1,812 research outputs found
Fundamentals of microcrack nucleation mechanics
A foundation for ultrasonic evaluation of microcrack nucleation mechanics is identified in order to establish a basis for correlations between plane strain fracture toughness and ultrasonic factors through the interaction of elastic waves with material microstructures. Since microcracking is the origin of (brittle) fracture, it is appropriate to consider the role of stress waves in the dynamics of microcracking. Therefore, the following topics are discussed: (1) microstress distributions with typical microstructural defects located in the stress field; (2) elastic wave scattering from various idealized defects; and (3) dynamic effective-properties of media with randomly distributed inhomogeneities
Sodium vacancy ordering and the co-existence of localized spins and itinerant charges in NaxCoO2
The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides
because the Co sits on a triangular lattice and its valence can be tuned over a
wide range by varying the Na concentration x. Up to now detailed modeling of
the rich phenomenology (which ranges from unconventional superconductivity to
enhanced thermopower) has been hampered by the difficulty of controlling pure
phases. We discovered that certain Na concentrations are specially stable and
are associated with superlattice ordering of the Na clusters. This leads
naturally to a picture of co-existence of localized spins and itinerant charge
carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our
picture brings coherence to a variety of measurements ranging from NMR to
optical to thermal transport. Our results also allow us to take the first step
towards modeling the mysterious ``Curie-Weiss'' metal state at x = 0.71. We
suggest the local moments may form a quantum spin liquid state and we propose
experimental test of our hypothesis.Comment: 16 pages, 5 figure
Quantum planes and quantum cylinders from Poisson homogeneous spaces
Quantum planes and a new quantum cylinder are obtained as quantization of
Poisson homogeneous spaces of two different Poisson structures on classical
Euclidean group E(2).Comment: 13 pages, plain Tex, no figure
Ballistic Annihilation Kinetics: The Case of Discrete Velocity Distributions
The kinetics of the annihilation process, , with ballistic particle
motion is investigated when the distribution of particle velocities is {\it
discrete}. This discreteness is the source of many intriguing phenomena. In the
mean field limit, the densities of different velocity species decay in time
with different power law rates for many initial conditions. For a
one-dimensional symmetric system containing particles with velocity 0 and , there is a particular initial state for which the concentrations of all
three species as decay as . For the case of a fast ``impurity'' in a
symmetric background of and particles, the impurity survival
probability decays as . In a symmetric
4-velocity system in which there are particles with velocities and
, there again is a special initial condition where the two species
decay at the same rate, t^{-\a}, with \a\cong 0.72. Efficient algorithms
are introduced to perform the large-scale simulations necessary to observe
these unusual phenomena clearly.Comment: 18 text pages, macro file included, hardcopy of 9 figures available
by email request to S
Quantized algebras of functions on homogeneous spaces with Poisson stabilizers
Let G be a simply connected semisimple compact Lie group with standard
Poisson structure, K a closed Poisson-Lie subgroup, 0<q<1. We study a
quantization C(G_q/K_q) of the algebra of continuous functions on G/K. Using
results of Soibelman and Dijkhuizen-Stokman we classify the irreducible
representations of C(G_q/K_q) and obtain a composition series for C(G_q/K_q).
We describe closures of the symplectic leaves of G/K refining the well-known
description in the case of flag manifolds in terms of the Bruhat order. We then
show that the same rules describe the topology on the spectrum of C(G_q/K_q).
