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

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

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