212 research outputs found
The Thermal Renormalization Group for Fermions, Universality, and the Chiral Phase-Transition
We formulate the thermal renormalization group, an implementation of the
Wilsonian RG in the real-time (CTP) formulation of finite temperature field
theory, for fermionic fields. Using a model with scalar and fermionic degrees
of freedom which should describe the two-flavor chiral phase-transition, we
discuss the mechanism behind fermion decoupling and universality at second
order transitions. It turns out that an effective mass-like term in the fermion
propagator which is due to thermal fluctuations and does not break chiral
symmetry is necessary for fermion decoupling to work. This situation is in
contrast to the high-temperature limit, where the dominance of scalar over
fermionic degrees of freedom is due to the different behavior of the
distribution functions. The mass-like contribution is the leading thermal
effect in the fermionic sector and is missed if a derivative expansion of the
fermionic propagator is performed. We also discuss results on the
phase-transition of the model considered where we find good agreement with
results from other methods.Comment: References added, minor typos correcte
Heisenberg frustrated magnets: a nonperturbative approach
Frustrated magnets are a notorious example where the usual perturbative
methods are in conflict. Using a nonperturbative Wilson-like approach, we get a
coherent picture of the physics of Heisenberg frustrated magnets everywhere
between and . We recover all known perturbative results in a single
framework and find the transition to be weakly first order in . We compute
effective exponents in good agreement with numerical and experimental data.Comment: 5 pages, Revtex, technical details available at
http://www.lpthe.jussieu.fr/~tissie
Structural Material Property Tailoring Using Deep Neural Networks
Advances in robotics, artificial intelligence, and machine learning are
ushering in a new age of automation, as machines match or outperform human
performance. Machine intelligence can enable businesses to improve performance
by reducing errors, improving sensitivity, quality and speed, and in some cases
achieving outcomes that go beyond current resource capabilities. Relevant
applications include new product architecture design, rapid material
characterization, and life-cycle management tied with a digital strategy that
will enable efficient development of products from cradle to grave. In
addition, there are also challenges to overcome that must be addressed through
a major, sustained research effort that is based solidly on both inferential
and computational principles applied to design tailoring of functionally
optimized structures. Current applications of structural materials in the
aerospace industry demand the highest quality control of material
microstructure, especially for advanced rotational turbomachinery in aircraft
engines in order to have the best tailored material property. In this paper,
deep convolutional neural networks were developed to accurately predict
processing-structure-property relations from materials microstructures images,
surpassing current best practices and modeling efforts. The models
automatically learn critical features, without the need for manual
specification and/or subjective and expensive image analysis. Further, in
combination with generative deep learning models, a framework is proposed to
enable rapid material design space exploration and property identification and
optimization. The implementation must take account of real-time decision cycles
and the trade-offs between speed and accuracy
Flow Equations without Mean Field Ambiguity
We compare different methods used for non-perturbative calculations in
strongly interacting fermionic systems. Mean field theory often shows a basic
ambiguity related to the possibility to perform Fierz transformations. The
results may then depend strongly on an unphysical parameter which reflects the
choice of the mean field, thus limiting the reliability. This ambiguity is
absent for Schwinger-Dyson equations or fermionic renormalization group
equations. Also renormalization group equations in a partially bosonized
setting can overcome the Fierz ambiguity if the truncation is chosen
appropriately. This is reassuring since the partially bosonized renormalization
group approach constitutes a very promising basis for the explicit treatment of
condensates and spontaneous symmetry breaking even for situations where the
bosonic correlation length is large.Comment: New version to match the one published in PRD. New title (former
title: Solving Mean Field Ambiguity by Flow Equations), added section IX and
appendix B. More explanations in the introduction and conclusions. 16 pages,
6 figures and 3 tables uses revtex
Effective average action in statistical physics and quantum field theory
An exact renormalization group equation describes the dependence of the free
energy on an infrared cutoff for the quantum or thermal fluctuations. It
interpolates between the microphysical laws and the complex macroscopic
phenomena. We present a simple unified description of critical phenomena for
