16,726 research outputs found
Creep rupture of materials: insights from a fiber bundle model with relaxation
I adapted a model recently introduced in the context of seismic phenomena, to
study creep rupture of materials. It consists of linear elastic fibers that
interact in an equal load sharing scheme, complemented with a local
viscoelastic relaxation mechanism. The model correctly describes the three
stages of the creep process, namely an initial Andrade regime of creep
relaxation, an intermediate regime of rather constant creep rate, and a
tertiary regime of accelerated creep towards final failure of the sample. In
the tertiary regime creep rate follows the experimentally observed one over
time-to-failure dependence. The time of minimum strain rate is systematically
observed to be about 60-65 % of the time to failure, in accordance with
experimental observations. In addition, burst size statistics of breaking
events display a -3/2 power law for events close to the time of failure, and a
steeper decay for the all-time distribution. Statistics of interevent times
shows a tendency of the events to cluster temporarily. This behavior should be
observable in acoustic emission experiments
Superconducting Gap Structure of kappa-(BEDT-TTF)2Cu(NCS)2 Probed by Thermal Conductivity Tensor
The thermal conductivity of organic superconductor kappa-(BEDT-TTF)2Cu(NCS)2
(Tc =10.4 K) has been studied in a magnetic field rotating within the 2D
superconducting planes with high alignment precision. At low temperatures (T <
0.5 K), a clear fourfold symmetry in the angular variation, which is
characteristic of a d-wave superconducting gap with nodes along the directions
rotated 45 degrees relative to the b and c axes of the crystal, was resolved.
The determined nodal structure is inconsistent with recent theoretical
predictions of superconductivity induced by the antiferromagnetic spin
fluctuation.Comment: 5 pages, 4 figures, to be published in Phys. Rev. Let
New Universality of Lyapunov Spectra in Hamiltonian Systems
A new universality of Lyapunov spectra {\lambda_i} is shown for Hamiltonian
systems. The universality appears in middle energy regime and is different from
another universality which can be reproduced by random matrices in the
following two points. One is that the new universality appears in a limited
range of large i/N rather than the whole range, where N is degrees of freedom.
The other is Lyapunov spectra do not behave linearly while random matrices give
linear behavior even on 3D lattice. Quadratic terms with smaller nonlinear
terms of potential functions play an intrinsic role in the new universality.Comment: 19 pages, 16 Encapsulated Postscript figures, LaTeX (100 kb
Quench Protection of DI-BSCCO Coil
AbstractQuench protection is one of the most important requirements for the practical application of high-temperature-superconducting (HTS) coils. Quench protection requires that early detection of a developing quench event is followed by rapid reduction of the operating current. However, such quench detection is very difficult because HTS wire produces heat only locally due to the very slow propagation velocity of a normal zone. Excellent high voltage insulation performance is required if the current is to be reduced rapidly in a large-scale superconducting application with very large inductance. Thus it is important to investigate the behavior of coils with various decay time constants, and to detect voltages on very short time scales. This goal remains to be achieved. In the present study we built test coil and a full-scale pole coil for a 20 MW motor for use in experiments on quench protection, and parameterized the relation between the decay time constant and the detecting voltage, using a conventional balance circuit to detect the quench, which was generated by gradually raising the temperature of the coils. The results verify that a balance circuit can be used for quench detection. For example, when the current decay time constant is 4seconds, the test coil can be protected even with a detecting voltage of 0.15 volts, despite a significant heat production rate of 126W. We also confirmed that the full-scale pole coil, with a decay time constant of 20seconds, can be protected with a detecting voltage of 0.06V
A self-consistent first-principles calculation scheme for correlated electron systems
A self-consistent calculation scheme for correlated electron systems is
created based on the density-functional theory (DFT). Our scheme is a
multi-reference DFT (MR-DFT) calculation in which the electron charge density
is reproduced by an auxiliary interacting Fermion system. A short-range
Hubbard-type interaction is introduced by a rigorous manner with a residual
term for the exchange-correlation energy. The Hubbard term is determined
uniquely by referencing the density fluctuation at a selected localized
orbital. This strategy to obtain an extension of the Kohn-Sham scheme provides
a self-consistent electronic structure calculation for the materials design.
Introducing an approximation for the residual exchange-correlation energy
functional, we have the LDA+U energy functional. Practical self-consistent
calculations are exemplified by simulations of Hydrogen systems, i.e. a
molecule and a periodic one-dimensional array, which is a proof of existence of
the interaction strength U as a continuous function of the local fluctuation
and structural parameters of the system.Comment: 23 pages, 8 figures, to appear in J. Phys. Condens. Matte
Effective Search Templates for a Primordial Stochastic Gravitational Wave Background
We calculate the signal-to-noise ratio (SNR) of the stochastic
gravitational-wave background in an extreme case that its spectrum has a sharp
falloff with its amplitude close to the detection threshold. Such a spectral
feature is a characteristic imprint of the change in the number of relativistic
degrees of freedom on the stochastic background generated during inflation in
the early Universe. We find that, although SNR is maximal with the correct
template which is proportional to the assumed real spectrum, its sensitivity to
the shape of template is fairly weak indicating that a simple power-law
template is sufficient to detect the signature.Comment: 13 pages, 4 figures, final version to be published in PR
Geometric Approach to Lyapunov Analysis in Hamiltonian Dynamics
As is widely recognized in Lyapunov analysis, linearized Hamilton's equations
of motion have two marginal directions for which the Lyapunov exponents vanish.
Those directions are the tangent one to a Hamiltonian flow and the gradient one
of the Hamiltonian function. To separate out these two directions and to apply
Lyapunov analysis effectively in directions for which Lyapunov exponents are
not trivial, a geometric method is proposed for natural Hamiltonian systems, in
particular. In this geometric method, Hamiltonian flows of a natural
Hamiltonian system are regarded as geodesic flows on the cotangent bundle of a
Riemannian manifold with a suitable metric. Stability/instability of the
geodesic flows is then analyzed by linearized equations of motion which are
related to the Jacobi equations on the Riemannian manifold. On some geometric
setting on the cotangent bundle, it is shown that along a geodesic flow in
question, there exist Lyapunov vectors such that two of them are in the two
marginal directions and the others orthogonal to the marginal directions. It is
also pointed out that Lyapunov vectors with such properties can not be obtained
in general by the usual method which uses linearized Hamilton's equations of
motion. Furthermore, it is observed from numerical calculation for a model
system that Lyapunov exponents calculated in both methods, geometric and usual,
coincide with each other, independently of the choice of the methods.Comment: 22 pages, 14 figures, REVTeX
Bubbling Calabi-Yau geometry from matrix models
We study bubbling geometry in topological string theory. Specifically, we
analyse Chern-Simons theory on both the 3-sphere and lens spaces in the
presence of a Wilson loop insertion of an arbitrary representation. For each of
these three manifolds we formulate a multi-matrix model whose partition
function is the vev of the Wilson loop and compute the spectral curve. This
spectral curve is the reduction to two dimensions of the mirror to a Calabi-Yau
threefold which is the gravitational dual of the Wilson loop insertion. For
lens spaces the dual geometries are new. We comment on a similar matrix model
which appears in the context of Wilson loops in AdS/CFT.Comment: 30 pages; v.2 reference added, minor correction
- …