7,310 research outputs found
A comparison of ground-based and space flight data: Atomic oxygen reactions with boron nitride and silicon nitride
The effects of atomic oxygen on boron nitride (BN) and silicon nitride (Si3N4) have been studied in low Earth orbit (LEO) flight experiments and in a ground-based simulation facility at Los Alamos National Laboratory. Both the in-flight and ground-based experiments employed the materials coated over thin (approx 250 Angstrom) silver films whose electrical resistance was measured in situ to detect penetration of atomic oxygen through the BN and Si3N4 materials. In the presence of atomic oxygen, silver oxidizes to form silver oxide, which has a much higher electrical resistance than pure silver. Permeation of atomic oxygen through BN, as indicated by an increase in the electrical resistance of the silver underneath, was observed in both the in-flight and ground-based experiments. In contrast, no permeation of atomic oxygen through Si3N4 was observed in either the in-flight or ground-based experiments. The ground-based results show good qualitative correlation with the LEO flight results, thus validating the simulation fidelity of the ground-based facility in terms of reproducing LEO flight results
Phonon quarticity induced by changes in phonon-tracked hybridization during lattice expansion and its stabilization of rutile TiO
Although the rutile structure of TiO is stable at high temperatures, the
conventional quasiharmonic approximation predicts that several acoustic phonons
decrease anomalously to zero frequency with thermal expansion, incorrectly
predicting a structural collapse at temperatures well below 1000\,K. Inelastic
neutron scattering was used to measure the temperature dependence of the phonon
density of states (DOS) of rutile TiO from 300 to 1373\,K. Surprisingly,
these anomalous acoustic phonons were found to increase in frequency with
temperature. First-principles calculations showed that with lattice expansion,
the potentials for the anomalous acoustic phonons transform from quadratic to
quartic, stabilizing the rutile phase at high temperatures. In these modes, the
vibrational displacements of adjacent Ti and O atoms cause variations in
hybridization of electrons of Ti and electrons of O atoms. With
thermal expansion, the energy variation in this "phonon-tracked hybridization"
flattens the bottom of the interatomic potential well between Ti and O atoms,
and induces a quarticity in the phonon potential.Comment: 7 pages, 6 figures, supplemental material (3 figures
The Escape Problem in a Classical Field Theory With Two Coupled Fields
We introduce and analyze a system of two coupled partial differential
equations with external noise. The equations are constructed to model
transitions of monovalent metallic nanowires with non-axisymmetric intermediate
or end states, but also have more general applicability. They provide a rare
example of a system for which an exact solution of nonuniform stationary states
can be found. We find a transition in activation behavior as the interval
length on which the fields are defined is varied. We discuss several
applications to physical problems.Comment: 24 page
Late Decaying Dark Matter, Bulk Viscosity and the Cosmic Acceleration
We discuss a cosmology in which cold dark matter begins to decay into
relativistic particles at a recent epoch (z < 1). We show that the large
entropy production and associated bulk viscosity from such decays leads to an
accelerating cosmology as required by observations. We investigate the effects
of decaying cold dark matter in a Lambda = 0, flat, initially matter dominated
cosmology. We show that this model satisfies the cosmological constraint from
the redshift-distance relation for type Ia supernovae. The age in such models
is also consistent with the constraints from the oldest stars and globular
clusters. Possible candidates for this late decaying dark matter are suggested
along with additional observational tests of this cosmological paradigm.Comment: 8 pages, 3 figures, 1 tabl
Spectral analysis of Gene co-expression network of Zebrafish
We analyze the gene expression data of Zebrafish under the combined framework
of complex networks and random matrix theory. The nearest neighbor spacing
distribution of the corresponding matrix spectra follows random matrix
predictions of Gaussian orthogonal statistics. Based on the eigenvector
analysis we can divide the spectra into two parts, first part for which the
eigenvector localization properties match with the random matrix theory
predictions, and the second part for which they show deviation from the theory
and hence are useful to understand the system dependent properties. Spectra
with the localized eigenvectors can be characterized into three groups based on
the eigenvalues. We explore the position of localized nodes from these
different categories. Using an overlap measure, we find that the top
contributing nodes in the different groups carry distinguished structural
features. Furthermore, the top contributing nodes of the different localized
eigenvectors corresponding to the lower eigenvalue regime form different
densely connected structure well separated from each other. Preliminary
biological interpretation of the genes, associated with the top contributing
nodes in the localized eigenvectors, suggests that the genes corresponding to
same vector share common features.Comment: 6 pages, four figures (accepted in EPL
Analytic Controllability of Time-Dependent Quantum Control Systems
The question of controllability is investigated for a quantum control system
in which the Hamiltonian operator components carry explicit time dependence
which is not under the control of an external agent. We consider the general
situation in which the state moves in an infinite-dimensional Hilbert space, a
drift term is present, and the operators driving the state evolution may be
unbounded. However, considerations are restricted by the assumption that there
exists an analytic domain, dense in the state space, on which solutions of the
controlled Schrodinger equation may be expressed globally in exponential form.
The issue of controllability then naturally focuses on the ability to steer the
quantum state on a finite-dimensional submanifold of the unit sphere in Hilbert
space -- and thus on analytic controllability. A relatively straightforward
strategy allows the extension of Lie-algebraic conditions for strong analytic
controllability derived earlier for the simpler, time-independent system in
which the drift Hamiltonian and the interaction Hamiltonia have no intrinsic
time dependence. Enlarging the state space by one dimension corresponding to
the time variable, we construct an augmented control system that can be treated
as time-independent. Methods developed by Kunita can then be implemented to
establish controllability conditions for the one-dimension-reduced system
defined by the original time-dependent Schrodinger control problem. The
applicability of the resulting theorem is illustrated with selected examples.Comment: 13 page
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