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Doping Nanocrystals And The Role Of Quantum Confinement
Recent progress in developing algorithms for solving the electronic structure problem for nanostructures is illustrated. Key ingredients in this approach include pseudopotentials implemented on a real space grid and the use of density functional theory. This procedure allows one to predict electronic properties for many materials across the nano-regime, i.e., from atoms to nanocrystals of sufficient size to replicate bulk properties. We will illustrate this method for doping silicon nanocrystals with phosphorous.Institute for Computational Engineering and Sciences (ICES
Time delays and energy transport velocities in three dimensional ideal cloaking
We obtained the energy transport velocity distribution for a three
dimensional ideal cloak explicitly. Near the operation frequency, the energy
transport velocity has rather peculiar distribution. The velocity along a line
joining the origin of the cloak is a constant, while the velocity approaches
zero at the inner boundary of the cloak. A ray pointing right into the origin
of the cloak will experience abrupt changes of velocities when it impinges on
the inner surface of the cloak. This peculiar distribution causes infinite time
delays for the ideal cloak within a geometric optics description.Comment: A scaling factor is added to convert the parameter \tau into the
physical tim
Space shuttle contamination due to backflow from control motor exhaust
Spacecraft contamination of the space shuttle orbiter and accompanying Spacelab payloads is studied. The scattering of molecules from the vernier engines and flash evaporator nozzle after impingement on the orbiter wing surfaces, and the backflow of molecules out of the flash evaporator nozzle plume flow field due to intermolecular collisions in the plume are the problems discussed. A method was formulated for dealing with these problems, and detailed results are given
Lattice model theory of the equation of state covering the gas, liquid, and solid phases
The three stable states of matter and the corresponding phase transitions were obtained with a single model. Patterned after Lennard-Jones and Devonshires's theory, a simple cubic lattice model containing two fcc sublattices (alpha and beta) is adopted. The interatomic potential is taken to be the Lennard-Jones (6-12) potential. Employing the cluster variation method, the Weiss and the pair approximations on the lattice gas failed to give the correct phase diagrams. Hybrid approximations were devised to describe the lattice term in the free energy. A lattice vibration term corresponding to a free volume correction is included semi-phenomenologically. The combinations of the lattice part and the free volume part yield the three states and the proper phase diagrams. To determine the coexistence regions, the equalities of the pressure and Gibbs free energy per molecule of the coexisting phases were utilized. The ordered branch of the free energy gives rise to the solid phase while the disordered branch yields the gas and liquid phases. It is observed that the triple point and the critical point quantities, the phase diagrams and the coexistence regions plotted are in good agreement with the experimental values and graphs for argon
Discovery of Griffiths phase in itinerant magnetic semiconductor Fe_{1-x}Co_xS_2
Critical points that can be suppressed to zero temperature are interesting
because quantum fluctuations have been shown to dramatically alter electron gas
properties. Here, the metal formed by Co doping the paramagnetic insulator
FeS, FeCoS, is demonstrated to order ferromagnetically at
where we observe unusual transport, magnetic, and
thermodynamic properties. We show that this magnetic semiconductor undergoes a
percolative magnetic transition with distinct similarities to the Griffiths
phase, including singular behavior at and zero temperature.Comment: 10 pages, 4 figure
Some exact results on the matter star-product in the half-string formalism
We show that the D25 sliver wavefunction, just as the D-instanton sliver,
factorizes when expressed in terms of half-string coordinates. We also
calculate analytically the star-product of two zero-momentum eigenstates of
using the vertex in the oscillator basis, thereby showing that the
star-product in the matter sector can indeed be seen as multiplication of
matrices acting on the space of functionals of half strings. We then use the
above results to establish that the matrices , conjectured by
Rastelli, Sen and Zwiebach to be left and right projectors on the sliver, are
indeed so.Comment: 27 pages; footnote adde
Toward RADSCAT measurements over the sea and their interpretation
Investigations into several areas which are essential to the execution and interpretation of suborbital observations by composite radiometer - scatterometer sensor (RADSCAT) are reported. Experiments and theory were developed to demonstrate the remote anemometric capability of the sensor over the sea through various weather conditions. It is shown that weather situations found in extra tropical cyclones are useful for demonstrating the all weather capability of the composite sensor. The large scale fluctuations of the wind over the sea dictate the observational coverage required to correlate measurements with the mean surface wind speed. Various theoretical investigations were performed to establish a premise for the joint interpretation of the experiment data. The effects of clouds and rains on downward radiometric observations over the sea were computed. A method of predicting atmospheric attenuation from joint observations is developed. In other theoretical efforts, the emission and scattering characteristics of the sea were derived. Composite surface theories with coherent and noncoherent assumptions were employed
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