5,611 research outputs found
Structure, compressibility factor and dynamics of highly size-asymmetric binary hard-disk liquids
By using event-driven molecular dynamics simulation, we investigate effects
of varying the area fraction of the smaller component on structure,
compressibility factor and dynamics of the highly size-asymmetric binary
hard-disk liquids. We find that the static pair correlations of the large disks
are only weakly perturbed by adding small disks. The higher-order static
correlations of the large disks, by contrast, can be strongly affected. The
compressibility factor of the system first decreases and then increases upon
increasing the area fraction of the small disks and separating different
contributions to it allows to rationalize this non-monotonic phenomenon.
Furthermore, adding small disks can influence dynamics of the system in
quantitative and qualitative ways. For the large disks, the structural
relaxation time increases monotonically with increasing the area fraction of
the small disks at low and moderate area fractions of the large disks. In
particular, "reentrant" behavior appears at sufficiently high area fractions of
the large disks, strongly resembling the reentrant glass transition in
short-ranged attractive colloids and the inverted glass transition in binary
hard spheres with large size disparity. By tuning the area fraction of the
small disks, relaxation process for the small disks shows concave-to-convex
crossover and logarithmic decay behavior, as found in other binary mixtures
with large size disparity. Moreover, diffusion of both species is suppressed by
adding small disks. Long-time diffusion for the small disks shows
power-law-like behavior at sufficiently high area fractions of the small disks,
which implies precursors of a glass transition for the large disks and a
localization transition for the small disks. Therefore, our results demonstrate
the generic dynamic features in highly size-asymmetric binary mixtures.Comment: 9 pages, 12 figure
Photodissociation and the Morphology of HI in Galaxies
Young massive stars produce Far-UV photons which dissociate the molecular gas
on the surfaces of their parent molecular clouds. Of the many dissociation
products which result from this ``back-reaction'', atomic hydrogen \HI is one
of the easiest to observe through its radio 21-cm hyperfine line emission. In
this paper I first review the physics of this process and describe a simplified
model which has been developed to permit an approximate computation of the
column density of photodissociated \HI which appears on the surfaces of
molecular clouds. I then review several features of the \HI morphology of
galaxies on a variety of length scales and describe how photodissociation might
account for some of these observations. Finally, I discuss several consequences
which follow if this view of the origin of HI in galaxies continues to be
successful.Comment: 18 pages, 7 figures in 8 files, invited review paper for the
conference "Penetrating Bars Through Masks of Cosmic Dust: The Hubble Tuning
Fork Strikes a New Note", South Africa, June 2004. Proceedings to be
published by Kluwer, eds. D.L. Block, K.C. Freeman, I. Puerari, R. Groess, &
E.K. Bloc
Extreme Electron-Phonon Coupling in Boron-based Layered Superconductors
The phonon-mode decomposition of the electron-phonon coupling in the
MgB2-like system Li_{1-x}BC is explored using first principles calculations. It
is found that the high temperature superconductivity of such systems results
from extremely strong coupling to only ~2% of the phonon modes. Novel
characteristics of E_2g branches include (1) ``mode lambda'' values of 25 and
greater compared to a mean of for other modes, (2) a precipitous
Kohn anomaly, and (3) E_2g phonon linewidths within a factor of ~2 of the
frequency itself, indicating impending breakdown of linear electron-phonon
theory. This behavior in borne out by recent inelastic x-ray scattering studies
of MgB2 by Shukla et al.Comment: 4 two-column pages, 4 figures. Equations simplified. Figure 4
changed. Comparison with new data include
Holographic optical disc
The holographic disc is a high capacity, disk-based data storage device that can provide the performance for next generation mass data storage needs. With a projected capacity approaching 1 terabit on a single 12 cm platter, the holographic disc has the potential to become a highly efficient storage hardware for data warehousing applications. The high readout rate of holographic disc makes it especially suitable for generating multiple, high bandwidth data streams such as required for network server computers. Multimedia applications such as interactive video and HDTV can also potentially benefit from the high capacity and fast data access of holographic memory
Proposal of Quantum Simulation of Pairing Model on an NMR Quantum Computer
We give out a proposal of quantum simulation of pairing model on an NMR
quantum computer. In our proposal, we choose an appropriate initial state which
can be easily prepared in experiment. Making use of feature of NMR measure and
the technology of the second (discrete) Fourier transformation, our theoretical
scheme can obtain the spectrum of paring model in principle. We concretely
discuss the case in the concerned subspaces of pairing model and then, as an
example, give out a simple initial state to get the gap of two the lowest
energy levels in the given subspace. The quantum simulation to get more
differences of energy levels is able to be discussed similarly.Comment: 5 pages, no figures, introducing and conclusion paragraphes are
revise
Exact Numerical Solution of the BCS Pairing Problem
We propose a new simulation computational method to solve the reduced BCS
Hamiltonian based on spin analogy and submatrix diagonalization. Then we
further apply this method to solve superconducting energy gap and the results
are well consistent with those obtained by Bogoliubov transformation method.
