20,104 research outputs found
Generally Covariant Conservative Energy-Momentum for Gravitational Anyons
We obtain a generally covariant conservation law of energy-momentum for
gravitational anyons by the general displacement transform. The energy-momentum
currents have also superpotentials and are therefore identically conserved. It
is shown that for Deser's solution and Clement's solution, the energy vanishes.
The reasonableness of the definition of energy-momentum may be confirmed by the
solution for pure Einstein gravity which is a limit of vanishing Chern-Simons
coulping of gravitational anyons.Comment: 12 pages, Latex, no figure
The Topological Structure of the Space-Time Disclination
The space-time disclination is studied by making use of the decomposition
theory of gauge potential in terms of antisymmetric tensor field and
-mapping method. It is shown that the self-dual and anti-self-dual parts
of the curvature compose the space-time disclinations which are classified in
terms of topological invariants--winding number. The projection of space-time
disclination density along an antisymmetric tensor field is quantized
topologically and characterized by Brouwer degree and Hopf index.Comment: 18 pages, Revte
Electrostatically Controlled Magnetization Rotation in Ferromagnet-Topological Insulator Planar Structures
An approach to the electrostatic control of magnetization
rotation in the hybrid structures composed of topological insulators (TIs) and
adjacent ferromagnetic insulators (FMI) is proposed and studied. The concept is
based on TI electron energy variation with in-plane to put-of plane FMI
magnetization turn. The calculations explicitly expose the effect of free
energy variability in the form of the electrically controlled uniaxial magnetic
anisotropy, which depends on proximate exchange interaction and TI surface
electron density. Combining with inherent anisotropy, the magnetization
rotation from in-plane to out-of-plane direction is shown to be realizable for
1.7~2.7 ns under the electrical variation of TI chemical potential in the range
100 meV around Dirac point. When bias is withdrawn a small signal current
can target the out-of-plane magnetization instable state to the desirable
direction of in-plane easy axis, thus the structure can lay the foundation for
low energy nonvolatile memory prototype
Topology of Knotted Optical Vortices
Optical vortices as topological objects exist ubiquitously in nature. In this
paper, by making use of the -mapping topological current theory, we
investigate the topology in the closed and knotted optical vortices. The
topological inner structure of the optical vortices are obtained, and the
linking of the knotted optical vortices is also given.Comment: 11 pages, no figures, accepted by Commun. Theor. Phys. (Beijing, P.
R. China
Nonlocal Entanglement Transformations Achievable by Separable Operations
For manipulations of multipartite quantum systems, it was well known that all
local operations assisted by classical communication (LOCC) constitute a proper
subset of the class of separable operations. Recently, Gheorghiu and Griffiths
found that LOCC and general separable operations are equally powerful for
transformations between bipartite pure states. In this letter we extend this
comparison to mixed states and show that in general separable operations are
strictly stronger than LOCC when transforming a mixed state to a pure entangled
state. A remarkable consequence of our finding is the existence of entanglement
monotone which may increase under separable operations.Comment: v2 has rephrased Theorem 1 and corrected Kraus operators in Theorem
2. Additional comments are welcome
Disclination in Lorentz Space-Time
The disclination in Lorentz space-time is studied in detail by means of
topological properties of -mapping. It is found the space-time
disclination can be described in term of a Dirac spinor. The size of the
disclination, which is proved to be the difference of two sets of su(2)% -like
monopoles expressed by two mixed spinors, is quantized topologically in terms
of topological invariantswinding number. The projection of space-time
disclination density along an antisymmetric tensor field is characterized by
Brouwer degree and Hopf index.Comment: Revtex, 7 page
Robust active magnetic dearing control using stabilizing dynamical compensators
The robust control of active magnetic bearings, based on a linearised interval model, is considered. Through robust stability analysis, all the first-order robust stabilizing dynamical compensators for the interval system are obtained. Disturbance attenuation and minimum control effort are also addressed. The approach is applied to a high-speed flywheel supported by two active and two passive magnetic bearings. Simulation and experimental results both show that it is simple, effective, and robust
Robust magnetic bearing control using stabilizing dynamical compensators
AbstractâThis paper considers the robust control of an active radial magnetic bearing system, having a homopolar, external rotor topology, which is used to support an annular fiber composite flywheel rim. A first-order dynamical compensator, which uses only position feedback information, is used for control, its design being based on a linearized one-dimensional second-order model which is treated as an interval system in order to cope with parameter uncertainties. Through robust stability analysis, a parameterization of all first-order robustly stabilizing dynamical compensators for the interval system is initially obtained. Then, by appropriate selection of the free parameters in the robust controller, the H2 norm of the disturbance-output transfer function is made arbitrarily small over the system parameter intervals, and the norm of the inputâoutput transfer function is made arbitrarily close to a lower bound. Simulation and experimental
results demonstrate both stability and performance robustness of the developed controller
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