7,682 research outputs found
Non-adiabatic effects in the phonon dispersion of Mg 1--x Al x B 2
Superconducting MgB shows an E zone center phonon, as measured
by Raman spectroscopy, that is very broad in energy and temperature dependent.
The Raman shift and lifetime show large differences with the values elsewhere
in the Brillouin Zone measured by Inelastic X-ray Scattering (IXS), where its
dispersion can be accounted for by standard harmonic phonon theory, adding only
a moderate electron-phonon coupling. Here we show that the effects rapidly
disappear when electron-phonon coupling is switched off by Al substitution on
the Mg sites. Moreover, using IXS with very high wave-vector resolution in
MgB, we can follow the dispersion connecting the Raman and the IXS signal,
in agreement with a theory using only electron-phonon coupling but without
strong anharmonic terms. The observation is important in order to understand
the effects of electron-phonon coupling on zone center phonons modes in
MgB, but also in all metals characterized by a small Fermi velocity in a
particular direction, typical for layered compounds
Brownian Entanglement
We show that for two classical brownian particles there exists an analog of
continuous-variable quantum entanglement: The common probability distribution
of the two coordinates and the corresponding coarse-grained velocities cannot
be prepared via mixing of any factorized distributions referring to the two
particles in separate. This is possible for particles which interacted in the
past, but do not interact in the present. Three factors are crucial for the
effect: 1) separation of time-scales of coordinate and momentum which motivates
the definition of coarse-grained velocities; 2) the resulting uncertainty
relations between the coordinate of the brownian particle and the change of its
coarse-grained velocity; 3) the fact that the coarse-grained velocity, though
pertaining to a single brownian particle, is defined on a common context of two
particles. The brownian entanglement is a consequence of a coarse-grained
description and disappears for a finer resolution of the brownian motion. We
discuss possibilities of its experimental realizations in examples of
macroscopic brownian motion.Comment: 18 pages, no figure
Robonaut 2 - Initial Activities On-Board the ISS
Robonaut 2, or R2, arrived on the International Space Station in February 2011 and is currently undergoing testing in preparation for it to become, initially, an Intra-Vehicular Activity (IVA) tool and then evolve into a system that can perform Extra-Vehicular Activities (EVA). After the completion of a series of system level checks to ensure that the robot traveled well on-board the Space Shuttle Atlantis, ground control personnel will remotely control the robot to perform free space tasks that will help characterize the differences between earth and zero-g control. For approximately one year, the fixed base R2 will perform a variety of experiments using a reconfigurable task board that was launched with the robot. While working side-by-side with human astronauts, Robonaut 2 will actuate switches, use standard tools, and manipulate Space Station interfaces, soft goods and cables. The results of these experiments will demonstrate the wide range of tasks a dexterous humanoid can perform in space and they will help refine the methodologies used to control dexterous robots both in space and here on earth. After the trial period that will evaluate R2 while on a fixed stanchion in the US Laboratory module, NASA plans to launch climbing legs that when attached to the current on-orbit R2 upper body will give the robot the ability to traverse through the Space Station and start assisting crew with general IVA maintenance activities. Multiple control modes will be evaluated in this extra-ordinary ISS test environment to prepare the robot for use during EVAs. Ground Controllers will remotely supervise the robot as it executes semi-autonomous scripts for climbing through the Space Station and interacting with IVA interfaces. IVA crew will locally supervise the robot using the same scripts and also teleoperate the robot to simulate scenarios with the robot working alone or as an assistant during space walks
A decomposition algorithm for feedback min-max model predictive control
Abstract-An algorithm for solving feedback min-max model predictive control for discrete time uncertain linear systems with constraints is presented in the paper. The algorithm solves the corresponding multi-stage min-max linear optimization problem. It is based on applying recursively a decomposition technique to solve the min-max problem via a sequence of low complexity linear programs. It is proved that the algorithm converges to the optimal solution in finite time. Simulation results are provided to compare the proposed algorithm with other approaches
Born's rule from measurements of classical signals by threshold detectors which are properly calibrated
The very old problem of the statistical content of quantum mechanics (QM) is
studied in a novel framework. The Born's rule (one of the basic postulates of
QM) is derived from theory of classical random signals. We present a
measurement scheme which transforms continuous signals into discrete clicks and
reproduces the Born's rule. This is the sheme of threshold type detection.
