131,881 research outputs found
Dynamics of a hole in the large--U Hubbard model: a Feynman diagram approach
We study the dynamics of a single hole in an otherwise half--filled
two--dimensional Hubbard model by introducing a nonlocal Bogolyubov
transformation in the antiferromagnetic state. This allows us to rewrite the
Hamiltonian in a form that makes a separation between high--energy processes
(involving double--occupancy) and low--energy physics possible. A diagrammatic
scheme is developped that allows for a systematic study of the different
processes delocalizing a carrier in the antiferromagnetic state. In particular,
the so--called Trugman process, important if transverse spin fluctuations are
neglected, is studied and is shown to be dominated by the leading vertex
corrections. We analyze the dynamics of a single hole both in the Ising limit
and with spin fluctuations. The results are compared with previous theories as
well as with recent exact small--cluster calculations, and we find good
agreement. The formalism establishes a link between weak and strong coupling
methodologies.Comment: Latex 34pages, Orsay Preprint, submitted to Phys. Rev.
Fuzzy Chance-constrained Programming Based Security Information Optimization for Low Probability of Identification Enhancement in Radar Network Systems
In this paper, the problem of low probability of identification (LPID) improvement for radar network systems is investigated. Firstly, the security information is derived to evaluate the LPID performance for radar network. Then, without any prior knowledge of hostile intercept receiver, a novel fuzzy chance-constrained programming (FCCP) based security information optimization scheme is presented to achieve enhanced LPID performance in radar network systems, which focuses on minimizing the achievable mutual information (MI) at interceptor, while the attainable MI outage probability at radar network is enforced to be greater than a specified confidence level. Regarding to the complexity and uncertainty of electromagnetic environment in the modern battlefield, the trapezoidal fuzzy number is used to describe the threshold of achievable MI at radar network based on the credibility theory. Finally, the FCCP model is transformed to a crisp equivalent form with the property of trapezoidal fuzzy number. Numerical simulation results demonstrating the performance of the proposed strategy are provided
Cutout reinforcements for shear loaded laminate and sandwich composite panels
This paper presents the numerical and experimental studies of shear loaded
laminated and sandwich carbon/epoxy composite panels with cutouts and
reinforcements aiming at reducing the cutout stress concentration and increasing
the buckling stability of the panels. The effect of different cutout sizes and
the design and materials of cutout reinforcements on the stress and buckling
behaviour of the panels are evaluated. For the sandwich panels with a range of
cutout size and a constant weight, an optimal ratio of the core to the face
thickness has been studied for the maximum buckling stability. The finite
element method and an analytical method are employed to perform parametric
studies. In both constant stress and constant displacement shear loading
conditions, the results are in very good agreement with those obtained from
experiment for selected cutout reinforcement cases. Conclusions are drawn on the
cutout reinforcement design and improvement of stress concentration and buckling
behaviour of shear loaded laminated and sandwich composite panels with cutouts
Comment on "Giant Plasticity of a Quantum Crystal"
In their Letter, Haziot et al. [Phys. Rev. Lett. 110 (2013) 035301] report a
novel phenomenon of giant plasticity for hcp Helium-4 quantum crystals. They
assert that Helium-4 exhibits mechanical properties not found in classical
plasticity theory. Specifically, they examine high-quality crystals as a
function of temperature and applied strain, where the shear modulus reaches a
plateau and dissipation becomes close to zero; both quantities are reported to
be independent of stress and strain, implying a reversible dissipation process
and quantum tunneling. In this Comment, we show that these signatures can be
explained with a classical model of thermally activated dislocation glide
without the need to invoke quantum tunneling or dissipationless motion.
Recently, we proposed a dislocation glide model in solid Helium-4 containing
the dissipation contribution in the presence of other dislocations with
qualitatively similar behavior [Zhou et al., Philos. Mag. Lett. 92 (2012) 608].Comment: 1 page, 1 figure, comment; minor revision
Conditions for Nondistortion Interrogation of Quantum System
Under some physical considerations, we present a universal formulation to
study the possibility of localizing a quantum object in a given region without
disturbing its unknown internal state. When the interaction between the object
and probe wave function takes place only once, we prove the necessary and
sufficient condition that the object's presence can be detected in an initial
state preserving way. Meanwhile, a conditioned optimal interrogation
probability is obtained.Comment: 5 pages, Revtex, 1 figures, Presentation improved, corollary 1 added.
To appear in Europhysics Letter
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