8,018 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
Observation of vortices and hidden pseudogap from scanning tunneling spectroscopic studies of electron-doped cuprate superconductor
We present the first demonstration of vortices in an electron-type cuprate
superconductor, the highest (= 43 K) electron-type cuprate
. Our spatially resolved quasiparticle tunneling spectra
reveal a hidden low-energy pseudogap inside the vortex core and unconventional
spectral evolution with temperature and magnetic field. These results cannot be
easily explained by the scenario of pure superconductivity in the ground state
of high- superconductivity.Comment: 6 pages, 4 figures. Two new graphs have been added into Figure 2.
Accepted for publication in Europhysics Letters. Corresponding author:
Nai-Chang Yeh (E-mail: [email protected]
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
Three-body problem at finite temperature and density
We derive practical three-body equations for the equal-time three-body Green
function in matter. Our equations describe both bosons and fermions at finite
density and temperature, and take into account all possible two-body
sub-processes allowed by the underlying Hamiltonian.Comment: 24 pages, revtex
Magnetic properties of the quasi-two-dimensional S = 1/2 Heisenberg antiferromagnet [Cu(pyz)2(HF2)]PF6
We report on high-field magnetization, specific-heat and electron spin
resonance (ESR) studies of the quasi-two-dimensional spin-1/2 Heisenberg
antiferromagnet [Cu(pyz)2(HF2)]PF6. The frequency-field diagram of ESR modes
below TN = 4.38 K is described in the frame of the meanfield theory, confirming
a collinear magnetic structure with an easy-plane anisotropy. The obtained
results allowed us to determine the anisotropy/exchange interaction ratio, A/J
= 0.003, and the upper limit for the inter/intra-plane exchange-interaction
ratio, J'/J = 1/16. It is argued that despite the onset of 3D long-range
magnetic ordering the magnetic properties of this material (including
high-magnetic-field magnetization and non-monotonic field dependence of the
Neel temperature) are strongly affected by two-dimensional spin correlations.Comment: 5 pages, 7 figure
Violations of Lorentz Covariance in Light Front Quark Models
Electromagnetic form factors of the nucleon from relativistic quark models
are analyzed: results from null-plane projection of the Feynman triangle
diagram are compared with a Bakamjian-Thomas model. The magnetic form factors
of the models differ by about 15% at spacelike momentum transfer 0.5 GeV^2,
while the charge form factors are much closer. Spurious contributions to
electromagnetic form factors due to violations of rotational symmetry are
eliminated from both models. One method changes magnetic form factors by about
10%, whereas the charge form factors stay nearly the same. Another one changes
the charge form factor of the Bakamjian-Thomas model by more than 50%.Comment: 19 pages, 9 figures, Late
Scanning tunneling spectroscopic evidence for magnetic field-induced microscopic orders in the high- superconductor YBaCuO
We report spatially resolved tunneling spectroscopic evidence for
field-induced microscopic orders in a high- superconductor . The spectral characteristics inside vortices reveal a
pseudogap () larger than the superconducting gap () as well as a subgap () smaller than ,
and the spectral weight shifts steadily from to
and upon increasing magnetic field. Additionally,
energy-independent conductance modulations at 3.6 and 7.1 lattice constants
along the Cu-O bonding directions and at 9.5 lattice constants along the nodal
directions are manifested in the vortex state. These wave-vectors differ
fundamentally from the strongly dispersive modes due to Bogoliubov
quasiparticle scattering interferences and may be associated with field-induced
microscopic orders of pair-, charge- and spin-density waves.Comment: Paper updated and accepted for publication in Europhysics Letters. 4
figures and 6 pages. Corresponding author: Nai-Chang Yeh (E-mail:
[email protected]
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
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