75,504 research outputs found
Physical Interpretations of Negative Imaginary Systems Theory
This paper presents some physical interpretations of recent stability results
on the feedback interconnection of negative imaginary systems. These
interpretations involve spring mass damper systems coupled together by springs
or RLC electrical networks coupled together via inductors or capacitors.Comment: To appear in the Proceedings of the 10th ASIAN CONTROL CONFERENCE
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Robust stability conditions for feedback interconnections of distributed-parameter negative imaginary systems
Sufficient and necessary conditions for the stability of positive feedback
interconnections of negative imaginary systems are derived via an integral
quadratic constraint (IQC) approach. The IQC framework accommodates
distributed-parameter systems with irrational transfer function
representations, while generalising existing results in the literature and
allowing exploitation of flexibility at zero and infinite frequencies to reduce
conservatism in the analysis. The main results manifest the important property
that the negative imaginariness of systems gives rise to a certain form of IQCs
on positive frequencies that are bounded away from zero and infinity. Two
additional sets of IQCs on the DC and instantaneous gains of the systems are
shown to be sufficient and necessary for closed-loop stability along a homotopy
of systems.Comment: Submitted to Automatica, A preliminary version of this paper appeared
in the Proceedings of the 2015 European Control Conferenc
Quasiparticle excitations in relativistic quantum field theory
We analyze the particle-like excitations arising in relativistic field
theories in states different than the vacuum. The basic properties
characterizing the quasiparticle propagation are studied using two different
complementary methods. First we introduce a frequency-based approach, wherein
the quasiparticle properties are deduced from the spectral analysis of the
two-point propagators. Second, we put forward a real-time approach, wherein the
quantum state corresponding to the quasiparticle excitation is explicitly
constructed, and the time-evolution is followed. Both methods lead to the same
result: the energy and decay rate of the quasiparticles are determined by the
real and imaginary parts of the retarded self-energy respectively. Both
approaches are compared, on the one hand, with the standard field-theoretic
analysis of particles in the vacuum and, on the other hand, with the
mean-field-based techniques in general backgrounds.Comment: 53 pages, 4 figures. Version accepted for publication in Ann. Phy
Exchange-correlation kernels for excited states in solids
The performance of several common approximations for the exchange-correlation
kernel within time-dependent density-functional theory is tested for elementary
excitations in the homogeneous electron gas. Although the adiabatic
local-density approximation gives a reasonably good account of the plasmon
dispersion, systematic errors are pointed out and traced to the neglect of the
wavevector dependence. Kernels optimized for atoms are found to perform poorly
in extended systems due to an incorrect behavior in the long-wavelength limit,
leading to quantitative deviations that significantly exceed the experimental
error bars for the plasmon dispersion in the alkali metals.Comment: 7 pages including 5 figures, RevTe
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