7,929 research outputs found
The Growth and Structure of Double - Diffusive Cells Adjacent to a Side - Wall in a Salt - Stratified Environment
August 15-21, 2004 Measurements are reported of the rate of horizontal extension of the cells in tanks of different lengths with a range of initial salinity gradients and cooling rates (which determine the vertical height of each cell). A simple model for the cell evolution is developed. It predicts that cell growth is dependent on tank length. The mean rate of increase of cell length decreases linearly in time, as does the density gradient inside the cells, supported by both temperature and salinity gradients. The results are found to agree quantitatively with the measurements
Microscopic Aspects of Stretched Exponential Relaxation (SER) in Homogeneous Molecular and Network Glasses and Polymers
Because the theory of SER is still a work in progress, the phenomenon itself
can be said to be the oldest unsolved problem in science, as it started with
Kohlrausch in 1847. Many electrical and optical phenomena exhibit SER with
probe relaxation I(t) ~ exp[-(t/{\tau}){\beta}], with 0 < {\beta} < 1. Here
{\tau} is a material-sensitive parameter, useful for discussing chemical
trends. The "shape" parameter {\beta} is dimensionless and plays the role of a
non-equilibrium scaling exponent; its value, especially in glasses, is both
practically useful and theoretically significant. The mathematical complexity
of SER is such that rigorous derivations of this peculiar function were not
achieved until the 1970's. The focus of much of the 1970's pioneering work was
spatial relaxation of electronic charge, but SER is a universal phenomenon, and
today atomic and molecular relaxation of glasses and deeply supercooled liquids
provide the most reliable data. As the data base grew, the need for a
quantitative theory increased; this need was finally met by the
diffusion-to-traps topological model, which yields a remarkably simple
expression for the shape parameter {\beta}, given by d*/(d* + 2). At first
sight this expression appears to be identical to d/(d + 2), where d is the
actual spatial dimensionality, as originally derived. The original model,
however, failed to explain much of the data base. Here the theme of earlier
reviews, based on the observation that in the presence of short-range forces
only d* = d = 3 is the actual spatial dimensionality, while for mixed short-
and long-range forces, d* = fd = d/2, is applied to four new spectacular
examples, where it turns out that SER is useful not only for purposes of
quality control, but also for defining what is meant by a glass in novel
contexts. (Please see full abstract in main text
Spectroscopic and Photometric Observations of Unidentified Ultraviolate Variable Objects in GUVV-2 Catalog
An NUV-optical diagram made for sources from the secend Galaxy Evolution
Explorer (GALEX) Ultraviolet Variability (GUVV-2) Catalog provide us a method
to tentatively classify the unknown GUVV2 sources by their NUV-optical
magnitudes. On the purpose of testing the correctness and generality of the
method, we carry out a program on the spectroscopic observations of the
unidentified GUVV2 sources. The spectroscopic identification for these 37
sources are 19 type -A to -F stars, 10 type -G to -K stars and 7 M dwarf stars
together with an AGN. We also present the light curves in R-band for two RR
Lyrae star candidates selected from the NUV-optical diagram, both of which
perform cyclic variations. Combining there light curves and colors, we classify
them as RR Lyrae stars. To confirm the results, we shows a color-color diagram
for the 37 newly spectroscopically identified objects compared with the
previously identified ones, which manifests good consistence with our former
results, indicating that the ultroviolet variable sources can be initially
classified by their NUV/optical color-color diagram.Comment: 10 pages, 7 figure
A few electrons per ion scenario for the B=0 metal-insulator transition in two dimensions
We argue on the basis of experimental numbers that the B=0 metal-insulator
transition in two dimensions, observed in Si-MOSFETs and in other
two-dimensional systems, is likely to be due to a few strongly interacting
electrons, which also interact strongly with the random positively ionized
impurities. At the insulating side the electrons are all bound in pairs to the
ions. On the metallic side free electrons exist which are scattered by ions
dressed with electron-pairs and therefore alter the bare scattering potential
of the ions. The physics at the metallic side of the transition is argued to be
controlled by the classical to quantum transport cross-over leading to the
observed non-monotonous dependence of the resistivity on temperature. This few
electrons per ion scenario appears to be an experimentally realistic and
testable scenario, which can also serve as a starting point for further
theoretical analysis of the two-dimensional metal-insulator transition.Comment: 8 pages, revised version, minor change
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