples (indirect observations). As a consequence, any
piece of exact or uncertain information about both input and output values is
consistently handled in the inference and learning stages. This ability,
extremely useful in certain situations, is not found in most alternative
methods. The proposed framework is formally justified from standard
probabilistic principles and illustrative examples are provided in the fields
of nonparametric pattern classification, nonlinear regression and pattern
completion. Finally, experiments on a real application and comparative results
over standard databases provide empirical evidence of the utility of the method
in a wide range of applications
Photoconductance of a submicron oxidized line in surface conductive single crystalline diamond
We report on sub-bandgap optoelectronic phenomena of hydrogen-terminated
diamond patterned with a submicron oxidized line. The line acts as an energy
barrier for the two-dimensional hole gas located below the hydrogenated diamond
surface. A photoconductive gain of the hole conductivity across the barrier is
measured for sub-bandgap illumination. The findings are consistent with
photogenerated electrons being trapped in defect levels within the barrier. We
discuss the spatial and energetic characteristics of the optoelectronic
phenomena, as well as possible photocurrent effects
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