1,623 research outputs found
Nonstoichiometric doping and Bi antisite defect in single crystal Bi2Se3
We studied the defects of Bi2Se3 generated from Bridgman growth of
stoichiometric and nonstoichiometric self-fluxes. Growth habit, lattice size,
and transport properties are strongly affected by the types of defect
generated. Major defect types of Bi_Se antisite and partial Bi_2-layer
intercalation are identified through combined studies of direct atomic-scale
imaging with scanning transmission electron microscopy (STEM) in conjunction
with energy-dispersive X-ray spectroscopy (STEM-EDX), X-ray diffraction, and
Hall effect measurements. We propose a consistent explanation to the origin of
defect type, growth morphology, and transport property.Comment: 5 pages, 5 figure
Quantum Cosmology and Open Universes
Quantum creation of Universes with compact spacelike sections that have
curvature either closed, flat or open, i.e. are studied. In the
flat and open cases, the superpotential of the Wheeler De Witt equation is
significantly modified, and as a result the qualitative behaviour of a typical
wavefunction differs from the traditional closed case. Using regularity
arguments, it is shown that the only consistent state for the wavefunction is
the Tunneling one. By computing the quantum probabilities for the curvature of
the sections, it is shown that quantum cosmology actually favours that the
Universe be open, . In all cases sufficient inflation
e-foldings is predicted: this is an improvement over classical measures that
generally are ambiguous as to whether inflation is certain to occur.Comment: 11 pages, Revtex, 7 figures. Accepted for publication in PRD. New
material and important corrections added in response to referee's repor
Computing the vertices of tropical polyhedra using directed hypergraphs
We establish a characterization of the vertices of a tropical polyhedron
defined as the intersection of finitely many half-spaces. We show that a point
is a vertex if, and only if, a directed hypergraph, constructed from the
subdifferentials of the active constraints at this point, admits a unique
strongly connected component that is maximal with respect to the reachability
relation (all the other strongly connected components have access to it). This
property can be checked in almost linear-time. This allows us to develop a
tropical analogue of the classical double description method, which computes a
minimal internal representation (in terms of vertices) of a polyhedron defined
externally (by half-spaces or hyperplanes). We provide theoretical worst case
complexity bounds and report extensive experimental tests performed using the
library TPLib, showing that this method outperforms the other existing
approaches.Comment: 29 pages (A4), 10 figures, 1 table; v2: Improved algorithm in section
5 (using directed hypergraphs), detailed appendix; v3: major revision of the
article (adding tropical hyperplanes, alternative method by arrangements,
etc); v4: minor revisio
Comparison of the Sachs-Wolfe Effect for Gaussian and Non-Gaussian Fluctuations
A consequence of non-Gaussian perturbations on the Sachs-Wolfe effect is
studied. For a particular power spectrum, predicted Sachs-Wolfe effects are
calculated for two cases: Gaussian (random phase) configuration, and a specific
kind of non-Gaussian configuration. We obtain a result that the Sachs-Wolfe
effect for the latter case is smaller when each temperature fluctuation is
properly normalized with respect to the corresponding mass fluctuation . The physical explanation and the generality of the result are
discussed.Comment: 16 page
Inflationary Initial Conditions Consistent with Causality
The initial condition problem of inflation is examined from the perspective
of both spacetime embedding and scalar field dynamics. The spacetime embedding
problem is solved for arbitrary initial spatial curvature Omega, which
generalizes previous works that primarily treat the flat case Omega=1. Scalar
field dynamics that is consistent with the embedding constraints are examined,
with the additional treatment of damping effects. The effects of
inhomogeneities on the embedding problem also are considered. A category of
initial conditions are identified that are not acausal and can develop into an
inflationary regime.Comment: 9 pages, 3 figures. Minor changes, matches version to appear in
Physical Review
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