1,440 research outputs found
Cooperative effects and disorder: A scaling analysis of the spectrum of the effective atomic Hamiltonian
We study numerically the spectrum of the non-Hermitian effective Hamiltonian
that describes the dipolar interaction of a gas of atoms with the
radiation field. We analyze the interplay between cooperative effects and
disorder for both scalar and vectorial radiation fields. We show that for dense
gases, the resonance width distribution follows, both in the scalar and
vectorial cases, a power law that originates
from cooperative effects between more than two atoms. This power law is
different from the behavior, which has been
considered as a signature of Anderson localization of light in random systems.
We show that in dilute clouds, the center of the energy distribution is
described by Wigner's semicircle law in the scalar and vectorial cases. For
dense gases, this law is replaced in the vectorial case by the Laplace
distribution. Finally, we show that in the scalar case the degree of resonance
overlap increases as a power law of the system size for dilute gases, but
decays exponentially with the system size for dense clouds.Comment: 11 pages, 12 figure
An assessment of warm fog: Nucleation, control, and recommended research
A state-of-the-art survey is given of warm fog research which has been performed up to, and including, 1974. Topics covered are nucleation, growth, coalescence, fog structures and visibility, effects of surface films, drop size spectrum, optical properties, instrumentation, liquid water content, condensation nuclei. Included is a summary of all reported fog modification experiments. Additional data is provided on air flow, turbulence, a summary of recommendations on instruments to be developed for determining turbulence, air flow, etc., as well as recommendations of various fog research tasks which should be performed for a better understanding of fog microphysics
Two-electron wavefunctions are matrix product states with bond dimension Three
We prove the statement in the title, for a suitable (wavefunction-dependent)
choice of the underlying orbitals, and show that Three is optimal. Thus for
two-electron systems, the QC-DMRG method with bond dimension Three combined
with fermionic mode optimization exactly recovers the FCI energy
Functional modelling in evolvable assembly systems
The design and reconfiguration of adaptive production systems is a key driver in modern advanced manufacturing. We summarise the use of an ap-proach from the field of functional modelling to capture the function, behaviour, and structure of a system. This model is an integral part of the Evolvable Assembly Systems architecture, allowing the system to adapt its behaviour in response to changing product requirements. The integrated approach is illustrated with an example taken from a real EAS instantiation
Roles of cooperative effects and disorder in photon localization: The case of a vector radiation field
We numerically study photon escape rates from three-dimensional atomic gases
and investigate the respective roles of cooperative effects and disorder in
photon localization, while taking into account the vectorial nature of light. A
scaling behavior is observed for the escape rates, and photons undergo a
crossover from delocalization toward localization as the optical thickness of
the cloud is increased. This result indicates that light localization is
dominated by cooperative effects rather than disorder. We compare our results
with those obtained in the case of a scalar radiation field and find no
significant differences. We conclude that the scalar model constitutes an
excellent approximation when considering photon escape rates from atomic gases.Comment: 9 pages, 9 figure
Giant Spin-splitting in the Bi/Ag(111) Surface Alloy
Surface alloying is shown to produce electronic states with a very large
spin-splitting. We discuss the long range ordered bismuth/silver(111) surface
alloy where an energy bands separation of up to one eV is achieved. Such strong
spin-splitting enables angular resolved photoemission spectroscopy to directly
observe the region close to the band edge, where the density of states shows
quasi-one dimensional behavior. The associated singularity in the local density
of states has been measured by low temperature scanning tunneling spectroscopy.
The implications of this new class of materials for potential spintronics
applications as well as fundamental issues are discussed.Comment: 4 pages, 4 figure
Efficient Algorithm for Asymptotics-Based Configuration-Interaction Methods and Electronic Structure of Transition Metal Atoms
Asymptotics-based configuration-interaction (CI) methods [G. Friesecke and B.
D. Goddard, Multiscale Model. Simul. 7, 1876 (2009)] are a class of CI methods
for atoms which reproduce, at fixed finite subspace dimension, the exact
Schr\"odinger eigenstates in the limit of fixed electron number and large
nuclear charge. Here we develop, implement, and apply to 3d transition metal
atoms an efficient and accurate algorithm for asymptotics-based CI.
Efficiency gains come from exact (symbolic) decomposition of the CI space
into irreducible symmetry subspaces at essentially linear computational cost in
the number of radial subshells with fixed angular momentum, use of reduced
density matrices in order to avoid having to store wavefunctions, and use of
Slater-type orbitals (STO's). The required Coulomb integrals for STO's are
evaluated in closed form, with the help of Hankel matrices, Fourier analysis,
and residue calculus.
Applications to 3d transition metal atoms are in good agreement with
experimental data. In particular we reproduce the anomalous magnetic moment and
orbital filling of Chromium in the otherwise regular series Ca, Sc, Ti, V, Cr.Comment: 14 pages, 1 figur
Photon localization and Dicke superradiance in atomic gases
Photon propagation in a gas of N atoms is studied using an effective
Hamiltonian describing photon mediated atomic dipolar interactions. The density
P(\Gamma) of photon escape rates is determined from the spectrum of the N x N
random matrix \Gamma_{ij} = \sin (x_{ij}) / x_{ij}, where x_{ij} is the
dimensionless random distance between any two atoms. Varying disorder and
system size, a scaling behavior is observed for the escape rates. It is
explained using microscopic calculations and a stochastic model which
emphasizes the role of cooperative effects in photon localization and provides
an interesting relation with statistical properties of "small world networks".Comment: 4 pages, 5 figure
Representational affordances in design, with examples from analogy making and optimization
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