80,922 research outputs found
Plug-and-Play Decentralized Model Predictive Control
In this paper we consider a linear system structured into physically coupled
subsystems and propose a decentralized control scheme capable to guarantee
asymptotic stability and satisfaction of constraints on system inputs and
states. The design procedure is totally decentralized, since the synthesis of a
local controller uses only information on a subsystem and its neighbors, i.e.
subsystems coupled to it. We first derive tests for checking if a subsystem can
be plugged into (or unplugged from) an existing plant without spoiling overall
stability and constraint satisfaction. When this is possible, we show how to
automatize the design of local controllers so that it can be carried out in
parallel by smart actuators equipped with computational resources and capable
to exchange information with neighboring subsystems. In particular, local
controllers exploit tube-based Model Predictive Control (MPC) in order to
guarantee robustness with respect to physical coupling among subsystems.
Finally, an application of the proposed control design procedure to frequency
control in power networks is presented.Comment: arXiv admin note: text overlap with arXiv:1210.692
Current-driven domain wall dynamics in ferrimagnets: Micromagnetic approach and collective coordinates model
[EN] Theoretical studies dealing with current-driven domain wall dynamics in ferrimagnetic
alloys and, by extension, other antiferromagnetically coupled systems
as some multilayers, are here presented. The analysis has been made by means
of micromagnetic simulations that consider these systems as constituted by two
subsystems coupled in terms of an additional exchange interlacing them. Both
subsystems differ in their respective gyromagnetic ratios and temperature dependence.
Other interactions, as for example anisotropic exchange or spin-orbit
torques, can be accounted for differently within each subsystem according to the
physical structure. Micromagnetic simulations are also endorsed by means of a
collective coordinates model which, in contrast with some previous approaches
to these antiferromagnetically coupled systems, based on effective parameters,
also considers them as formed by two coupled subsystems with experimentally
definite parameters. Both simulations and the collective model reinforce the
angular moment compensation argument as accountable for the linear increase
with current of domain wall velocities in these alloys at a certain temperature
or composition. Importantly, the proposed approach by means of two coupled
subsystems permits to infer relevant results in the development of future experimental
setups that are unattainable by means of effective models.MAT2017-87072-C4-1-P from the Spanish government
SA299P18 from the Junta de Castillay León
Non-linear feedback effects in coupled Boson-Fermion systems
We address ourselves to a class of systems composed of two coupled subsystems
without any intra-subsystem interaction: itinerant Fermions and localized
Bosons on a lattice. Switching on an interaction between the two subsystems
leads to feedback effects which result in a rich dynamical structure in both of
them. Such feedback features are studied on the basis of the flow equation
technique - an infinite series of infinitesimal unitary transformations - which
leads to a gradual elimination of the inter-subsystem interaction. As a result
the two subsystems get decoupled but their renormalized kinetic energies become
mutually dependent on each other. Choosing for the inter - subsystem
interaction a charge exchange term (the Boson-Fermion model) the initially
localized Bosons acquire itinerancy through their dependence on the
renormalized Fermion dispersion. This latter evolves from a free particle
dispersion into one showing a pseudogap structure near the chemical potential.
Upon lowering the temperature both subsystems simultaneously enter a
macroscopic coherent quantum state. The Bosons become superfluid, exhibiting a
soundwave like dispersion while the Fermions develop a true gap in their
dispersion. The essential physical features described by this technique are
already contained in the renormalization of the kinetic terms in the respective
Hamiltonians of the two subsystems. The extra interaction terms resulting in
the process of iteration only strengthen this physics. We compare the results
with previous calculations based on selfconsistent perturbative approaches.Comment: 14 pages, 16 figures, accepted for publication in Phys. Rev.
About low field memory and negative magnetization in semiconductors and polymers
Ginzburg-Landau bulk magnetization of itinerant electrons can provide a
negative effective field in the Weiss model by coupling to localized magnetic
moments. The coupling enforces remnant magnetization, which can be negative or
positive depending on the sample magnetic history. Stable magnetic
susceptibility of coupled nonequilibrium subsystems with magnetization reversal
is always positive. Gauss-scale fields could be expected for switching between
negative and positive remnant moments in semiconductors with coupling at
ambient temperatures. Negative magnetization in ultra-high conducting polymers
is also discussed within the developed framework.Comment: 8 pages, no figure
Non-Abelian quantum holonomy of hydrogen-like atoms
We study the Uhlmann holonomy [Rep. Math. Phys. 24, 229 (1986)] of quantum
states for hydrogen-like atoms where the intrinsic spin and orbital angular
momentum are coupled by the spin-orbit interaction and subject to a slowly
varying magnetic field. We show that the holonomy for the orbital angular
momentum and spin subsystems is non-Abelian, while the holonomy of the whole
system is Abelian. Quantum entanglement in the states of the whole system is
crucially related to the non-Abelian gauge structure of the subsystems. We
analyze the phase of the Wilson loop variable associated with the Uhlmann
holonomy, and find a relation between the phase of the whole system with
corresponding marginal phases. Based on the result for the model system we
provide evidence that the phase of the Wilson loop variable and the mixed-state
geometric phase [E. Sj\"oqvist {\it et al.} Phys. Rev. Lett. 85, 2845 (2000)]
are in general inequivalent.Comment: Shortened version; journal reference adde
Entanglement between two subsystems, the Wigner semicircle and extreme value statistics
The entanglement between two arbitrary subsystems of random pure states is
studied via properties of the density matrix's partial transpose,
. The density of states of is close to the
semicircle law when both subsystems have dimensions which are not too small and
are of the same order. A simple random matrix model for the partial transpose
is found to capture the entanglement properties well, including a transition
across a critical dimension. Log-negativity is used to quantify entanglement
between subsystems and analytic formulas for this are derived based on the
simple model. The skewness of the eigenvalue density of is
derived analytically, using the average of the third moment over the ensemble
of random pure states. The third moment after partial transpose is also shown
to be related to a generalization of the Kempe invariant. The smallest
eigenvalue after partial transpose is found to follow the extreme value
statistics of random matrices, namely the Tracy-Widom distribution. This
distribution, with relevant parameters obtained from the model, is found to be
useful in calculating the fraction of entangled states at critical dimensions.
These results are tested in a quantum dynamical system of three coupled
standard maps, where one finds that if the parameters represent a strongly
chaotic system, the results are close to those of random states, although there
are some systematic deviations at critical dimensions.Comment: Substantially improved version (now 43 pages, 10 figures) that is
accepted for publication in Phys. Rev.
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