131,752 research outputs found
The metal-insulator transition in disordered solids: How theoretical prejudices influence its characterization. A critical review of analyses of experimental data
In a recent experiment, Siegrist et al. [Nature Materials 10, 202 (2011)]
investigated the metal-insulator transition (MIT) of GeSb_2Te_4 on increasing
annealing temperature. The authors conclude that this material exhibits a
discontinuous MIT with a finite minimum metallic conductivity. The striking
contrast to reports on other disordered substances motivates the present
in-depth study of the influence of the MIT criterion used on the
characterization of the MIT. First, we discuss in detail the inherent biases of
the various available approaches to locating the MIT. Second, reanalyzing the
GeSb_2Te_4 measurements, we show that this material resembles other disordered
solids to a large extent: according to a widely-used approach, these data may
also be interpreted in terms of a continuous MIT. Checking the justification of
the respective fits, however, uncovers inconsistencies in the experimental
data. Third, comparing with previous experimental studies of crystalline Si:As,
Si:P, Si:B, Ge:Ga, CdSe:In, n-Cd_{0.95}Mn$_{0.05}Se,
Cd_{0.95}Mn_{0.05}Te_{0.97}Se_{0.03}:In, disordered Gd, and nano-granular Pt-C,
we show that such an inconclusive behavior occurs frequently: the analysis of
the logarithmic temperature derivative of the conductivity highlights serious
inconsistencies in the original interpretations in terms of a continuous MIT.
In part, they are common to all these studies and seem to be generic, in part,
they vary from experiment to experiment and may arise from measurement
problems. Thus, the question for the character of the MIT of these materials
has to be considered as yet open. The challenges now lie in improving the
measurement precision and in developing a microscopic theory capable of
explaining the seemingly generic features.Comment: Revtex-file + 23 figures -> 51 pages. Revisions: Some arguments
completed; structure slightly modified: mathematical part of former
Subsection II.E is now presented as Appendix B. This version was accepted for
publ. by Critical Reviews in Solid State and Materials Sciences at July 18,
2017. It differs from this publication concerning citation style, abstract,
and some very minor modification
Spacecraft Attitude Stabilization with Piecewise-constant Magnetic Dipole Moment
In actual implementations of magnetic control laws for spacecraft attitude
stabilization, the time in which Earth magnetic field is measured must be
separated from the time in which magnetic dipole moment is generated. The
latter separation translates into the constraint of being able to genere only
piecewise-constant magnetic dipole moment. In this work we present attitude
stabilization laws using only magnetic actuators that take into account of the
latter aspect. Both a state feedback and an output feedback are presented, and
it is shown that the proposed design allows for a systematic selection of the
sampling period.Comment: arXiv admin note: text overlap with arXiv:1411.275
Cooperative Adaptive Control for Cloud-Based Robotics
This paper studies collaboration through the cloud in the context of
cooperative adaptive control for robot manipulators. We first consider the case
of multiple robots manipulating a common object through synchronous centralized
update laws to identify unknown inertial parameters. Through this development,
we introduce a notion of Collective Sufficient Richness, wherein parameter
convergence can be enabled through teamwork in the group. The introduction of
this property and the analysis of stable adaptive controllers that benefit from
it constitute the main new contributions of this work. Building on this
original example, we then consider decentralized update laws, time-varying
network topologies, and the influence of communication delays on this process.
Perhaps surprisingly, these nonidealized networked conditions inherit the same
benefits of convergence being determined through collective effects for the
group. Simple simulations of a planar manipulator identifying an unknown load
are provided to illustrate the central idea and benefits of Collective
Sufficient Richness.Comment: ICRA 201
Nonlinear dynamics of phase separation in thin films
We present a long-wavelength approximation to the Navier-Stokes Cahn-Hilliard
equations to describe phase separation in thin films. The equations we derive
underscore the coupled behaviour of free-surface variations and phase
separation. We introduce a repulsive substrate-film interaction potential and
analyse the resulting fourth-order equations by constructing a Lyapunov
functional, which, combined with the regularizing repulsive potential, gives
rise to a positive lower bound for the free-surface height. The value of this
lower bound depends on the parameters of the problem, a result which we compare
with numerical simulations. While the theoretical lower bound is an obstacle to
the rupture of a film that initially is everywhere of finite height, it is not
sufficiently sharp to represent accurately the parametric dependence of the
observed dips or `valleys' in free-surface height. We observe these valleys
across zones where the concentration of the binary mixture changes sharply,
indicating the formation of bubbles. Finally, we carry out numerical
simulations without the repulsive interaction, and find that the film ruptures
in finite time, while the gradient of the Cahn--Hilliard concentration develops
a singularity.Comment: 26 pages, 20 figures, PDFLaTeX with RevTeX4 macros. A thorough
analysis of the equations is presented in arXiv:0805.103
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