Calibration of detectors plays a crucial role.Comment: The problem of double clicks is resolved; hence, one can proceed in
purely wave framework, i.e., the wave-partcile duality has been resolved in
favor of the wave picture of prequantum realit
Changes in the firn structure of the western Greenland Ice Sheet caused by recent warming
Atmospheric warming over the Greenland Ice Sheet during the last 2 decades has increased the amount of surface meltwater production, resulting in the migration of melt and percolation regimes to higher altitudes and an increase in the amount of ice content from refrozen meltwater found in the firn above the superimposed ice zone. Here we present field and airborne radar observations of buried ice layers within the near-surface (0–20 m) firn in western Greenland, obtained from campaigns between 1998 and 2014. We find a sharp increase in firn-ice content in the form of thick widespread layers in the percolation zone, which decreases the capacity of the firn to store meltwater. The estimated total annual ice content retained in the near-surface firn in areas with positive surface mass balance west of the ice divide in Greenland reached a maximum of 74 ± 25 Gt in 2012, compared to the 1958–1999 average of 13 ± 2 Gt, while the percolation zone area more than doubled between 2003 and 2012. Increased melt and column densification resulted in surface lowering averaging −0.80 ± 0.39 m yr−1 between 1800 and 2800 m in the accumulation zone of western Greenland. Since 2007, modeled annual melt and refreezing rates in the percolation zone at elevations below 2100 m surpass the annual snowfall from the previous year, implying that mass gain in the region is retained after melt in the form of refrozen meltwater. If current melt trends over high elevation regions continue, subsequent changes in firn structure will have implications for the hydrology of the ice sheet and related abrupt seasonal densification could become increasingly significant for altimetry-derived ice sheet mass balance estimates
MPC for tracking periodic reference signals
This paper is devoted to the design of a predictive controller for constrained linear systems to track periodic references. The only assumption on the dynamics of the reference is that it is periodic and its period is known. It is also assumed that the reference signal is a priori known by the controller. Inspired in the hierarchical control scheme based on the trajectory planification, the ideas of the MPC for tracking [Limon et al., 2008] are extended to this case. The proposed predictive controller has the future sequence of inputs and an artificial reference as decision variables. The cost function is divided into two terms: one penalizes the tracking error with the artificial reference and other penalizes the deviation of the artificial reference to the reference to be tracked. Stability is ensured thanks to the addition of two constraints: a terminal constraint on the predicted trajectory and a constraint that enforces the artificial reference to be periodic. It is proved that the proposed controller is recursively feasible and the controlled system satisfies the hard constraints, is asymptotically stable and converges to the best possible reachable trajectory. The properties of the proposed controller are illustrated in an example
Schroedingers equation with gauge coupling derived from a continuity equation
We consider a statistical ensemble of particles of mass m, which can be
described by a probability density \rho and a probability current \vec{j} of
the form \rho \nabla S/m. The continuity equation for \rho and \vec{j} implies
a first differential equation for the basic variables \rho and S. We further
assume that this system may be described by a linear differential equation for
a complex state variable \chi. Using this assumptions and the simplest possible
Ansatz \chi(\rho,S) Schroedingers equation for a particle of mass m in an
external potential V(q,t) is deduced. All calculations are performed for a
single spatial dimension (variable q) Using a second Ansatz \chi(\rho,S,q,t)
which allows for an explict q,t-dependence of \chi, one obtains a generalized
Schroedinger equation with an unusual external influence described by a
time-dependent Planck constant. All other modifications of Schroeodingers
equation obtained within this Ansatz may be eliminated by means of a gauge
transformation. Thus, this second Ansatz may be considered as a generalized
gauging procedure. Finally, making a third Ansatz, which allows for an
non-unique external q,t-dependence of \chi, one obtains Schroedingers equation
with electromagnetic potentials \vec{A}, \phi in the familiar gauge coupling
form. A possible source of the non-uniqueness is pointed out.Comment: 25 pages, no figure
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