49,521 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
Universality and correlations in individuals wandering through an online extremist space
The 'out of the blue' nature of recent terror attacks and the diversity of
apparent motives, highlight the importance of understanding the online
trajectories that individuals follow prior to developing high levels of
extremist support. Here we show that the physics of stochastic walks, with and
without temporal correlation, provides a unifying description of these online
trajectories. Our unique dataset comprising all users of a global social media
site, reveals universal characteristics in individuals' online lifetimes. Our
accompanying theory generates analytical and numerical solutions that describe
the characteristics shown by individuals that go on to develop high levels of
extremist support, and those that do not. The existence of these temporal and
also many-body correlations suggests that existing physics machinery can be
used to quantify and perhaps mitigate the risk of future events
Shuttle-promoted nano-mechanical current switch
We investigate electron shuttling in three-terminal nanoelectromechanocal
device built on a movable metallic rod oscillating between two drains. The
device shows a double-well shaped electromechanical potential tunable by a
source-drain bias voltage. Four stationary regimes controllable by the bias are
found for this device: (i) single stable fixed point, (ii) two stable fixed
points, (iii) two limiting cycles, and (iv) single limiting cycle. In the
presence of perpendicular magnetic field the Lorentz force makes possible
switching from one electromechanical state to another. The mechanism of tunable
transitions between various stable regimes based on the interplay between
voltage controlled electromechanical instability and magnetically controlled
switching is suggested. The switching phenomenon is implemented for achieving
both a reliable \emph{active} current switch and sensoring of small variations
of magnetic field.Comment: 11 pages, 4 figure
Self-sustained oscillations in nanoelectromechanical systems induced by Kondo resonance
We investigate instability and dynamical properties of nanoelectromechanical
systems represented by a single-electron device containing movable quantum dot
attached to a vibrating cantilever via asymmetric tunnel contact. The Kondo
resonance in electron tunneling between source and shuttle facilitates
self-sustained oscillations originated from strong coupling of mechanical and
electronic/spin degrees of freedom. We analyze stability diagram for
two-channel Kondo shuttling regime due to limitations given by the
electromotive force acting on a moving shuttle and find that the saturation
amplitude of oscillation is associated with the retardation effect of
Kondo-cloud. The results shed light on possible ways of experimental
realization of dynamical probe for the Kondo-cloud by using high tunability of
mechanical dissipation as well as supersensitive detection of mechanical
displacement
q-Deformation of W(2,2) Lie algebra associated with quantum groups
An explicit realization of the W(2,2) Lie algebra is presented using the
famous bosonic and fermionic oscillators in physics, which is then used to
construct the q-deformation of this Lie algebra. Furthermore, the quantum group
structures on the q-deformation of this Lie algebra are completely determined.Comment: 12 page
Consistency test of general relativity from large scale structure of the Universe
We construct a consistency test of General Relativity (GR) on cosmological
scales. This test enables us to distinguish between the two alternatives to
explain the late-time accelerated expansion of the universe, that is, dark
energy models based on GR and modified gravity models without dark energy. We
derive the consistency relation in GR which is written only in terms of
observables - the Hubble parameter, the density perturbations, the peculiar
velocities and the lensing potential. The breakdown of this consistency
relation implies that the Newton constant which governs large-scale structure
is different from that in the background cosmology, which is a typical feature
in modified gravity models. We propose a method to perform this test by
reconstructing the weak lensing spectrum from measured density perturbations
and peculiar velocities. This reconstruction relies on Poisson's equation in GR
to convert the density perturbations to the lensing potential. Hence any
inconsistency between the reconstructed lensing spectrum and the measured
lensing spectrum indicates the failure of GR on cosmological scales. The
difficulties in performing this test using actual observations are discussed.Comment: 7 pages, 1 figur
Energy and momentum deposited into a QCD medium by a jet shower
Hard partons moving through a dense QCD medium lose energy by radiative
emissions and elastic scatterings. Deposition of the radiative contribution
into the medium requires rescattering of the radiated gluons. We compute the
total energy loss and its deposition into the medium self-consistently within
the same formalism, assuming perturbative interaction between probe and medium.
The same transport coefficients that control energy loss of the hard parton
determine how the energy is deposited into the medium; this allows a parameter
free calculation of the latter once the former have been computed or extracted
from experimental energy loss data. We compute them for a perturbative medium
in hard thermal loop (HTL) approximation. Assuming that the deposited
energy-momentum is equilibrated after a short relaxation time, we compute the
medium's hydrodynamical response and obtain a conical pattern that is strongly
enhanced by showering.Comment: 4 pages, 3 figures, revtex4, intro modified, typos correcte
Magneto-infrared spectroscopy of Landau levels and Zeeman splitting of three-dimensional massless Dirac Fermions in ZrTe
We present a magneto-infrared spectroscopy study on a newly identified
three-dimensional (3D) Dirac semimetal ZrTe. We observe clear transitions
between Landau levels and their further splitting under magnetic field. Both
the sequence of transitions and their field dependence follow quantitatively
the relation expected for 3D \emph{massless} Dirac fermions. The measurement
also reveals an exceptionally low magnetic field needed to drive the compound
into its quantum limit, demonstrating that ZrTe is an extremely clean
system and ideal platform for studying 3D Dirac fermions. The splitting of the
Landau levels provides a direct and bulk spectroscopic evidence that a
relatively weak magnetic field can produce a sizeable Zeeman effect on the 3D
Dirac fermions, which lifts the spin degeneracy of Landau levels. Our analysis
indicates that the compound evolves from a Dirac semimetal into a topological
line-node semimetal under current magnetic field configuration.Comment: Editors' Suggestio
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