6,409 research outputs found
Investigations of solutions of Einstein's field equations close to lambda-Taub-NUT
We present investigations of a class of solutions of Einstein's field
equations close to the family of lambda-Taub-NUT spacetimes. The studies are
done using a numerical code introduced by the author elsewhere. One of the main
technical complication is due to the S3-topology of the Cauchy surfaces.
Complementing these numerical results with heuristic arguments, we are able to
yield some first insights into the strong cosmic censorship issue and the
conjectures by Belinskii, Khalatnikov, and Lifschitz in this class of
spacetimes. In particular, the current investigations suggest that strong
cosmic censorship holds in this class. We further identify open issues in our
current approach and point to future research projects.Comment: 24 pages, 12 figures, uses psfrag and hyperref; replaced with
published version, only minor corrections of typos and reference
L-H transition dynamics in fluid turbulence simulations with neoclassical force balance
Spontaneous transport barrier generation at the edge of a magnetically
confined plasma is investigated. To this end, a model of electrostatic
turbulence in three-dimensional geometry is extended to account for the impact
of friction between trapped and passing particles on the radial electric field.
Non-linear flux-driven simulations are carried out, and it is shown that
considering the radial and temporal variations of the neoclassical friction
coefficients allows for a transport barrier to be generated above a threshold
of the input power
Analysis of Different Types of Regret in Continuous Noisy Optimization
The performance measure of an algorithm is a crucial part of its analysis.
The performance can be determined by the study on the convergence rate of the
algorithm in question. It is necessary to study some (hopefully convergent)
sequence that will measure how "good" is the approximated optimum compared to
the real optimum. The concept of Regret is widely used in the bandit literature
for assessing the performance of an algorithm. The same concept is also used in
the framework of optimization algorithms, sometimes under other names or
without a specific name. And the numerical evaluation of convergence rate of
noisy algorithms often involves approximations of regrets. We discuss here two
types of approximations of Simple Regret used in practice for the evaluation of
algorithms for noisy optimization. We use specific algorithms of different
nature and the noisy sphere function to show the following results. The
approximation of Simple Regret, termed here Approximate Simple Regret, used in
some optimization testbeds, fails to estimate the Simple Regret convergence
rate. We also discuss a recent new approximation of Simple Regret, that we term
Robust Simple Regret, and show its advantages and disadvantages.Comment: Genetic and Evolutionary Computation Conference 2016, Jul 2016,
Denver, United States. 201
Lazy Abstraction-Based Controller Synthesis
We present lazy abstraction-based controller synthesis (ABCS) for
continuous-time nonlinear dynamical systems against reach-avoid and safety
specifications. State-of-the-art multi-layered ABCS pre-computes multiple
finite-state abstractions of varying granularity and applies reactive synthesis
to the coarsest abstraction whenever feasible, but adaptively considers finer
abstractions when necessary. Lazy ABCS improves this technique by constructing
abstractions on demand. Our insight is that the abstract transition relation
only needs to be locally computed for a small set of frontier states at the
precision currently required by the synthesis algorithm. We show that lazy ABCS
can significantly outperform previous multi-layered ABCS algorithms: on
standard benchmarks, lazy ABCS is more than 4 times faster
Effective T-odd P-even hadronic interactions from quark models
Tests of time reversal symmetry at low and medium energies may be analyzed in
the framework of effective hadronic interactions. Here, we consider the quark
structure of hadrons to make a connection to the more fundamental degrees of
freedom. It turns out that for P-even T-odd interactions hadronic matrix
elements evaluated in terms of quark models give rise to factors of 2 to 5.
Also, it is possible to relate the strength of the anomalous part of the
effective rho-type T-odd P-even tensor coupling to quark structure effects.Comment: 6 pages, 1 figure, RevTe
Quasilinear hyperbolic Fuchsian systems and AVTD behavior in T2-symmetric vacuum spacetimes
We set up the singular initial value problem for quasilinear hyperbolic
Fuchsian systems of first order and establish an existence and uniqueness
theory for this problem with smooth data and smooth coefficients (and with even
lower regularity). We apply this theory in order to show the existence of
smooth (generally not analytic) T2-symmetric solutions to the vacuum Einstein
equations, which exhibit AVTD (asymptotically velocity term dominated) behavior
in the neighborhood of their singularities and are polarized or half-polarized.Comment: 78 page
CP violation in the decay mode
Within the framework of Standard Model, the exclusive decay mode is studied. Although the usual short distance contribution is
small compared to the similar mode, the process offers the
possibility of studying the CP violation, a feature absent in the
counterpart.Comment: 11 page latex file including 2 ps figures. Typos corrected, minor
changes. To appear in PR
In-medium nucleon-nucleon potentials in configuration space
Based on the thermodynamic Green function approach two-nucleon correlations
in nuclear matter at finite temperatures are revisited. To this end, we derive
phase equivalent effective -space potentials that include the effect of the
Pauli blocking at a given temperature and density. These potentials enter into
a Schr\"odinger equation that is the -space representation of the
Galitskii-Feynman equation for two nucleons. We explore the analytical
structure of the equation in the complex -plane by means of Jost functions.
We find that despite the Mott effect the correlation with deuteron quantum
numbers are manifested as antibound states, i.e., as zeros of the Jost function
on the negative imaginary axis of the complex momentum space. The analysis
presented here is also suited for Coulombic systems.Comment: 6 pages, 1 table, 4 figure
Comparative studies of the scanning tunneling spectra in cuprate and iron-arsenide superconductors
We report scanning tunneling spectroscopic studies of cuprate and
iron-arsenic superconductors, including YBa_2Cu_3O_{7-\delta} (Y-123, T_c = 93
K), Sr_{0.9}La_{0.1}CuO_2 (La-112, T_c = 43 K), and the "122" compounds
Ba(Fe_{1-x}Co_x)_2As_2 (Co-122 with x = 0.06, 0.08, 0.12 for T_c = 14, 24, 20
K). For H > 0, pseudogap (\Delta_{PG}) features are revealed inside the
vortices, with \Delta_{PG} = [(\Delta_{eff})^2+(\Delta_{SC})^2]^{1/2} >
\Delta_{SC} in Y-123 and \Delta_{PG} < \Delta_{SC} in La-112, suggesting that
the physical origin of \Delta_{PG} is a competing order coexisting with
superconductivity. Additionally, Fourier transformation (FT) of the Y-123
spectra exhibits two types of spectral peaks, one type is associated with
energy (\omega)-dependent quasiparticle interference (QPI) wave-vectors and the
other consists of \omega-independent wave-vectors due to competing orders and
(\pi,\pi) magnetic resonances. For the multi-band Co-122 compounds, two-gap
superconductivity is found for all doping levels. Magnetic resonant modes that
follow the temperature dependence of the superconducting gaps are also
identified. These findings, together with the \omega- and x-dependent QPI
spectra, are consistent with a sign-changing s-wave pairing symmetry in the
Co-122 iron arsenides. Our comparative studies suggest that the commonalities
among the cuprate and the ferrous superconductors include the proximity to
competing orders, antiferromagnetic (AFM) spin fluctuations and magnetic
resonances in the superconducting (SC) state, and the unconventional pairing
symmetries with sign-changing order parameters on different parts of the Fermi
surface.Comment: 6 pages, 3 figures. Submitted to Journal of Physics: Conference
Proceedings for the 26th International Low-Temperature Conference (2011).
Corresponding author: Nai-Chang Yeh ([email protected]
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