8,724 research outputs found
Precision laser range finder system design for Advanced Technology Laboratory applications
Preliminary system design of a pulsed precision ruby laser rangefinder system is presented which has a potential range resolution of 0.4 cm when atmospheric effects are negligible. The system being proposed for flight testing on the advanced technology laboratory (ATL) consists of a modelocked ruby laser transmitter, course and vernier rangefinder receivers, optical beacon retroreflector tracking system, and a network of ATL tracking retroreflectors. Performance calculations indicate that spacecraft to ground ranging accuracies of 1 to 2 cm are possible
Degenerate ground states and nonunique potentials: breakdown and restoration of density functionals
The Hohenberg-Kohn (HK) theorem is one of the most fundamental theorems of
quantum mechanics, and constitutes the basis for the very successful
density-functional approach to inhomogeneous interacting many-particle systems.
Here we show that in formulations of density-functional theory (DFT) that
employ more than one density variable, applied to systems with a degenerate
ground state, there is a subtle loophole in the HK theorem, as all mappings
between densities, wave functions and potentials can break down. Two weaker
theorems which we prove here, the joint-degeneracy theorem and the
internal-energy theorem, restore the internal, total and exchange-correlation
energy functionals to the extent needed in applications of DFT to atomic,
molecular and solid-state physics and quantum chemistry. The joint-degeneracy
theorem constrains the nature of possible degeneracies in general many-body
systems
Electron Localization in the Insulating State
The insulating state of matter is characterized by the excitation spectrum,
but also by qualitative features of the electronic ground state. The insulating
ground wavefunction in fact: (i) sustains macroscopic polarization, and (ii) is
localized. We give a sharp definition of the latter concept, and we show how
the two basic features stem from essentially the same formalism. Our approach
to localization is exemplified by means of a two--band Hubbard model in one
dimension. In the noninteracting limit the wavefunction localization is
measured by the spread of the Wannier orbitals.Comment: 5 pages including 3 figures, submitted to PR
Band mass anisotropy and the intrinsic metric of fractional quantum Hall systems
It was recently pointed out that topological liquid phases arising in the
fractional quantum Hall effect (FQHE) are not required to be rotationally
invariant, as most variational wavefunctions proposed to date have been.
Instead, they possess a geometric degree of freedom corresponding to a shear
deformation that acts like an intrinsic metric. We apply this idea to a system
with an anisotropic band mass, as is intrinsically the case in many-valley
semiconductors such as AlAs and Si, or in isotropic systems like GaAs in the
presence of a tilted magnetic field, which breaks the rotational invariance. We
perform exact diagonalization calculations with periodic boundary conditions
(torus geometry) for various filling fractions in the lowest, first and second
Landau levels. In the lowest Landau level, we demonstrate that FQHE states
generally survive the breakdown of rotational invariance by moderate values of
the band mass anisotropy. At 1/3 filling, we generate a variational family of
Laughlin wavefunctions parametrized by the metric degree of freedom. We show
that the intrinsic metric of the Laughlin state adjusts as the band mass
anisotropy or the dielectric tensor are varied, while the phase remains robust.
In the n=1 Landau level, mass anisotropy drives transitions between
incompressible liquids and compressible states with charge density wave
ordering. In n>=2 Landau levels, mass anisotropy selects and enhances stripe
ordering with compatible wave vectors at partial 1/3 and 1/2 fillings.Comment: 9 pages, 8 figure
The Decay Properties of the Finite Temperature Density Matrix in Metals
Using ordinary Fourier analysis, the asymptotic decay behavior of the density
matrix F(r,r') is derived for the case of a metal at a finite electronic
temperature. An oscillatory behavior which is damped exponentially with
increasing distance between r and r' is found. The decay rate is not only
determined by the electronic temperature, but also by the Fermi energy. The
theoretical predictions are confirmed by numerical simulations
External field control of donor electron exchange at the Si/SiO2 interface
We analyze several important issues for the single- and two-qubit operations
in Si quantum computer architectures involving P donors close to a SiO2
interface. For a single donor, we investigate the donor-bound electron
manipulation (i.e. 1-qubit operation) between the donor and the interface by
electric and magnetic fields. We establish conditions to keep a donor-bound
state at the interface in the absence of local surface gates, and estimate the
maximum planar density of donors allowed to avoid the formation of a
2-dimensional electron gas at the interface. We also calculate the times
involved in single electron shuttling between the donor and the interface. For
a donor pair, we find that under certain conditions the exchange coupling (i.e.
