70,024 research outputs found
Nonlinear robust controller design for multi-robot systems with unknown payloads
This work is concerned with the control problem of a multi-robot system handling a payload with unknown mass properties. Force constraints at the grasp points are considered. Robust control schemes are proposed that cope with the model uncertainty and achieve asymptotic path tracking. To deal with the force constraints, a strategy for optimally sharing the task is suggested. This strategy basically consists of two steps. The first detects the robots that need help and the second arranges that help. It is shown that the overall system is not only robust to uncertain payload parameters, but also satisfies the force constraints
Constraints and evolution in cosmology
We review some old and new results about strict and non strict hyperbolic
formulations of the Einstein equations.Comment: To appear in the proceedings of the first Aegean summer school in
General Relativity, S. Cotsakis ed. Springer Lecture Notes in Physic
Mean Free Path in Disordered Multichannel Tight-Binding Wires
Transport in a disordered tight-binding wire involves a collection of
different mean free paths resulting from the distinct fermi points, which
correspond to the various scattering channels of the wire. The generalization
of Thouless' relation between the mean free path and the localization length
permits to define an average channel mean free path,, such that
in an -channel system. The averaged mean free path
is expressed exactly in terms of the total reflection coefficient of
the wire and compared with the mean free path defined in the maximum entropy
approach
Theory of Impurity Effects on the Spin Nematic State
The effect of magnetic bond disorder in otherwise antiferro nematic ordered
system is investigated. We introduced triangular-shaped ferromagnetic bond
disorder in the S=1 bilinear-biquadratic model on a triangular lattice. It is
shown that the coupling between the impurity magnetic moment and nonmagnetic
excitation in the bulk yields single-moment anisotropy and long-range
anisotropic interaction between impurity magnetic moments. This interaction can
induce unconventional spin-freezing phenomena observed in triangular magnet,
NiGa2S4.Comment: 19 pages, 14 figure
Thresholds for epidemic spreading in networks
We study the threshold of epidemic models in quenched networks with degree
distribution given by a power-law. For the susceptible-infected-susceptible
(SIS) model the activity threshold lambda_c vanishes in the large size limit on
any network whose maximum degree k_max diverges with the system size, at odds
with heterogeneous mean-field (HMF) theory. The vanishing of the threshold has
not to do with the scale-free nature of the connectivity pattern and is instead
originated by the largest hub in the system being active for any spreading rate
lambda>1/sqrt{k_max} and playing the role of a self-sustained source that
spreads the infection to the rest of the system. The
susceptible-infected-removed (SIR) model displays instead agreement with HMF
theory and a finite threshold for scale-rich networks. We conjecture that on
quenched scale-rich networks the threshold of generic epidemic models is
vanishing or finite depending on the presence or absence of a steady state.Comment: 5 pages, 4 figure
Antiferromagnetic Quantum Spins on the Pyrochlore Lattice
The ground state of the S=1/2 Heisenberg antiferromagnet on the pyrochlore
lattice is theoretically investigated. Starting from the limit of isolated
tetrahedra, I include interactions between the tetrahedra and obtain an
effective model for the spin-singlet ground state multiplet by third-order
perturbation. I determine its ground state using the mean-field approximation
and found a dimerized state with a four-sublattice structure, which agrees with
the proposal by Harris et al. I also discuss chirality correlations and spin
correlations for this state.Comment: 4 pages in 2-column format, 5 figures; To appear in J. Phys. Soc.
Jpn. (Mar, 2001
A theory of non-local linear drift wave transport
Transport events in turbulent tokamak plasmas often exhibit non-local or
non-diffusive action at a distance features that so far have eluded a
conclusive theoretical description. In this paper a theory of non-local
transport is investigated through a Fokker-Planck equation with fractional
velocity derivatives. A dispersion relation for density gradient driven linear
drift modes is derived including the effects of the fractional velocity
derivative in the Fokker-Planck equation. It is found that a small deviation (a
few percent) from the Maxwellian distribution function alters the dispersion
relation such that the growth rates are substantially increased and thereby may
cause enhanced levels of transport.Comment: 22 pages, 2 figures. Manuscript submitted to Physics of Plasma
Uniform Mixing of High-Tc Superconductivity and Antiferromagnetism on a Single CuO2 Plane in Hg-based Five-layered Cuprate
We report a site selective Cu-NMR study on under-doped Hg-based five-layered
high- cuprate HgBa2Ca4Cu5Oy with a Tc=72 K. Antiferromagnetism (AF)
has been found to take place at TN=290 K, exhibiting a large antiferromagnetic
moment of 0.67-0.69uB at three inner planes (IP's). This value is comparable to
the values reported for non-doped cuprates, suggesting that the IP may be in a
nearly non-doped regime. Most surprisingly, the AF order is also detected with
M(OP)=0.1uB even at two outer planes (OP's) that are responsible for the onset
of superconductivity (SC). The high-Tc SC at Tc = 72 K can uniformly coexist on
a microscopic level with the AF at OP's. This is the first microscopic evidence
for the uniform mixed phase of AF and SC on a single CuO2 plane in a simple
environment without any vortex lattice and/or stripe order.Comment: 4 pages, 4 figures. To be published in Phys.Rev.Let
Conformal ``thin sandwich'' data for the initial-value problem of general relativity
The initial-value problem is posed by giving a conformal three-metric on each
of two nearby spacelike hypersurfaces, their proper-time separation up to a
multiplier to be determined, and the mean (extrinsic) curvature of one slice.
The resulting equations have the {\it same} elliptic form as does the
one-hypersurface formulation. The metrical roots of this form are revealed by a
conformal ``thin sandwich'' viewpoint coupled with the transformation
properties of the lapse function.Comment: 7 pages, RevTe
Effect of nuclear quadrupole interactions on the dynamics of two-level systems in glasses
The standard tunneling model describes quite satisfactorily the thermal
properties of amorphous solids at temperatures in terms of an ensemble
of two-level systems possessing logarithmically uniform distribution over their
tunneling amplitudes and uniform distribution over their asymmetry energies. In
particular, this distribution explains the observable logarithmic temperature
dependence of the dielectric constant. Yet, experiments have shown that at
ultralow temperatures such a temperature behavior breaks down and the
dielectric constant becomes temperature independent (plateau effect). In this
letter we suggest an explanation of this behavior exploiting the effect of the
nuclear quadrupole interaction on tunneling. We show that below a temperature
corresponding to the characteristic energy of the nuclear quadrupole
interaction the effective tunneling amplitude is reduced by a small overlap
factor of the nuclear quadrupole ground states in the left and right potential
wells of the tunneling system. It is just this reduction that explains the
plateau effect . We predict that the application of a sufficiently large
magnetic field should restore the logarithmic dependence because of the
suppression of the nuclear quadrupole interaction.Comment: To appear in the Physical Review Letter
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