1,046 research outputs found
Relation of agronomic and multispectral reflectance characteristics of spring wheat canopies
The relationships between crop canopy variables such as leaf area index (LAI) and their multispectral reflectance properties were investigated along with the potential for estimating canopy variables from remotely sensed reflectance measurements. Reflectance spectra over the 0.4 to 2.5 micron wavelength range were acquired during each of the major development stages of spring wheat canopies at Williston, North Dakota, during three seasons. Treatments included planting date, N fertilization, cultivar, and soil moisture. Agronomic measurements included development stage, biomass, LAI, and percent soil cover. High correlations were found between reflectance and percent cover, LAI, and biomass. A near infrared wavelength band, 0.76 to 0.90 microns, was most important in explaining variation in LAI and percent cover, while a middle infrared band, 2.08 to 2.35 microns, explained the most variation in biomass and plant water content. Transformations, including the near infrared/red reflectance ratio and greenness index, were also highly correlated to canopy variables. The relationship of canopy variables to reflectance decreased as the crop began to ripen. the canopy variables could be accurately predicted using measurements from three to five wavelength bands. The wavelength bands proposed for the thematic mapper sensor were more strongly related to the canopy variables than the LANDSAT MSS bands
A new model for simulating colloidal dynamics
We present a new hybrid lattice-Boltzmann and Langevin molecular dynamics
scheme for simulating the dynamics of suspensions of spherical colloidal
particles. The solvent is modeled on the level of the lattice-Boltzmann method
while the molecular dynamics is done for the solute. The coupling between the
two is implemented through a frictional force acting both on the solvent and on
the solute, which depends on the relative velocity. A spherical colloidal
particle is represented by interaction sites at its surface. We demonstrate
that this scheme quantitatively reproduces the translational and rotational
diffusion of a neutral spherical particle in a liquid and show preliminary
results for a charged spherical particle. We argue that this method is
especially advantageous in the case of charged colloids.Comment: For a movie click on the link below Fig
A Numerical Model for Brownian Particles Fluctuating in Incompressible Fluids
We present a numerical method that consistently implements thermal
fluctuations and hydrodynamic interactions to the motion of Brownian particles
dispersed in incompressible host fluids. In this method, the thermal
fluctuations are introduced as random forces acting on the Brownian particles.
The hydrodynamic interactions are introduced by directly resolving the fluid
motions with the particle motion as a boundary condition to be satisfied. The
validity of the method has been examined carefully by comparing the present
numerical results with the fluctuation-dissipation theorem whose analytical
form is known for dispersions of a single spherical particle. Simulations are
then performed for more complicated systems, such as a dispersion composed of
many spherical particles and a single polymeric chain in a solvent.Comment: 6 pages, 8 figure
Optimizing end-labeled free-solution electrophoresis by increasing the hydrodynamic friction of the drag-tag
We study the electrophoretic separation of polyelectrolytes of varying
lengths by means of end-labeled free-solution electrophoresis (ELFSE). A
coarse-grained molecular dynamics simulation model, using full electrostatic
interactions and a mesoscopic Lattice Boltzmann fluid to account for
hydrodynamic interactions, is used to characterize the drag coefficients of
different label types: linear and branched polymeric labels, as well as
transiently bound micelles.
It is specifically shown that the label's drag coefficient is determined by
its hydrodynamic size, and that the drag per label monomer is largest for
linear labels. However, the addition of side chains to a linear label offers
the possibility to increase the hydrodynamic size, and therefore the label
efficiency, without having to increase the linear length of the label, thereby
simplifying synthesis. The third class of labels investigated, transiently
bound micelles, seems very promising for the usage in ELFSE, as they provide a
significant higher hydrodynamic drag than the other label types.
