11,186 research outputs found
(13)C NMR investigation of the superconductor MgCNi_3 up to 800K
We report (13)C NMR characterization of the new superconductor MgCNi_3 (He et
al., Nature (411), 54 (2001)). We found that both the uniform spin
susceptibility and the spin fluctuations show a strong enhancement with
decreasing temperature, and saturate below ~50K and ~20K respectively. The
nuclear spin-lattice relaxation rate 1/(13)T_1T exhibits typical behaviour for
isotropic s-wave superconductivity with a coherence peak below Tc=7.0K that
grows with decreasing magnetic field.Comment: Accepted for publication in Physical Review Letter
Gravitational radiation from dynamical black holes
An effective energy tensor for gravitational radiation is identified for
uniformly expanding flows of the Hawking mass-energy. It appears in an energy
conservation law expressing the change in mass due to the energy densities of
matter and gravitational radiation, with respect to a Killing-like vector
encoding a preferred flow of time outside a black hole. In a spin-coefficient
formulation, the components of the effective energy tensor can be understood as
the energy densities of ingoing and outgoing, transverse and longitudinal
gravitational radiation. By anchoring the flow to the trapping horizon of a
black hole in a given sequence of spatial hypersurfaces, there is a locally
unique flow and a measure of gravitational radiation in the strong-field
regime.Comment: 5 revtex4 pages. Additional comment
Black holes, cosmological singularities and change of signature
There exists a widespread belief that signature type change could be used to
avoid spacetime singularities. We show that signature change cannot be utilised
to this end unless the Einstein equation is abandoned at the suface of
signature type change. We also discuss how to solve the initial value problem
and show to which extent smooth and discontinuous signature changing solutions
are equivalent.Comment: 14pages, Latex, no figur
The Magnetization of Cu_2(C_5H_{12}N_2)_2Cl_4 : A Heisenberg Spin Ladder System
We study the magnetization of a Heisenberg spin ladder using exact
diagonalization techniques, finding three distinct magnetic phases. We consider
the results in relation to the experimental behaviour of the new copper
compound Cu_2(C_5H_{12}N_2)_2Cl_4 and deduce that the compound is well
described by such a model with a ratio of `chain' to `rung' bond strengths
(J/J^\prime) of the order of 0.2, consistent with results from the magnetic
susceptibility. The effects of temperature, spin impurities and additional
diagonal bonds are presented and we give evidence that these diagonal bonds are
indeed of a ferromagnetic nature.Comment: Latex file (4 pages), related figures (encapsulated postscript)
appende
A Note on trapped Surfaces in the Vaidya Solution
The Vaidya solution describes the gravitational collapse of a finite shell of
incoherent radiation falling into flat spacetime and giving rise to a
Schwarzschild black hole. There has been a question whether closed trapped
surfaces can extend into the flat region (whereas closed outer trapped surfaces
certainly can). For the special case of self-similar collapse we show that the
answer is yes, if and only if the mass function rises fast enough.Comment: 14 pages, 4 figures; minor polish added to version
Construction and enlargement of traversable wormholes from Schwarzschild black holes
Analytic solutions are presented which describe the construction of a
traversable wormhole from a Schwarzschild black hole, and the enlargement of
such a wormhole, in Einstein gravity. The matter model is pure radiation which
may have negative energy density (phantom or ghost radiation) and the
idealization of impulsive radiation (infinitesimally thin null shells) is
employed.Comment: 22 pages, 7 figure
SU(2) Cosmological Solitons
We present a class of numerical solutions to the SU(2) nonlinear
-model coupled to the Einstein equations with cosmological constant
in spherical symmetry. These solutions are characterized by the
presence of a regular static region which includes a center of symmetry. They
are parameterized by a dimensionless ``coupling constant'' , the sign of
the cosmological constant, and an integer ``excitation number'' . The
phenomenology we find is compared to the corresponding solutions found for the
Einstein-Yang-Mills (EYM) equations with positive (EYM). If
we choose positive and fix , we find a family of static spacetimes
with a Killing horizon for . As a limiting solution
for we find a {\em globally} static spacetime with
, the lowest excitation being the Einstein static universe. To
interpret the physical significance of the Killing horizon in the cosmological
context, we apply the concept of a trapping horizon as formulated by Hayward.
For small values of an asymptotically de Sitter dynamic region contains
the static region within a Killing horizon of cosmological type. For strong
coupling the static region contains an ``eternal cosmological black hole''.Comment: 20 pages, 6 figures, Revte
Spatiotemporal characterization of the transition from sinus rhythm to ventricular fibrillation during an acute ischemic event in the intact human heart by whole-heart sock-mapping
Optimization of the global re-entry vulnerability index to minimise cycle length dependency and prediction of ventricular arrhythmias during human epicardial sock mapping
The re-entry vulnerability index (RVI) is an activation-repolarization marker recently proposed to predict sites of ventricular tachycardia (VT) initiation. RVI is inversely related to the probability of establishing a re-entry. The aim of this study was to characterize the CL dependency of RVI, assess different methods for CL-dependency corrections and test the capability of RVI to predict ventricular arrhythmias. Twenty-four subjects underwent whole heart epicardial mapping using a multi-electrode sock enabling the recording of 240 unipolar electrograms. Ventricular pacing was delivered at CLs decreasing from 600 to 350 ms in steps of 50 ms. In a separate study, 1 patient went into VT during steady state pacing. Predisposition to VT was assessed by using the 10th percentile RVI, termed global RVI. The results show that own to CL dependency of local repolarization, there was a strong positive association between RVI and CL. Local repolarization detrending and correction with the Bazett's formula eliminated the CL dependency, while a weak association was found after correction with the Fredericia's formula. In the patient who developed VT, global RVI was significantly lower than in the patients who did not develop VT. Corrections for CL dependency enhanced these differences. In conclusion, de-trending and Bazett's corrections effectively compensated for the CL dependency of RVI and global RVI may reveal predisposition to ventricular arrhythmias. Further analysis is necessary to establish the role of RVI for risk stratification
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