972 research outputs found
On Factor Universality in Symbolic Spaces
The study of factoring relations between subshifts or cellular automata is
central in symbolic dynamics. Besides, a notion of intrinsic universality for
cellular automata based on an operation of rescaling is receiving more and more
attention in the literature. In this paper, we propose to study the factoring
relation up to rescalings, and ask for the existence of universal objects for
that simulation relation. In classical simulations of a system S by a system T,
the simulation takes place on a specific subset of configurations of T
depending on S (this is the case for intrinsic universality). Our setting,
however, asks for every configurations of T to have a meaningful interpretation
in S. Despite this strong requirement, we show that there exists a cellular
automaton able to simulate any other in a large class containing arbitrarily
complex ones. We also consider the case of subshifts and, using arguments from
recursion theory, we give negative results about the existence of universal
objects in some classes
Analysis of Downhole Logs, Leg 57, Deep Sea Drilling Project
All sites drilled on Leg 57 were logged as weather
conditions permitted. The logging was done by the Singapore
office of Schlumberger, and because of short
notice the special logging tools required to pass through
the Challenger drill pipe were not available. The standard
centralizer, excentralizer, and caliper tools were
not appropriate, and those modified aboard ship were
not completely satisfactory. In addition, we suspected
that vertical motion from the ship's heave degraded the
quality of logs. Both the sonic and neutron density tools
were operated beyond their normal working ranges. Despite
these operational difficulties, the logs from different
runs in a hole, both repeat sections and runs with
different tools, show generally the same features. Over
intervals of 5 to 10 meters the field logs seem to be good
indicators of the physical character of the section penetrated
(Carson and Bruns, this volume).
The recovery of continuous core as well as logs differs
from normal industrial drilling practice, in which a
major purpose of logging is to establish lithology. Since
lithology is best established by direct examination, the
log analysis focused less on identifying a specific lithology
than on continuous measurement of in situ
physical properties, especially at Sites 438, 439, and 440,
where recovery was good. However, the logs were used
to establish the lithology of missing intervals, especially
at Site 441, where recovery was poor. Lithology and
physical properties aspects of the log analysis are covered
in the individual site chapters and in a comparative
analysis of laboratory measurements of physical properties
and logged physical parameters (Carson and Bruns,
this volume). In the present chapter we concentrate on
log precision, correlation of key horizons in seismic
records and core lithology, and on fracturing as indicated
by the logs. We do this by using the results of a
study of corrected logs and the derivative products from
computer programs developed by the Scientific Software
Corporation (SSC) and Technology Research Center
and of corrected logs from Schlumberger (available
from DSDP)
Tc for heavy Fermion superconductors linked with other physical properties at zero and applied pressure
The superconducting transition temperature Tc has earlier been correlated
with coherence length and effective mass for a series of heavy Fermion (HF)
materials at atmospheric pressure. Here, a further physical property, the dc
electrical conductivity sigma(Tc), is one focal point, another being the
pressure dependence of both Tc and sigma(Tc) for several HF materials. The
relaxation time tau(Tc) is also studied in relation to an Uncertainty Principle
limit, involving only the thermal energy kB Tc and Planck's constant.Comment: Supercond. Sci. Tech., to appea
Phase Transition in a One-Dimensional Extended Peierls-Hubbard Model with a Pulse of Oscillating Electric Field: I. Threshold Behavior in Ionic-to-Neutral Transition
Photoinduced dynamics of charge density and lattice displacements is
calculated by solving the time-dependent Schr\"odinger equation for a
one-dimensional extended Peierls-Hubbard model with alternating potentials for
the mixed-stack organic charge-transfer complex, TTF-CA. A pulse of oscillating
electric field is incorporated into the Peierls phase of the transfer integral.
The frequency, the amplitude, and the duration of the pulse are varied to study
the nonlinear and cooperative character of the photoinduced transition. When
the dimerized ionic phase is photoexcited, the threshold behavior is clearly
observed by plotting the final ionicity as a function of the increment of the
total energy. Above the threshold photoexcitation, the electronic state reaches
the neutral one with equidistant molecules after the electric field is turned
off. The transition is initiated by nucleation of a metastable neutral domain,
for which an electric field with frequency below the linear absorption peak is
more effective than that at the peak. When the pulse is strong and short, the
charge transfer takes place on the same time scale with the disappearance of
dimerization. As the pulse becomes weak and long, the dimerization-induced
polarization is disordered to restore the inversion symmetry on average before
the charge transfer takes place to bring the system neutral. Thus, a
paraelectric ionic phase is transiently realized by a weak electric field. It
is shown that infrared light also induces the ionic-to-neutral transition,
which is characterized by the threshold behavior.Comment: 24 pages, 11 figure
A possible mechanism of ultrafast amorphization in phase-change memory alloys: an ion slingshot from the crystalline to amorphous position
We propose that the driving force of an ultrafast crystalline-to-amorphous
transition in phase-change memory alloys are strained bonds existing in the
(metastable) crystalline phase. For the prototypical example of GST, we
demonstrate that upon breaking of long Ge-Te bond by photoexcitation Ge ion
shot from an octahedral crystalline to a tetrahedral amorphous position by the
uncompensated force of strained short bonds. Subsequent lattice relaxation
stabilizes the tetrahedral surroundings of the Ge atoms and ensures the
long-term stability of the optically induced phase.Comment: 6 pages, 3 figure
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