899 research outputs found
Transvaal and Natal Iron Age settlement revealed by aerial photography and excavation
African Studies Seminar series. Paper presented 1968In 1820 John Campbell visited a thriving Iron Age settlement built by the Hurutse
tribe at that time under the regent Liqueling, and known as Kurrichane or Kaditshwene
(spelling uncertain), believed to be near the present town of Zeerust in the S.W. Transvaal. …
In 1828 Robert Moffat, travelling to the east of Kurrichane, described remains of innumerable, recently destroyed settlements similar to Kurrichane. Moffat’s record was the start of Iron Age research in the Transvaal. Most, or all, of the Iron Age structures discussed in the present paper probably predate
the tribal wars of the 1820's. My subject in the present paper is confined to Iron Age structures
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ANTHEM: a two-dimensional multicomponent self-consistent hydro-electron transport code for laser-matter interaction studies
The ANTHEM code for the study of CO/sub 2/-laser-generated transport is outlined. ANTHEM treats the background plasma as coupled Eulerian thermal and ion fluids, and the suprathermal electrons as either a third fluid or a body of evolving collisional PIC particles. The electrons scatter off the ions; the suprathermals drag against the thermal background. Self-consistent E- and B-fields are computed by the Implicit Moment Method. The current status of the code is described. Typical output from ANTHEM is discussed with special application to Augmented-Return-Current CO/sub 2/-laser-driven targets
Averaging rheological quantities in descriptions of soft glassy materials
Many mean-field models have been introduced to describe the mechanical
behavior of glassy materials. They often rely on averages performed over
distributions of elements or states. We here underline that averaging is a more
intricate procedure in mechanics than in more classical situations such as
phase transitions in magnetic systems. This leads us to modify the predictions
of the recently proposed SGR model for soft glassy materials, for which we
suggest that the viscosity should diverge at the glass transition temperature
with an exponential form .Comment: 4 pages, Latex, 1 eps figur
An Exact Diagonalization Demonstration of Incommensurability and Rigid Band Filling for N Holes in the t-J Model
We have calculated S(q) and the single particle distribution function
for N holes in the t - J model on a non--square sqrt{8} X sqrt{32} 16--site
lattice with periodic boundary conditions; we justify the use of this lattice
in compariosn to those of having the full square symmetry of the bulk. This new
cluster has a high density of vec k points along the diagonal of reciprocal
space, viz. along k = (k,k). The results clearly demonstrate that when the
single hole problem has a ground state with a system momentum of vec k =
(pi/2,pi/2), the resulting ground state for N holes involves a shift of the
peak of the system's structure factor away from the antiferromagnetic state.
This shift effectively increases continuously with N. When the single hole
problem has a ground state with a momentum that is not equal to k =
(pi/2,pi/2), then the above--mentioned incommensurability for N holes is not
found. The results for the incommensurate ground states can be understood in
terms of rigid--band filling: the effective occupation of the single hole k =
(pi/2,pi/2) states is demonstrated by the evaluation of the single particle
momentum distribution function . Unlike many previous studies, we show
that for the many hole ground state the occupied momentum states are indeed k =
(+/- pi/2,+/- pi/2) states.Comment: Revtex 3.0; 23 pages, 1 table, and 13 figures, all include
Numerical approximation of the Euler-Poisson-Boltzmann model in the quasineutral limit
This paper analyzes various schemes for the Euler-Poisson-Boltzmann (EPB)
model of plasma physics. This model consists of the pressureless gas dynamics
equations coupled with the Poisson equation and where the Boltzmann relation
relates the potential to the electron density. If the quasi-neutral assumption
is made, the Poisson equation is replaced by the constraint of zero local
charge and the model reduces to the Isothermal Compressible Euler (ICE) model.
