3,936 research outputs found
Compression of Atomic Phase Space Using an Asymmetric One-Way Barrier
We show how to construct asymmetric optical barriers for atoms. These
barriers can be used to compress phase space of a sample by creating a confined
region in space where atoms can accumulate with heating at the single photon
recoil level. We illustrate our method with a simple two-level model and then
show how it can be applied to more realistic multi-level atoms
Theory and Simulation of the diffusion of kinks on dislocations in bcc metals
Isolated kinks on thermally fluctuating (1/2) screw, edge and
(1/2) edge dislocations in bcc iron are simulated under zero stress
conditions using molecular dynamics (MD). Kinks are seen to perform stochastic
motion in a potential landscape that depends on the dislocation character and
geometry, and their motion provides fresh insight into the coupling of
dislocations to a heat bath. The kink formation energy, migration barrier and
friction parameter are deduced from the simulations. A discrete
Frenkel-Kontorova-Langevin (FKL) model is able to reproduce the coarse grained
data from MD at a fraction of the computational cost, without assuming an a
priori temperature dependence beyond the fluctuation-dissipation theorem.
Analytic results reveal that discreteness effects play an essential r\^ole in
thermally activated dislocation glide, revealing the existence of a crucial
intermediate length scale between molecular and dislocation dynamics. The model
is used to investigate dislocation motion under the vanishingly small stress
levels found in the evolution of dislocation microstructures in irradiated
materials
Direct observation of size scaling and elastic interaction between nano-scale defects in collision cascades
Using in-situ transmission electron microscopy, we have directly observed
nano-scale defects formed in ultra-high purity tungsten by low-dose high energy
self-ion irradiation at 30K. At cryogenic temperature lattice defects have
reduced mobility, so these microscope observations offer a window on the
initial, primary damage caused by individual collision cascade events. Electron
microscope images provide direct evidence for a power-law size distribution of
nano-scale defects formed in high-energy cascades, with an upper size limit
independent of the incident ion energy, as predicted by Sand et al. [Eur. Phys.
Lett., 103:46003, (2013)]. Furthermore, the analysis of pair distribution
functions of defects observed in the micrographs shows significant
intra-cascade spatial correlations consistent with strong elastic interaction
between the defects
Statistical Mechanics of an Optical Phase Space Compressor
We describe the statistical mechanics of a new method to produce very cold
atoms or molecules. The method results from trapping a gas in a potential well,
and sweeping through the well a semi-permeable barrier, one that allows
particles to leave but not to return. If the sweep is sufficiently slow, all
the particles trapped in the well compress into an arbitrarily cold gas. We
derive analytical expressions for the velocity distribution of particles in the
cold gas, and compare these results with numerical simulations.Comment: 7 pages, 3 figure
Correlation effects in the ground state charge density of Mott-insulating NiO: a comparison of ab-initio calculations and high-energy electron diffraction measurements
Accurate high-energy electron diffraction measurements of structure factors
of NiO have been carried out to investigate how strong correlations in the Ni
3d shell affect electron charge density in the interior area of nickel ions and
whether the new ab-initio approaches to the electronic structure of strongly
correlated metal oxides are in accord with experimental observations. The
generalized gradient approximation (GGA) and the local spin density
approximation corrected by the Hubbard U term (LSDA+U) are found to provide the
closest match to experimental measurements. The comparison of calculated and
observed electron charge densities shows that correlations in the Ni 3d shell
suppress covalent bonding between the oxygen and nickel sublattices.Comment: 6 pages, LaTeX and 5 figures in the postscript forma
Quantum switches and quantum memories for matter-wave lattice solitons
We study the possibility of implementing a quantum switch and a quantum
memory for matter wave lattice solitons by making them interact with
"effective" potentials (barrier/well) corresponding to defects of the optical
lattice. In the case of interaction with an "effective" potential barrier, the
bright lattice soliton experiences an abrupt transition from complete
transmission to complete reflection (quantum switch) for a critical height of
the barrier. The trapping of the soliton in an "effective" potential well and
its release on demand, without loses, shows the feasibility of using the system
as a quantum memory. The inclusion of defects as a way of controlling the
interactions between two solitons is also reported
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