666 research outputs found
Numerical Study of Wave Propagation in Uniaxially Anisotropic Lorentzian Backward Wave Slabs
The propagation and refraction of a cylindrical wave created by a line
current through a slab of backward wave medium, also called left-handed medium,
is numerically studied with FDTD. The slab is assumed to be uniaxially
anisotropic. Several sets of constitutive parameters are considered and
comparisons with theoretical results are made. Electric field distributions are
studied inside and behind the slab. It is found that the shape of the
wavefronts and the regions of real and complex wave vectors are in agreement
with theoretical results.Comment: 6 pages, figure
Three-Dimensional SU(3) gauge theory and the Spatial String Tension of the (3+1)-Dimensional Finite Temperature SU(3) Gauge Theory
We establish a close relation between the spatial string tension of the
(3+1)-dimensional gauge theory at finite temperature () and
the string tension of the 3-dimensional gauge theory () which
is similar to what has been found previously for . We obtain
and , respectively. For temperatures larger than twice the critical
temperature results are consistent with a temperature dependent coupling
running according to the two-loop -function with .Comment: 11 pages (4 figures
Density Functional Theory of Multicomponent Quantum Dots
Quantum dots with conduction electrons or holes originating from several
bands are considered. We assume the particles are confined in a harmonic
potential and assume the electrons (or holes) belonging to different bands to
be different types of fermions with isotropic effective masses. The density
functional method with the local density approximation is used. The increased
number of internal (Kohn-Sham) states leads to a generalisation of Hund's first
rule at high densities. At low densitites the formation of Wigner molecules is
favored by the increased internal freedom.Comment: 11 pages, 5 figure
Deep convolutional neural networks for estimating porous material parameters with ultrasound tomography
We study the feasibility of data based machine learning applied to ultrasound
tomography to estimate water-saturated porous material parameters. In this
work, the data to train the neural networks is simulated by solving wave
propagation in coupled poroviscoelastic-viscoelastic-acoustic media. As the
forward model, we consider a high-order discontinuous Galerkin method while
deep convolutional neural networks are used to solve the parameter estimation
problem. In the numerical experiment, we estimate the material porosity and
tortuosity while the remaining parameters which are of less interest are
successfully marginalized in the neural networks-based inversion. Computational
examples confirms the feasibility and accuracy of this approach
A really simple approximation of smallest grammar
In this paper we present a really simple linear-time algorithm constructing a
context-free grammar of size O(g log (N/g)) for the input string, where N is
the size of the input string and g the size of the optimal grammar generating
this string. The algorithm works for arbitrary size alphabets, but the running
time is linear assuming that the alphabet Sigma of the input string can be
identified with numbers from 1,ldots, N^c for some constant c. Algorithms with
such an approximation guarantee and running time are known, however all of them
were non-trivial and their analyses were involved. The here presented algorithm
computes the LZ77 factorisation and transforms it in phases to a grammar. In
each phase it maintains an LZ77-like factorisation of the word with at most l
factors as well as additional O(l) letters, where l was the size of the
original LZ77 factorisation. In one phase in a greedy way (by a left-to-right
sweep and a help of the factorisation) we choose a set of pairs of consecutive
letters to be replaced with new symbols, i.e. nonterminals of the constructed
grammar. We choose at least 2/3 of the letters in the word and there are O(l)
many different pairs among them. Hence there are O(log N) phases, each of them
introduces O(l) nonterminals to a grammar. A more precise analysis yields a
bound O(l log(N/l)). As l \leq g, this yields the desired bound O(g log(N/g)).Comment: Accepted for CPM 201
The Spatial String Tension in High Temperature Lattice Gauge Theories
We develop some techniques which allow an analytic evaluation of space-like
observables in high temperature lattice gauge theories. We show that such
variables are described extremely well by dimensional reduction. In particular,
by using results obtained in the context of ``Induced QCD'', we evaluate the
contributions to space-like observables coming from the Higgs sector of the
dimensionally reduced action, we find that they are of higher order in the
coupling constant compared to those coming from the space-like action and hence
neglegible near the continuum limit. In the case of SU(2) gauge theory our
results agree with those obtained through Montecarlo simulations both in (2+1)
and (3+1) dimensions and they also indicate a possible way of removing the gap
between the two values of recently appeared in the literature.Comment: 17 pages, (Latex), DFTT 8/9
SU(3) Lattice Gauge Theory With Adjoint Action At Nonzero Temperature
We study the thermal phase diagram of pure SU(3) gauge theory with
fundamental and adjoint couplings. We improve previous estimates of the
position of the bulk transition line and determine the thermal deconfinement
transition lines for and 8. For the deconfinement
transition line splits cleanly away from the bulk transition line. With
increasing the thermal deconfinement transition lines shift to
increasingly weaker coupling, joining onto the bulk transition line at
increasingly larger in a pattern consistent with the usual
universality picture of lattice gauge theories.Comment: Talk presented by U. M. Heller at Lat94 conference, September 27 -
October 1, 1994, Bielefeld, Germany. self unwrapping postscript fil
Screening Masses of Hot SU(2) Gauge Theory from the 3D Adjoint Higgs Model
We study the Landau gauge propagators of the lattice SU(2) 3d adjoint Higgs
model, considered as an effective theory of high temperature 4d SU(2) gauge
theory. From the long distance behaviour of the propagators we extract the
screening masses. It is shown that the pole masses extracted from the
propagators agree well with the screening masses obtained recently in finite
temperature SU(2) theory. The relation of the propagator masses to the masses
extracted from gauge invariant correlators is also discussed. In so-called
lambda gauges non-perturbative evidence is given for the gauge independence of
pole masses within this class of gauges.Comment: Talk given at SEWM98 Conference, Copenhagen, December 199
Lattice-continuum relations for 3d SU(N)+Higgs theories
3d lattice studies have recently attracted a lot of attention, especially in
connection with finite temperature field theories. One ingredient in these
studies is a perturbative computation of the 2-loop lattice counterterms, which
are exact in the continuum limit. We extend previous such results to SU(N)
gauge theories with Higgs fields in the fundamental and adjoint
representations. The fundamental SU(3)xSU(2) case might be relevant for the
electroweak phase transition in the MSSM, and the adjoint case for the GUT
phase transition and for QCD in the high temperature phase. We also revisit the
standard SU(2)xU(1) and U(1) theories.Comment: 21 page
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