We compare a numerical strategy based on the EPB model to a strategy using a
reformulation (called REPB formulation). The REPB scheme captures the
quasi-neutral limit more accurately
Beyond electoralism: reflections on anarchy, populism, and the crisis of electoral politics
This paper is comprised of a series of short, conversational or polemical
interventions reflecting on the political ‘moment’ that has emerged in the wake of
the rise of right-populist politics, particularly in the Global North. We position the
UK’s ‘Brexit’ vote and the election of Donald Trump as US President as
emblematic of this shift, which has a longer genesis and a wider scale than these
events alone. In particular, we draw on anarchist principles and approaches to
consider opportunities for re-energising and re-orienting our academic and activist
priorities in the wake of these turbulent times. Following a short introductory
section, in which we collectively discuss key questions, challenges and tensions,
each contributor individually draws from their own research or perspective to
explore the possibilities of a politics beyond electoralism
Direct Observation of a One Dimensional Static Spin Modulation in Insulating La1.95Sr0.05CuO4
We report the results of an extensive elastic neutron scattering study of the
incommensurate (IC) static spin correlations in La1.95Sr0.05CuO4 which is an
insulating spin glass at low temperatures. The present neutron scattering
experiments on the same x=0.05 crystal employ a narrower instrumental
Q-resolution and thereby have revealed that the crystal has only two
orthorhombic twins at low temperatures with relative populations of 2:1. We
find that, in a single twin, only two satellites are observed at (1, +/-0.064,
L)(ortho) and (0, 1+/-0.064, L)(ortho), that is, the modulation vector is only
along the orthorhombic b*-axis. This demonstrates unambiguously that
La1.95Sr0.05CuO4 has a one-dimensional static diagonal spin modulation at low
temperatures, consistent with certain stripe models. We have also reexamined
the x=0.04 crystal that previously was reported to show a single commensurate
peak. By mounting the sample in the (H, K, 0) zone, we have discovered that the
x=0.04 sample in fact has the same IC structure as the sample. The
incommensurability parameter d for x=0.04 and 0.05, where d is the distance
from (1/2, 1/2) in tetragonal reciprocal lattice units, follows the linear
relation d=x. These results demonstrate that the insulator to superconductor
transition in the under doped regime (0.05 </= x </= 0.06) in La2-xSrxCuO4 is
coincident with a transition from diagonal to collinear static stripes at low
temperatures thereby evincing the intimate coupling between the one dimensional
spin density modulation and the superconductivity.Comment: 9 pages 8 figure
Quantisation of twistor theory by cocycle twist
We present the main ingredients of twistor theory leading up to and including
the Penrose-Ward transform in a coordinate algebra form which we can then
`quantise' by means of a functorial cocycle twist. The quantum algebras for the
conformal group, twistor space CP^3, compactified Minkowski space CMh and the
twistor correspondence space are obtained along with their canonical quantum
differential calculi, both in a local form and in a global *-algebra
formulation which even in the classical commutative case provides a useful
alternative to the formulation in terms of projective varieties. We outline how
the Penrose-Ward transform then quantises. As an example, we show that the
pull-back of the tautological bundle on CMh pulls back to the basic instanton
on S^4\subset CMh and that this observation quantises to obtain the
Connes-Landi instanton on \theta-deformed S^4 as the pull-back of the
tautological bundle on our \theta-deformed CMh. We likewise quantise the
fibration CP^3--> S^4 and use it to construct the bundle on \theta-deformed
CP^3 that maps over under the transform to the \theta-deformed instanton.Comment: 68 pages 0 figures. Significant revision now has detailed formulae
for classical and quantum CP^
A Gene Network Perspective on Axonal Regeneration
The regenerative capacity of injured neurons in the central nervous system is limited due to the absence of a robust neuron-intrinsic injury-induced gene response that supports axon regeneration. In peripheral neurons axotomy induces a large cohort of regeneration-associated genes (RAGs). The forced expression of some of these RAGs in injured neurons has some beneficial effect on axon regeneration, but the reported effects are rather small. Transcription factors (TFs) provide a promising class of RAGs. TFs are hubs in the regeneration-associated gene network, and potentially control the coordinate expression of many RAGs simultaneously. Here we discuss the use of combined experimental and computational methods to identify novel regeneration-associated TFs with a key role in initiating and maintaining the RAG-response in injured neurons. We propose that a relatively small number of hub TFs with multiple functional connections in the RAG network might provide attractive new targets for gene-based and/or pharmacological approaches to promote axon regeneration in the central nervous system
Understanding the impact of cavitation on hydrocarbons in the middle distillate range
NOTICE: this is the author’s version of a work that was accepted for publication in Fuel. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Fuel, 156, September 2015, pp. 30-39, http://dx/doi.org/10.1016/j.fuel.2015.04.026Hydrocarbons in the middle distillate range (C8 - C26) have been treated with ultrasound at 20 kHz - a frequency sufficient to drive acoustic cavitation. The high temperatures experienced as a result of the implosion of fuel vapour bubbles are sufficient to produce pyrolytic degradation and dehydrogenation, as well as a growth mechanism that results in the formation of small particles that have similarities with the primary soot particles produced during diesel combustion. These nanosized particles agglomerate as a result of kinetically driven collisions during cavitation to form a dispersion of micron sized particles in the treated hydrocarbon. The particles are carbonaceous in character, being a mixture of amorphous and graphitic-like carbon. The mass of material produced increases with the C/H atomic ratio of the hydrocarbon undergoing cavitation and is decreased through the addition (1 - 3 %v/v) of low boiling paraffinic hydrocarbons, possibly as a result of lowering the temperature developed inside imploding cavities. Dispersions of microparticles contain equilibrated levels of nanoparticles. If sufficiently high numbers of these smaller primary particles are present they agglomerate due to thermally driven collisions during post-cavitation storage. When this happened a sharp rise in the number of 1 - 2 µm particles was seen after only a few days. Some evidence is presented for the behaviour of ultrasonically treated hydrocarbons being related to the degradation of diesel fuel exposed to hydrodynamic cavitation in the fuel systems of modern common rail direct injection diesel engines.Shell Global Solution
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