The results are compared to theoretical predictions, and we investigate how
the efficiency of the ELFSE method can be improved by using smartly designed
drag-tags.Comment: 32 pages, 11 figures, submitted to Macromolecule
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Slow and fast single photons from a quantum dot interacting with the excited state hyperfine structure of the Cesium D1-line
Hybrid interfaces between distinct quantum systems play a major role in the implementation of quantum networks. Quantum states have to be stored in memories to synchronize the photon arrival times for entanglement swapping by projective measurements in quantum repeaters or for entanglement purification. Here, we analyze the distortion of a single-photon wave packet propagating through a dispersive and absorptive medium with high spectral resolution. Single photons are generated from a single In(Ga)As quantum dot with its excitonic transition precisely set relative to the Cesium D1 transition. The delay of spectral components of the single-photon wave packet with almost Fourier-limited width is investigated in detail with a 200 MHz narrow-band monolithic Fabry-Pérot resonator. Reflecting the excited state hyperfine structure of Cesium, “slow light” and “fast light” behavior is observed. As a step towards room-temperature alkali vapor memories, quantum dot photons are delayed for 5 ns by strong dispersion between the two 1.17 GHz hyperfine-split excited state transitions. Based on optical pumping on the hyperfine-split ground states, we propose a simple, all-optically controllable delay for synchronization of heralded narrow-band photons in a quantum network
Electron correlations for ground state properties of group IV semiconductors
Valence energies for crystalline C, Si, Ge, and Sn with diamond structure
have been determined using an ab-initio approach based on information from
cluster calculations. Correlation contributions, in particular, have been
evaluated in the coupled electron pair approximation (CEPA), by means of
increments obtained for localized bond orbitals and for pairs and triples of
such bonds. Combining these results with corresponding Hartree-Fock (HF) data,
we recover about 95 % of the experimental cohesive energies. Lattice constants
are overestimated at the HF level by about 1.5 %; correlation effects reduce
these deviations to values which are within the error bounds of this method. A
similar behavior is found for the bulk modulus: the HF values which are
significantly too high are reduced by correlation effects to about 97 % of the
experimental values.Comment: 22 pages, latex, 2 figure
Electrons in High-Tc Compounds: Ab-Initio Correlation Results
Electronic correlations in the ground state of an idealized infinite-layer
high-Tc compound are computed using the ab-initio method of local ansatz.
Comparisons are made with the local-density approximation (LDA) results, and
the correlation functions are analyzed in detail. These correlation functions
are used to determine the effective atomic-interaction parameters for model
Hamiltonians. On the resulting model, doping dependencies of the relevant
correlations are investigated. Aside from the expected strong atomic
correlations, particular spin correlations arise. The dominating contribution
is a strong nearest neighbor correlation that is Stoner-enhanced due to the
closeness of the ground state to the magnetic phase. This feature depends
moderately on doping, and is absent in a single-band Hubbard model. Our
calculated spin correlation function is in good qualitative agreement with that
determined from the neutron scattering experiments for a metal.Comment: 21pp, 5fig, Phys. Rev. B (Oct. 98
Excited States of Proton-bound DNA/RNA Base Homo-dimers: Pyrimidines
We are presenting the electronic photo fragment spectra of the protonated
pyrimidine DNA bases homo-dimers. Only the thymine dimer exhibits a well
structured vibrational progression, while protonated monomer shows broad
vibrational bands. This shows that proton bonding can block some non radiative
processes present in the monomer.Comment: We acknowledge the use of the computing facility cluster GMPCS of the
LUMAT federation (FR LUMAT 2764
Multi-Particle Collision Dynamics -- a Particle-Based Mesoscale Simulation Approach to the Hydrodynamics of Complex Fluids
In this review, we describe and analyze a mesoscale simulation method for
fluid flow, which was introduced by Malevanets and Kapral in 1999, and is now
called multi-particle collision dynamics (MPC) or stochastic rotation dynamics
(SRD). The method consists of alternating streaming and collision steps in an
ensemble of point particles. The multi-particle collisions are performed by
grouping particles in collision cells, and mass, momentum, and energy are
locally conserved. This simulation technique captures both full hydrodynamic
interactions and thermal fluctuations. The first part of the review begins with
a description of several widely used MPC algorithms and then discusses
important features of the original SRD algorithm and frequently used
variations. Two complementary approaches for deriving the hydrodynamic
equations and evaluating the transport coefficients are reviewed. It is then
shown how MPC algorithms can be generalized to model non-ideal fluids, and
binary mixtures with a consolute point. The importance of angular-momentum
conservation for systems like phase-separated liquids with different
viscosities is discussed. The second part of the review describes a number of
recent applications of MPC algorithms to study colloid and polymer dynamics,
the behavior of vesicles and cells in hydrodynamic flows, and the dynamics of
viscoelastic fluids
Scale-free static and dynamical correlations in melts of monodisperse and Flory-distributed homopolymers: A review of recent bond-fluctuation model studies
It has been assumed until very recently that all long-range correlations are
screened in three-dimensional melts of linear homopolymers on distances beyond
the correlation length characterizing the decay of the density
fluctuations. Summarizing simulation results obtained by means of a variant of
the bond-fluctuation model with finite monomer excluded volume interactions and
topology violating local and global Monte Carlo moves, we show that due to an
interplay of the chain connectivity and the incompressibility constraint, both
static and dynamical correlations arise on distances . These
correlations are scale-free and, surprisingly, do not depend explicitly on the
compressibility of the solution. Both monodisperse and (essentially)
Flory-distributed equilibrium polymers are considered.Comment: 60 pages, 49 figure
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