49,282 research outputs found
Atom Lithography with Near-Resonant Light Masks: Quantum Optimization Analysis
We study the optimal focusing of two-level atoms with a near resonant
standing wave light, using both classical and quantum treatments of the
problem. Operation of the focusing setup is considered as a nonlinear spatial
squeezing of atoms in the thin- and thick-lens regimes. It is found that the
near-resonant standing wave focuses the atoms with a reduced background in
comparison with far-detuned light fields. For some parameters, the quantum
atomic distribution shows even better localization than the classical one.
Spontaneous emission effects are included via the technique of quantum Monte
Carlo wave function simulations. We investigate the extent to which
non-adiabatic and spontaneous emission effects limit the achievable minimal
size of the deposited structures.Comment: 10 pages including 11 figures in Revte
First-principles thermoelasticity of bcc iron under pressure
We investigate the elastic and isotropic aggregate properties of
ferromagnetic bcc iron as a function of temperature and pressure by computing
the Helmholtz free energies for the volume-conserving strained structures using
the first-principles linear response linear-muffin-tin-orbital method and the
generalized-gradient approximation. We include the electronic excitation
contributions to the free energy from the band structures, and phonon
contributions from quasi-harmonic lattice dynamics. We make detailed
comparisons between our calculated elastic moduli and their temperature and
pressure dependences with available experimental and theoretical data.Comment: 5 figures, 2 table
Lower Bounds in the Preprocessing and Query Phases of Routing Algorithms
In the last decade, there has been a substantial amount of research in
finding routing algorithms designed specifically to run on real-world graphs.
In 2010, Abraham et al. showed upper bounds on the query time in terms of a
graph's highway dimension and diameter for the current fastest routing
algorithms, including contraction hierarchies, transit node routing, and hub
labeling. In this paper, we show corresponding lower bounds for the same three
algorithms. We also show how to improve a result by Milosavljevic which lower
bounds the number of shortcuts added in the preprocessing stage for contraction
hierarchies. We relax the assumption of an optimal contraction order (which is
NP-hard to compute), allowing the result to be applicable to real-world
instances. Finally, we give a proof that optimal preprocessing for hub labeling
is NP-hard. Hardness of optimal preprocessing is known for most routing
algorithms, and was suspected to be true for hub labeling
Viscosity of Colloidal Suspensions
Simple expressions are given for the Newtonian viscosity as
well as the viscoelastic behavior of the viscosity of
neutral monodisperse hard sphere colloidal suspensions as a function of volume
fraction and frequency over the entire fluid range, i.e., for
volume fractions . These expressions are based on an
approximate theory which considers the viscosity as composed as the sum of two
relevant physical processes: , where is the
infinite frequency (or very short time) viscosity, with the solvent
viscosity, the equilibrium hard sphere radial distribution
function at contact, and the contribution due to the
diffusion of the colloidal particles out of cages formed by their neighbors, on
the P\'{e}clet time scale , the dominant physical process in
concentrated colloidal suspensions. The Newtonian viscosity agrees very well with the extensive experiments of Van
der Werff et al and others. Also, the asymptotic behavior for large is
of the form , in agreement
with these experiments, but the theoretical coefficient differs by a
constant factor from the exact coefficient, computed from the
Green-Kubo formula for . This still enables us to predict
for practical purposes the visco-elastic behavior of monodisperse spherical
colloidal suspensions for all volume fractions by a simple time rescaling.Comment: 51 page
The effect of real-air properties upon aerodynamic forces, moments, and heat transfer rates for reentry vehicles
Eulerian Statistically Preserved Structures in Passive Scalar Advection
We analyze numerically the time-dependent linear operators that govern the
dynamics of Eulerian correlation functions of a decaying passive scalar
advected by a stationary, forced 2-dimensional Navier-Stokes turbulence. We
show how to naturally discuss the dynamics in terms of effective compact
operators that display Eulerian Statistically Preserved Structures which
determine the anomalous scaling of the correlation functions. In passing we
point out a bonus of the present approach, in providing analytic predictions
for the time-dependent correlation functions in decaying turbulent transport.Comment: 10 pages, 10 figures. Submitted to Phys. Rev.
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