The exponential problem of 2^{N}-dimension matrix is reduced to the polynomial
problem of N-dimension matrix. It is essential to validate this method on a
real quantumComment: 7 pages, 3 figure
A Response Regulator from a Soil Metagenome Enhances Resistance to the β-lactam Antibiotic Carbenicillin in \u3cem\u3eEscherichia Coli\u3c/em\u3e
Functional metagenomic analysis of soil metagenomes is a method for uncovering as-yet unidentified mechanisms for antibiotic resistance. Here we report an unconventional mode by which a response regulator derived from a soil metagenome confers resistance to the β-lactam antibiotic carbenicillin in Escherichia coli. A recombinant clone (βlr16) harboring a 5,169 bp DNA insert was selected from a metagenomic library previously constructed from a remote Alaskan soil. The βlr16 clone conferred specific resistance to carbenicillin, with limited increases in resistance to other tested antibiotics, including other β-lactams (penicillins and cephalosporins), rifampin, ciprofloxacin, erythromycin, chloramphenicol, nalidixic acid, fusidic acid, and gentamicin. Resistance was more pronounced at 24°C than at 37°C. Zone-of-inhibition assays suggested that the mechanism of carbenicillin resistance was not due to antibiotic inactivation. The DNA insert did not encode any genes known to confer antibiotic resistance, but did have two putative open reading frames (ORFs) that were annotated as a metallopeptidase and a two-component response regulator. Transposon mutagenesis and subcloning of the two ORFs followed by phenotypic assays showed that the response regulator gene was necessary and sufficient to confer the resistance phenotype. Quantitative reverse transcriptase PCR showed that the response regulator suppressed expression of the ompF porin gene, independently of the small RNA regulator micF, and enhanced expression of the acrD, mdtA, and mdtB efflux pump genes. This work demonstrates that antibiotic resistance can be achieved by the modulation of gene regulation by heterologous DNA. Functional analyses such as these can be important for making discoveries in antibiotic resistance gene biology and ecology
Hole-doped, High-Temperature Superconductors Li_{x}BC, Na_{x}BC and C_{x} : A Coherent-Potential-Based Prediction
Using density-functional-based methods, we show that the hole-doped
Li_{x}BC and Na_{x}BC in P6_{3}/mmc crystal structure and C_{x} in graphite
structure are capable of showing superconductivity, possibly with a T_{c} much
higher than that of
MgB_{2}. We use full-potential methods to obtain the optimized lattice
constants a and c, coherent-potential approximation to describe the effects of
disorder, Gaspari-Gyorffy formalism to obtain the electron-phonon coupling
constant , and Allen-Dynes equation to calculate T_{c} as a function
of hole concentration in these alloys.Comment: 4 pages, 5 figure
11β,17,21-Trihydroxy-6α-methyl-3,20-dioxopregna-1,4-dien-21-yl 3-carboxypropionate
The molecule of the title compound, C26H34O8, a prednisolone derivative, contains three six-membered rings (A, B and C) and one five-membered ring (D). Ring A is planar and rings B and C adopt chair conformations, while ring D adopts an envelope conformation with the C atom bonded to the methyl group at the flap. The crystal structure is stabilized by intermolecular O—H⋯O hydrogen bond
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