12,836 research outputs found
A Note on TeV Cerenkov Events as Bose-Einstein Gamma Condensations
The idea that the TeV air showers, thought to be produced by >10 TeV gamma
rays from Mrk 501, can be mimicked by coherent bunches of sub-TeV photons is
reexamined, focusing on fundamental considerations. In particular, it is shown
that the minimum spot size of the beam of pulsed TeV photons arriving at Earth
is on the order of a few kilometers, unless a lens with certain characteristics
is placed between the TeV laser and Earth. The viability of the laser
production mechanism proposed by Harwit et al. (2000) is also reassessed.Comment: 4 page
Nonlinear inverse problem by T-matrix completion. I. Theory
We propose a conceptually new method for solving nonlinear inverse scattering
problems (ISPs) such as are commonly encountered in tomographic ultrasound
imaging, seismology and other applications. The method is inspired by the
theory of nonlocality of physical interactions and utilizes the relevant
formalism. We formulate the ISP as a problem whose goal is to determine an
unknown interaction potential from external scattering data. Although we
seek a local (diagonally-dominated) as the solution to the posed problem,
we allow to be nonlocal at the intermediate stages of iterations. This
allows us to utilize the one-to-one correspondence between and the T-matrix
of the problem, . Here it is important to realize that not every
corresponds to a diagonal and we, therefore, relax the usual condition of
strict diagonality (locality) of . An iterative algorithm is proposed in
which we seek that is (i) compatible with the measured scattering data and
(ii) corresponds to an interaction potential that is as
diagonally-dominated as possible. We refer to this algorithm as to the
data-compatible T-matrix completion (DCTMC). This paper is Part I in a two-part
series and contains theory only. Numerical examples of image reconstruction in
a strongly nonlinear regime are given in Part II. The method described in this
paper is particularly well suited for very large data sets that become
increasingly available with the use of modern measurement techniques and
instrumentation.Comment: This is Part I of a paper series containing theory only. Part II
contains simulations and is available as arXiv:1505.06777 [math-ph]. Accepted
in this form to Phys. Rev.
Solution of the inverse scattering problem by T-matrix completion. II. Simulations
This is Part II of the paper series on data-compatible T-matrix completion
(DCTMC), which is a method for solving nonlinear inverse problems. Part I of
the series contains theory and here we present simulations for inverse
scattering of scalar waves. The underlying mathematical model is the scalar
wave equation and the object function that is reconstructed is the medium
susceptibility. The simulations are relevant to ultrasound tomographic imaging
and seismic tomography. It is shown that DCTMC is a viable method for solving
strongly nonlinear inverse problems with large data sets. It provides not only
the overall shape of the object but the quantitative contrast, which can
correspond, for instance, to the variable speed of sound in the imaged medium.Comment: This is Part II of a paper series. Part I contains theory and is
available at arXiv:1401.3319 [math-ph]. Accepted in this form to Phys. Rev.
The treatment of depression and simple phobia through an interpreter in the North East of England : a case study.
A 35-year-old Middle Eastern woman, experiencing moderate depression compounded by animal phobia was referred to an Improving Access to Psychological Therapies (IAPT) service. Shared understandings were gradually developed using written functional analyses translated in session. Activity scheduling was integrated with graded exposure to increase access to positive reinforcement. Questionnaires and subjective data indicated a reduction in phobic avoidance and functioning increased. Despite the complexity of working as a triad, a positive therapeutic relationship was achieved with increased mutual cultural understanding. Indirect communication led to difficulties maintaining guided discovery and focus. There is limited evidence to support CBT when delivered through an interpreter. IAPT recommendations suggest staff reflect the community; the North East has one of the lowest foreign-born populations in the UK indicating that IAPT services may be ill prepared to work with ethnic minorities. Learning points for the therapist were: maintain simplicity, take time to formulate incorporation of cultural difference, and use transcultural interventions. The interpreter brought advantages; providing means of communication and understanding of cultural differences. Disadvantages were the potential for bias or lost information, increased time and complexity of delivering therapy. This case indicates a deficit in high intensity training and lack of literature to support therapists
Giant isotope effect in the incoherent tunneling specific heat of the molecular nanomagnet Fe8
Time-dependent specific heat experiments on the molecular nanomagnet Fe8 and
the isotopic enriched analogue 57Fe8 are presented. The inclusion of the 57Fe
nuclear spins leads to a huge enhancement of the specific heat below 1 K,
ascribed to a strong increase in the spin-lattice relaxation rate Gamma arising
from incoherent, nuclear-spin-mediated magnetic quantum tunneling in the
ground-doublet. Since Gamma is found comparable to the expected tunneling rate,
the latter process has to be inelastic. A model for the coupling of the
tunneling levels to the lattice is presented. Under transverse field, a
crossover from nuclear-spin-mediated to phonon-induced tunneling is observed.Comment: Replaced with version accepted for publication in Physical Review
Letter
Quantum dot dephasing by edge states
We calculate the dephasing rate of an electron state in a pinched quantum
dot, due to Coulomb interactions between the electron in the dot and electrons
in a nearby voltage biased ballistic nanostructure. The dephasing is caused by
nonequilibrium time fluctuations of the electron density in the nanostructure,
which create random electric fields in the dot. As a result, the electron level
in the dot fluctuates in time, and the coherent part of the resonant
transmission through the dot is suppressed
Transmission of two interacting electrons
The transmission of two electrons through a region where they interact is
found to be enhanced by a renormalization of the repulsive interaction. For a
specific example of the single-particle Hamiltonian, which includes a strongly
attractive potential, the renormalized interaction becomes attractive, and the
transmission has a pronounced maximum as function of the depth of the
single-electron attractive potential. The results apply directly to a simple
model of scattering of two interacting electrons by a quantum dot.Comment: 12 pages, 2 figure
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