729 research outputs found
Electronic structure and optical properties of ZnX (X=O, S, Se, Te)
Electronic band structure and optical properties of zinc monochalcogenides
with zinc-blende- and wurtzite-type structures were studied using the ab initio
density functional method within the LDA, GGA, and LDA+U approaches.
Calculations of the optical spectra have been performed for the energy range
0-20 eV, with and without including spin-orbit coupling. Reflectivity,
absorption and extinction coefficients, and refractive index have been computed
from the imaginary part of the dielectric function using the Kramers--Kronig
transformations. A rigid shift of the calculated optical spectra is found to
provide a good first approximation to reproduce experimental observations for
almost all the zinc monochalcogenide phases considered. By inspection of the
calculated and experimentally determined band-gap values for the zinc
monochalcogenide series, the band gap of ZnO with zinc-blende structure has
been estimated.Comment: 17 pages, 10 figure
From white elephant to Nobel Prize: Dennis Gaborâs wavefront reconstruction
Dennis Gabor devised a new concept for optical imaging in 1947 that went by a variety of names over the following decade: holoscopy, wavefront reconstruction, interference microscopy, diffraction microscopy and Gaboroscopy. A well-connected and creative research engineer, Gabor worked actively to publicize and exploit his concept, but the scheme failed to capture the interest of many researchers. Gaborâs theory was repeatedly deemed unintuitive and baffling; the technique was appraised by his contemporaries to be of dubious practicality and, at best, constrained to a narrow branch of science. By the late 1950s, Gaborâs subject had been assessed by its handful of practitioners to be a white elephant. Nevertheless, the concept was later rehabilitated by the research of Emmett Leith and Juris Upatnieks at the University of Michigan, and Yury Denisyuk at the Vavilov Institute in Leningrad. What had been judged a failure was recast as a success: evaluations of Gaborâs work were transformed during the 1960s, when it was represented as the foundation on which to construct the new and distinctly different subject of holography, a re-evaluation that gained the Nobel Prize for Physics for Gabor alone in 1971. This paper focuses on the difficulties experienced in constructing a meaningful subject, a practical application and a viable technical community from Gaborâs ideas during the decade 1947-1957
Coulomb correlation effects in zinc monochalcogenides
Electronic structure and band characteristics for zinc monochalcogenides with
zinc-blende- and wurtzite-type structures are studied by first-principles
density-functional-theory calculations with different approximations. It is
shown that the local-density approximation underestimates the band gap and
energy splitting between the states at the top of the valence band, misplaces
the energy levels of the Zn-3d states, and overestimates the
crystal-field-splitting energy. Regardless of the structure type considered,
the spin-orbit-coupling energy is found to be overestimated for ZnO and
underestimated for ZnS with wurtzite-type structure, and more or less correct
for ZnSe and ZnTe with zinc-blende-type structure. The order of the states at
the top of the valence band is found to be anomalous for ZnO in both
zinc-blende- and wurtzite-type structure, but is normal for the other zinc
monochalcogenides considered. It is shown that the Zn-3d electrons and their
interference with the O-2p electrons are responsible for the anomalous order.
The typical errors in the calculated band gaps and related parameters for ZnO
originate from strong Coulomb correlations, which are found to be highly
significant for this compound. The LDA+U approach is by and large found to
correct the strong correlation of the Zn-3d electrons, and thus to improve the
agreement with the experimentally established location of the Zn-3d levels
compared with that derived from pure LDA calculations
Laser-stimulated Synthesis of Large Nanostructured Fractal Silver Aggregates
A Laser-stimulated synthesis of large silver nanoaggregates consisting of
hundreds to thousands of nanoparticles is investigated. It is shown that the
morphology of the synthesized nanostructures can be controlled with light and
monitored via the evolution of the colloid absorption spectra. A solution
method is demonstrated that enables production of a bulk amount of metal
nanoaggregates, which are of paramount importance for subwavelength
concentration and dramatic enhancement of the electromagnetically-induced
processes at the nanoscale.Comment: 10 pages, 8 figures, 13 reference
The RKKY interactions and the Mott Transition
A two-site cluster generalization of the Hubbard model in large dimensions is
examined in order to study the role of short-range spin correlations near the
metal-insulator transition (MIT). The model is mapped to a two-impurity
Kondo-Anderson model in a self-consistently determined bath, making it possible
to directly address the competition between the Kondo effect and RKKY
interactions in a lattice context. Our results indicate that the RKKY
interactions lead to qualitative modifications of the MIT scenario even in the
absence of long range antiferromagnetic ordering.Comment: 10 pages, 10 figures; to appear in Phys. Rev. B (1999
Square lattice site percolation at increasing ranges of neighbor interactions
We report site percolation thresholds for square lattice with neighbor
interactions at various increasing ranges. Using Monte Carlo techniques we
found that nearest neighbors (N), next nearest neighbors (N), next next
nearest neighbors (N) and fifth nearest neighbors (N) yield the same
. At odds, fourth nearest neighbors (N) give .
These results are given an explanation in terms of symmetry arguments. We then
consider combinations of various ranges of interactions with (N+N),
(N+N), (N+N+N) and (N+N). The calculated associated
thresholds are respectively . The
existing Galam--Mauger universal formula for percolation thresholds does not
reproduce the data showing dimension and coordination number are not sufficient
to build a universal law which extends to complex lattices.Comment: 4 pages, revtex
Quantum Nondemolition State Measurement via Atomic Scattering in Bragg Regime
We suggest a quantum nondemolition scheme to measure a quantized cavity field
state using scattering of atoms in general Bragg regime. Our work extends the
QND measurement of a cavity field from Fock state, based on first order Bragg
deflection [9], to any quantum state based on Bragg deflection of arbitrary
order. In addition a set of experimental parameters is provided to perform the
experiment within the frame work of the presently available technology.Comment: 11 pages text, 4 eps figures, to appear in letter section of journal
of physical society of Japa
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