O(N)-symmetric scalar models in two, three or four dimensions, including
essential scaling for the Kosterlitz-Thouless transition.Comment: 34 pages,5 figures,LaTe
Dual superfluid-Bose glass critical point in two dimensions and the universal conductivity
We study the continuum version of the dual theory for a system of
two-dimensional, zero temperature, disordered bosons, interacting with short
range repulsion and at a commensurate density. The dual theory, which describes
vortices in the bosonic ground state, and has a form of 3D classical scalar
electrodynamics in random, correlated magnetic field, admits a new disordered
critical point within RG calculation at fixed dimension. The universal
conductivity and the critical exponents at the superfluid-Bose glass critical
point are calculated as series in fixed-point values of the dual coupling
constants, to the lowest non-trivial order: ,
and . The comparison with numerical results and experiments
is discussed.Comment: 8 pages, LaTex, some clarifications and references adde
A non perturbative approach of the principal chiral model between two and four dimensions
We investigate the principal chiral model between two and four dimensions by
means of a non perturbative Wilson-like renormalization group equation. We are
thus able to follow the evolution of the effective coupling constants within
this whole range of dimensions without having recourse to any kind of small
parameter expansion. This allows us to identify its three dimensional critical
physics and to solve the long-standing discrepancy between the different
perturbative approaches that characterizes the class of models to which the
principal chiral model belongs.Comment: 5 pages, 1 figure, Revte
Spatial distribution of photoelectrons participating in formation of x-ray absorption spectra
Interpretation of x-ray absorption near-edge structure (XANES) experiments is
often done via analyzing the role of particular atoms in the formation of
specific peaks in the calculated spectrum. Typically, this is achieved by
calculating the spectrum for a series of trial structures where various atoms
are moved and/or removed. A more quantitative approach is presented here, based
on comparing the probabilities that a XANES photoelectron of a given energy can
be found near particular atoms. Such a photoelectron probability density can be
consistently defined as a sum over squares of wave functions which describe
participating photoelectron diffraction processes, weighted by their normalized
cross sections. A fine structure in the energy dependence of these
probabilities can be extracted and compared to XANES spectrum. As an
illustration of this novel technique, we analyze the photoelectron probability
density at the Ti K pre-edge of TiS2 and at the Ti K-edge of rutile TiO2.Comment: Journal abstract available on-line at
http://link.aps.org/abstract/PRB/v65/e20511
Critical properties of the topological Ginzburg-Landau model
We consider a Ginzburg-Landau model for superconductivity with a Chern-Simons
term added. The flow diagram contains two charged fixed points corresponding to
the tricritical and infrared stable fixed points. The topological coupling
controls the fixed point structure and eventually the region of first order
transitions disappears. We compute the critical exponents as a function of the
topological coupling. We obtain that the value of the exponent does not
vary very much from the XY value, . This shows that the
Chern-Simons term does not affect considerably the XY scaling of
superconductors. We discuss briefly the possible phenomenological applications
of this model.Comment: RevTex, 7 pages, 8 figure
How fast can the wall move? A study of the electroweak phase transition dynamics
We consider the dynamics of bubble growth in the Minimal Standard Model at
the electroweak phase transition and determine the shape and the velocity of
the phase boundary, or bubble wall. We show that in the semi-classical
approximation the friction on the wall arises from the deviation of massive
particle populations from thermal equilibrium. We treat these with Boltzmann
equations in a fluid approximation. This approximation is reasonable for the
top quarks and the light species while it underestimates the friction from the
infrared bosons and Higgs particles. We use the two-loop finite temperature
effective potential and find a subsonic bubble wall for the whole range of
Higgs masses GeV. The result is weakly dependent on : the wall
velocity falls in the range , while the wall thickness is
in the range . The wall is thicker than the phase equilibrium
value because out of equilibrium particles exert more friction on the back than
on the base of a moving wall. We also consider the effect of an infrared gauge
condensate which may exist in the symmetric phase; modelling it simplemindedly,
we find that the wall may become supersonic, but not ultrarelativistic.Comment: 42 pages, plain latex, with three figures. Minor editing August 1 (we
figured out how to do analytically some integrals we previously did
numerically, made corresponding (slight) changes to numerical results, and
corrected some typos.
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