2-qubit operation) between the respective electron pair at the interface may be
of the same order of magnitude as the coupling in GaAs-based two-electron
double quantum dots where coherent spin manipulation and control has been
recently demonstrated (for example for donors ~10 nm below the interface and
\~40 nm apart, J~10^{-4} meV), opening the perspective for similar experiments
to be performed in Si.Comment: 11 pages, 15 figures. Changes in Eq. 24 plus minor typo
Domain structure of bulk ferromagnetic crystals in applied fields near saturation
We investigate the ground state of a uniaxial ferromagnetic plate with
perpendicular easy axis and subject to an applied magnetic field normal to the
plate. Our interest is the asymptotic behavior of the energy in macroscopically
large samples near the saturation field. We establish the scaling of the
critical value of the applied field strength below saturation at which the
ground state changes from the uniform to a branched domain magnetization
pattern and the leading order scaling behavior of the minimal energy.
Furthermore, we derive a reduced sharp-interface energy giving the precise
asymptotic behavior of the minimal energy in macroscopically large plates under
a physically reasonable assumption of small deviations of the magnetization
from the easy axis away from domain walls. On the basis of the reduced energy,
and by a formal asymptotic analysis near the transition, we derive the precise
asymptotic values of the critical field strength at which non-trivial
minimizers (either local or global) emerge. The non-trivial minimal energy
scaling is achieved by magnetization patterns consisting of long slender
needle-like domains of magnetization opposing the applied fieldComment: 38 pages, 7 figures, submitted to J. Nonlin. Sci
Disorder and Interaction in 2D: Exact diagonalization study of the Anderson-Hubbard-Mott model
We investigate, by numerically calculating the charge stiffness, the effects
of random diagonal disorder and electron-electron interaction on the nature of
the ground state in the 2D Hubbard model through the finite size exact
diagonalization technique. By comparing with the corresponding 1D Hubbard model
results and by using heuristic arguments we conclude that it is
\QTR{it}{unlikely} that there is a 2D metal-insulator quantum phase transition
although the effect of interaction in some range of parameters is to
substantially enhance the non-interacting charge stiffness.Comment: 13 pages, 2 figures Revised version. Accepted for publication in
Phys. Rev. Let
Flutuação populacional do ácaro-da-ferrugem-da-videira em vinhedo comercial em Candiota, RS, com diferentes métodos de amostragem.
Nos vinhedos da Região da Campanha do Rio Grande do Sul, o ácaro-da-ferrugem-da-videira, Calepitrimerus vitis (Nalepa, 1905) (Acari: Eriophyidae), tem sido encontrado com frequência desde a safra 2004/2005 associado com sintomas de bronzeamento nas folhas. A flutuação populacional de C. vitis na cultivar ?Cabernet Sauvignon? foi estudada em vinhedo comercial localizado em Candiota, RS, durante as safras agrícolas 2007/2008 (de novembro a junho) e 2008/2009 (de outubro a maio). A amostragem foi realizada nas folhas e através de armadilhas constituídas por fitas adesivas de dupla face instaladas nos ramos de produção. O pico populacional, na primeira safra, ocorreu em março de 2008 quando foram registrados 0,34 indivíduos por cm² da face abaxial das folhas medianas e 29,48 indivíduos por armadilhas. Na segunda safra, o pico populacional foi menos intenso e ocorreu em outubro de 2008, quando foram detectados 0,11 indivíduos por cm² da face abaxial das folhas medianas e 0,43 indivíduos por armadilhas. Foi detectado que o início do deslocamento de C. vitis para os locais de hibernação ocorre no verão, a partir de fevereiro. As armadilhas adesivas foram mais eficientes para identificar a presença de C. vitis no vinhedo do que a avaliação direta nas folhas. Uma correlação positiva foi encontrada entre o número de C. vitis na face abaxial das folhas e o percentual de folhas com infestação
Ferromagnetism in the two dimensional t-t' Hubbard model at the Van Hove density
Using an improved version of the projection quantum Monte Carlo technique, we
study the square-lattice Hubbard model with nearest-neighbor hopping t and
next-nearest-neighbor hopping t', by simulation of lattices with up to 20 X 20
sites. For a given R=2t'/t, we consider that filling which leads to a singular
density of states of the noninteracting problem. For repulsive interactions, we
find an itinerant ferromagnet (antiferromagnet) for R=0.94 (R=0.2). This is
consistent with the prediction of the T-matrix approximation, which sums the
most singular set of diagrams.Comment: 10 pages, RevTeX 3.0 + a single postscript file with all figure
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