42,760 research outputs found
Wavefront sets in algebraic quantum field theory
The investigation of wavefront sets of n-point distributions in quantum field
theory has recently acquired some attention stimulated by results obtained with
the help of concepts from microlocal analysis in quantum field theory in curved
spacetime. In the present paper, the notion of wavefront set of a distribution
is generalized so as to be applicable to states and linear functionals on nets
of operator algebras carrying a covariant action of the translation group in
arbitrary dimension. In the case where one is given a quantum field theory in
the operator algebraic framework, this generalized notion of wavefront set,
called "asymptotic correlation spectrum", is further investigated and several
of its properties for physical states are derived. We also investigate the
connection between the asymptotic correlation spectrum of a physical state and
the wavefront sets of the corresponding Wightman distributions if there is a
Wightman field affiliated to the local operator algebras. Finally we present a
new result (generalizing known facts) which shows that certain spacetime points
must be contained in the singular supports of the 2n-point distributions of a
non-trivial Wightman field.Comment: 34 pages, LaTex2
Structured random measurements in signal processing
Compressed sensing and its extensions have recently triggered interest in
randomized signal acquisition. A key finding is that random measurements
provide sparse signal reconstruction guarantees for efficient and stable
algorithms with a minimal number of samples. While this was first shown for
(unstructured) Gaussian random measurement matrices, applications require
certain structure of the measurements leading to structured random measurement
matrices. Near optimal recovery guarantees for such structured measurements
have been developed over the past years in a variety of contexts. This article
surveys the theory in three scenarios: compressed sensing (sparse recovery),
low rank matrix recovery, and phaseless estimation. The random measurement
matrices to be considered include random partial Fourier matrices, partial
random circulant matrices (subsampled convolutions), matrix completion, and
phase estimation from magnitudes of Fourier type measurements. The article
concludes with a brief discussion of the mathematical techniques for the
analysis of such structured random measurements.Comment: 22 pages, 2 figure
STM and RHEED study of the Si(001)-c(8x8) surface
The Si(001) surface deoxidized by short annealing at T~925C in the ultrahigh
vacuum molecular beam epitaxy chamber has been in situ investigated by high
resolution scanning tunnelling microscopy (STM) and reflected high energy
electron diffraction (RHEED). RHEED patterns corresponding to (2x1) and (4x4)
structures were observed during sample treatment. The (4x4) reconstruction
arose at T<600C after annealing. The reconstruction was observed to be
reversible: the (4x4) structure turned into the (2x1) one at T>600C, the (4x4)
structure appeared again at recurring cooling. The c(8x8) reconstruction was
revealed by STM at room temperature on the same samples. A fraction of the
surface area covered by the c(8x8) structure decreased as the sample cooling
rate was reduced. The (2x1) structure was observed on the surface free of the
c(8x8) one. The c(8x8) structure has been evidenced to manifest itself as the
(4x4) one in the RHEED patterns. A model of the c(8x8) structure formation has
been built on the basis of the STM data. Origin of the high-order structure on
the Si(001) surface and its connection with the epinucleation phenomenon are
discussed.Comment: 26 pages, 12 figure
A New Derivation of the CPT and Spin-Statistics Theorems in Non-Commutative Field Theories
We propose an alternative axiomatic description for non-commutative field
theories (NCFT) based on some ideas by Soloviev to nonlocal quantum fields. The
local commutativity axiom is replaced by the weaker condition that the fields
commute at sufficiently large spatial separations, called asymptotic
commutativity, formulated in terms of the theory of analytic functionals. The
question of a possible violation of the CPT and Spin-Statistics theorems caused
by nonlocality of the commutation relations
is investigated. In spite of this
inherent nonlocality, we show that the modification aforementioned is
sufficient to ensure the validity of these theorems for NCFT. We restrict
ourselves to the simplest model of a scalar field in the case of only
space-space non-commutativity.Comment: The title is new, and the analysis in the manuscript has been made
more precise. This revised version is to be published in J.Math.Phy
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