18,348 research outputs found
Understanding Hot-Electron Generation and Plasmon Relaxation in Metal Nanocrystals: Quantum and Classical Mechanisms
Generation of energetic (hot) electrons is an intrinsic property of any
plasmonic nanostructure under illumination. Simultaneously, a striking
advantage of metal nanocrystals over semiconductors lies in their very large
absorption cross sections. Therefore, metal nanostructures with strong and
tailored plasmonic resonances are very attractive for photocatalytic
applications. However, the central questions regarding plasmonic hot electrons
are how to quantify and extract the optically-excited energetic electrons in a
nanocrystal. We develop a theory describing the generation rates and the
energy-distributions of hot electrons in nanocrystals with various geometries.
In our theory, hot electrons are generated owing to surfaces and hot spots. The
formalism predicts that large optically-excited nanocrystals show the
excitation of mostly low-energy Drude electrons, whereas plasmons in small
nanocrystals involve mostly hot electrons. The energy distributions of
electrons in an optically-excited nanocrystal show how the quantum many-body
state in small particles evolves towards the classical state described by the
Drude model when increasing nanocrystal size. We show that the rate of surface
decay of plasmons in nanocrystals is directly related to the rate of generation
of hot electrons. Based on a detailed many-body theory involving kinetic
coefficients, we formulate a simple scheme describing the plasmon's dephasing.
In most nanocrystals, the main decay mechanism of a plasmon is the Drude
friction-like process and the secondary path comes from generation of hot
electrons due to surfaces and electromagnetic hot spots. This latter path
strongly depends on the size, shape and material of the nanocrystal,
correspondingly affecting its efficiency of hot-electron production. The
results in the paper can be used to guide the design of plasmonic nanomaterials
for photochemistry and photodetectors.Comment: 90 pages, 21 figures, including Supplementary Informatio
Recurrent flow analysis in spatiotemporally chaotic 2-dimensional Kolmogorov flow
Motivated by recent success in the dynamical systems approach to transitional
flow, we study the efficiency and effectiveness of extracting simple invariant
sets (recurrent flows) directly from chaotic/turbulent flows and the potential
of these sets for providing predictions of certain statistics of the flow.
Two-dimensional Kolmogorov flow (the 2D Navier-Stokes equations with a
sinusoidal body force) is studied both over a square [0, 2{\pi}]2 torus and a
rectangular torus extended in the forcing direction. In the former case, an
order of magnitude more recurrent flows are found than previously (Chandler &
Kerswell 2013) and shown to give improved predictions for the dissipation and
energy pdfs of the chaos via periodic orbit theory. Over the extended torus at
low forcing amplitudes, some extracted states mimick the statistics of the
spatially-localised chaos present surprisingly well recalling the striking
finding of Kawahara & Kida (2001) in low-Reynolds-number plane Couette flow. At
higher forcing amplitudes, however, success is limited highlighting the
increased dimensionality of the chaos and the need for larger data sets.
Algorithmic developments to improve the extraction procedure are discussed
Clustering of spectra and fractals of regular graphs
We exhibit a characteristic structure of the class of all regular graphs of
degree d that stems from the spectra of their adjacency matrices. The structure
has a fractal threadlike appearance. Points with coordinates given by the mean
and variance of the exponentials of graph eigenvalues cluster around a line
segment that we call a filar. Zooming-in reveals that this cluster splits into
smaller segments (filars) labeled by the number of triangles in graphs. Further
zooming-in shows that the smaller filars split into subfilars labelled by the
number of quadrangles in graphs, etc. We call this fractal structure,
discovered in a numerical experiment, a multifilar structure. We also provide a
mathematical explanation of this phenomenon based on the Ihara-Selberg trace
formula, and compute the coordinates and slopes of all filars in terms of
Bessel functions of the first kind.Comment: 10 pages, 5 figure
Efimov Physics in 6Li Atoms
A new narrow 3-atom loss resonance associated with an Efimov trimer crossing
the 3-atom threshold has recently been discovered in a many-body system of
ultracold 6Li atoms in the three lowest hyperfine spin states at a magnetic
field near 895 G. O'Hara and coworkers have used measurements of the 3-body
recombination rate in this region to determine the complex 3-body parameter
associated with Efimov physics. Using this parameter as the input, we calculate
the universal predictions for the spectrum of Efimov states and for the 3-body
recombination rate in the universal region above 600 G where all three
scattering lengths are large. We predict an atom-dimer loss resonance at (672
+/- 2) G associated with an Efimov trimer disappearing through an atom-dimer
threshold. We also predict an interference minimum in the 3-body recombination
rate at (759 +/- 1) G where the 3-spin mixture may be sufficiently stable to
allow experimental study of the many-body system.Comment: 27 pages, 9 figures, REVTeX4, published versio
Three-body Recombination of Lithium-6 Atoms with Large Negative Scattering Lengths
The 3-body recombination rate at threshold for distinguishable atoms with
large negative pair scattering lengths is calculated in the zero-range
approximation. The only parameters in this limit are the 3 scattering lengths
and the Efimov parameter, which can be complex valued. We provide semi-analytic
expressions for the cases of 2 or 3 equal scattering lengths and we obtain
numerical results for the general case of 3 different scattering lengths. Our
general result is applied to the three lowest hyperfine states of Lithium-6
atoms. Comparisons with recent experiments provide indications of loss features
associated with Efimov trimers near the 3-atom threshold.Comment: 4 pages, 4 figures, agrees with published versio
Land classification of south-central Iowa from computer enhanced images
The author has identified the following significant results. Enhanced LANDSAT imagery was most useful for land classification purposes, because these images could be photographically printed at large scales such as 1:63,360. The ability to see individual picture elements was no hindrance as long as general image patterns could be discerned. Low cost photographic processing systems for color printings have proved to be effective in the utilization of computer enhanced LANDSAT products for land classification purposes. The initial investment for this type of system was very low, ranging from 200 beyond a black and white photo lab. The technical expertise can be acquired from reading a color printing and processing manual
Land Classification of South-central Iowa from Computer Enhanced Images
The author has identified the following significant results. Two enhanced false color negatives from multispectral scanner scenes, dated 15 April 1974 and 29 August 1972, were printed at a scale of 1:125,000 to form the basis for land use interpretations in the Wapello County, Iowa test site. The use of geomorphic principles proved valuable in the interpretation of the April scene to form valuable generalizations for planning purposes on soil associations, topography, alluvial valleys, and agricultural land use. The August scene was superior in providing information on urban extent, transportation networks, forest cover, and water bodies
Land classification of south-central Iowa from computer enhanced images
There are no author-identified singificant results in this report
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