Next we show that the family of C*-algebras C(G_q/K_q), 0<q\le1, has a
canonical structure of a continuous field of C*-algebras and provides a strict
deformation quantization of the Poisson algebra \C[G/K]. Finally, extending a
result of Nagy, we show that C(G_q/K_q) is canonically KK-equivalent to C(G/K).Comment: 23 pages; minor changes, typos correcte
The first products made in space: Monodisperse latex particles
The preparation of large particle size 3 to 30 micrometer monodisperse latexes in space confirmed that original rationale unequivocally. The flight polymerizations formed negligible amounts of coagulum as compared to increasing amounts for the ground-based polymerizations. The number of offsize large particles in the flight latexes was smaller than in the ground-based latexes. The particle size distribution broadened and more larger offsize particles were formed when the polymerizations of the partially converted STS-4 latexes were completed on Earth. Polymerization in space also showed other unanticipated advantages. The flight latexes had narrower particle size distributions than the ground-based latexes. The particles of the flight latexes were more perfect spheres than those of the ground-based latexes. The superior uniformity of the flight latexes was confirmed by the National Bureau of Standards acceptance of the 10 micrometer STS-6 latex and the 30 micrometer STS-11 latexes as Standard Reference Materials, the first products made in space for sale on Earth. The polymerization rates in space were the same as those on Earth within experimental error. Further development of the ground-based polymerization recipes gave monodisperse particles as large as 100 micrometer with tolerable levels of coagulum, but their uniformity was significantly poorer than the flight latexes. Careful control of the polymerization parameters gave uniform nonspherical particles: symmetrical and asymmetrical doublets, ellipsoids, egg-shaped, ice cream cone-shaped, and popcorn-shaped particles
Searching for Stable Na-ordered Phases in Single Crystal Samples of gamma-NaxCoO2
We report on the preparation and characterization of single crystal gamma
phase NaxCoO2 with 0.25 < x < 0.84 using a non-aqueous electrochemical
chronoamperemetry technique. By carefully mapping the overpotential versus x
(for x < 0.84), we find six distinct stable phases with Na levels corresponding
to x ~ 0.75, 0.71, 0.50, 0.43, 0.33 and 0.25. The composition with x ~0.55
appears to have a critical Na concentration which separates samples with
different magnetic behavior as well as different Na ion diffusion mechanisms.
Chemical analysis of an aged crystal reveals different Na ion diffusion
mechanisms above and below x_c ~ 0.53, where the diffusion process above x_c
has a diffusion coefficient about five times larger than that below x_c. The
series of crystals were studied with X-ray diffraction, susceptibility, and
transport measurements. The crystal with x = 0.5 shows a weak ferromagnetic
transition below T=27 K in addition to the usual transitions at T = 51 K and 88
K. The resistivity of the Curie-Weiss metallic Na0.71CoO2 composition has a
very low residual resistivity, which attests to the high homogeneity of the
crystals prepared by this improved electrochemical method. Our results on the
various stable crystal compositions point to the importance of Na ion ordering
across the phase diagram.Comment: 9 pages, 9 figure
Hot and repulsive traffic flow
We study a message passing model, applicable also to traffic problems. The
model is implemented in a discrete lattice, where particles move towards their
destination, with fluctuations around the minimal distance path. A repulsive
interaction between particles is introduced in order to avoid the appearance of
traffic jam. We have studied the parameter space finding regions of fluid
traffic, and saturated ones, being separated by abrupt changes. The improvement
of the system performance is also explored, by the introduction of a
non-constant potential acting on the particles. Finally, we deal with the
behavior of the system when temporary failures in the transmission occurs.Comment: 22 pages, uuencoded gzipped postscript file. 11 figures include
An Empirical Charge Transfer Potential with Correct Dissociation Limits
The empirical valence bond (EVB) method [J. Chem. Phys. 52, 1262 (1970)] has
always embodied charge transfer processes. The mechanism of that behavior is
examined here and recast for use as a new empirical potential energy surface
for large-scale simulations. A two-state model is explored. The main features
of the model are: (1) Explicit decomposition of the total system electron
density is invoked; (2) The charge is defined through the density decomposition
into constituent contributions; (3) The charge transfer behavior is controlled
through the resonance energy matrix elements which cannot be ignored; and (4) A
reference-state approach, similar in spirit to the EVB method, is used to
define the resonance state energy contributions in terms of "knowable"
quantities. With equal validity, the new potential energy can be expressed as a
nonthermal ensemble average with a nonlinear but analytical charge dependence
in the occupation number. Dissociation to neutral species for a gas-phase
process is preserved. A variant of constrained search density functional theory
is advocated as the preferred way to define an energy for a given charge.Comment: Submitted to J. Chem. Phys. 11/12/03. 14 pages, 8 figure
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