7,810 research outputs found
Charge Fluctuations in YBa2Cu3O6+x Superconductors
Striped phases in which spin and charge separate into different regions in
the material have been proposed to account for the unusual properties of the
high- cuprate superconductors. The driving force for a striped phase is
the charge distribution, which self-organizes itself into linear regions. In
the highest materials such regions are not static but fluctuate in time.
Neutrons, having no charge, can not directly observe these fluctuations but
they can be observed indirectly by their effect on the phonons. Neutron
scattering measurements have been made using a specialized technique to study
the phonon line shapes in four crystals with oxygen doping levels varying from
highly underdoped to optimal doping. It is shown that fluctuating charge
stripes exist over the whole doping range, and become visible below
temperatures somewhat higher than the pseudogap temperature.Comment: 10 pages, 4 figures,Proceedings of Third International Conference on
Theories, Discoveries, and Applications of Superconductors, to be published
in Physica
Neutron Scattering Studies of the Magnetic Fluctuations in YBa_2Cu_3O_{7-\delta}
Neutron scattering measurements have been made on the spin fluctuations in
YBa_2Cu_3O_{7-\delta} for different oxygen doping levels. Incommensurability is
clearly observed for oxygen concentrations of 6.6 and 6.7 and is suggested for
the 6.93. Measurements of the resonance for the O_{6.6} concentration show that
it exists in a broadened and less intense form at temperatures much higher than
T_c.Comment: 9 pages, 4 gif figures, Proceedings of Spectroscopies in Novel
Superconductors, Cape Cod, Massachusetts, Sept, 199
Optimal post-experiment estimation of poorly modeled dynamic systems
Recently, a novel strategy for post-experiment state estimation of discretely-measured dynamic systems has been developed. The method accounts for errors in the system dynamic model equations in a more general and rigorous manner than do filter-smoother algorithms. The dynamic model error terms do not require the usual process noise assumptions of zero-mean, symmetrically distributed random disturbances. Instead, the model error terms require no prior assumptions other than piecewise continuity. The resulting state estimates are more accurate than filters for applications in which the dynamic model error clearly violates the typical process noise assumptions, and the available measurements are sparse and/or noisy. Estimates of the dynamic model error, in addition to the states, are obtained as part of the solution of a two-point boundary value problem, and may be exploited for numerous reasons. In this paper, the basic technique is explained, and several example applications are given. Included among the examples are both state estimation and exploitation of the model error estimates
Predicting Human Lifespan Limits
Recent discoveries show steady improvements in life expectancy during modern
decades. Does this support that humans continue to live longer in future? We
recently put forward the maximum survival tendency, as found in survival curves
of industrialized countries, which is described by extended Weibull model with
age-dependent stretched exponent. The maximum survival tendency suggests that
human survival dynamics may possess its intrinsic limit, beyond which survival
is inevitably forbidden. Based on such tendency, we develop the model and
explore the patterns in the maximum lifespan limits from industrialized
countries during recent three decades. This analysis strategy is simple and
useful to interpret the complicated human survival dynamics.Comment: 11 pages, 3 figures, 2 tables; Natural Science (in press
Magnetic excitations in the stripe phase of high-T_c superconductors
The magnetic excitations in the stripe phase of high-T_c superconductors are
investigated in a model of spin ladders which are effectively coupled via
charged stripes. Starting from the effective single-triplon model for the
isolated spin ladder, the quasi-one-dimensional spin system can be described
straightforwardly. Very good agreement is obtained with recent neutron
scattering data on La_(15/8)Ba_(1/8)CuO_4 (no spin gap) and YBa_2Cu_3O_(6.6)
(gapped). The signature of quasi-one-dimensional spin physics in a
single-domain stripe phase is predicted.Comment: 3 pages, 3 figures included, submitted to the proceedings of JEMS
200
Phasing the Kastro: Abandonment and the Archaeological Record
While the processes of settlement abandonment have been a topic of long-standing interest to anthropologists and archaeologists working in the American Southwest, their implications are seldom acknowledged by Aegeanists. On the Kastro in East Crete, a settlement inhabited from Late Minoan IIIC into the Archaic period, variations in artifact type, size, and distribution across the site have not been adequately explained, but may now be reassessed with respect to abandonment processes. Recognition of these processes affects the interpretation of remains associated with both abandonment and deposition resulting from nonabandonment behavior, and also provides criteria for phasing
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