817 research outputs found
Dense packing crystal structures of physical tetrahedra
We present a method for discovering dense packings of general convex hard
particles and apply it to study the dense packing behavior of a one-parameter
family of particles with tetrahedral symmetry representing a deformation of the
ideal mathematical tetrahedron into a less ideal, physical, tetrahedron and all
the way to the sphere. Thus, we also connect the two well studied problems of
sphere packing and tetrahedron packing on a single axis. Our numerical results
uncover a rich optimal-packing behavior, compared to that of other continuous
families of particles previously studied. We present four structures as
candidates for the optimal packing at different values of the parameter,
providing an atlas of crystal structures which might be observed in systems of
nano-particles with tetrahedral symmetry
Nanowire metamaterials with extreme optical anisotropy
We study perspectives of nanowire metamaterials for negative-refraction
waveguides, high-performance polarizers, and polarization-sensitive biosensors.
We demonstrate that the behavior of these composites is strongly influenced by
the concentration, distribution, and geometry of the nanowires, derive an
analytical description of electromagnetism in anisotropic nanowire-based
metamaterials, and explore the limitations of our approach via
three-dimensional numerical simulations. Finally, we illustrate the developed
approach on the examples of nanowire-based high energy-density waveguides and
non-magnetic negative index imaging systems with far-field resolution of
one-sixth of vacuum wavelength.Comment: Updated version; accepted to Appl.Phys.Let
Recursion and Path-Integral Approaches to the Analytic Study of the Electronic Properties of
The recursion and path-integral methods are applied to analytically study the
electronic structure of a neutral molecule. We employ a tight-binding
Hamiltonian which considers both the and valence electrons of carbon.
From the recursion method, we obtain closed-form {\it analytic} expressions for
the and eigenvalues and eigenfunctions, including the highest
occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) states, and the Green's functions. We also present the local densities
of states around several ring clusters, which can be probed experimentally by
using, for instance, a scanning tunneling microscope. {}From a path-integral
method, identical results for the energy spectrum are also derived. In
addition, the local density of states on one carbon atom is obtained; from this
we can derive the degree of degeneracy of the energy levels.Comment: 19 pages, RevTex, 6 figures upon reques
International Federation of Clinical Chemistry (IFCC): Scientific Division, Committee on Enzymes. IFCC methods for the measurement of catalytic concentration of enzymes. Part 7. IFCC method for creatine kinase (ATP: creatine (N-phosphotransferase, EC 2.7.3.2). IFCC Recommendation
Rules for Computing Symmetry, Density and Stoichiometry in a Quasi-Unit-Cell Model of Quasicrystals
The quasi-unit cell picture describes the atomic structure of quasicrystals
in terms of a single, repeating cluster which overlaps neighbors according to
specific overlap rules. In this paper, we discuss the precise relationship
between a general atomic decoration in the quasi-unit cell picture atomic
decorations in the Penrose tiling and in related tiling pictures. Using these
relations, we obtain a simple, practical method for determining the density,
stoichiometry and symmetry of a quasicrystal based on the atomic decoration of
the quasi-unit cell taking proper account of the sharing of atoms between
clusters.Comment: 14 pages, 8 figure
Spin tunneling in the Kagom\'e antiferromagnet
The collective tunneling of a small cluster of spins between two degenerate
ground state configurations of the Kagom\'{e}-lattice quantum Heisenberg
antiferromagnet is \mbox{studied}. The cluster consists of the six spins on a
hexagon of the lattice. The resulting tunnel splitting energy is
calculated in detail, including the prefactor to the exponential \exp(- \SSo /
\hbar). This is done by setting up a coherent spin state path integral in
imaginary time and evaluating it by the method of steepest descent. The hexagon
tunneling problem is mapped onto a much simpler tunneling problem, involving
only one collective degree of freedom, which can be treated by known methods.
It is found that for half-odd-integer spins, the tunneling amplitude and the
tunnel splitting energy are exactly zero, because of destructive interference
between symmetry-related -instanton and -instanton tunneling paths.
This destructive interference is shown to occur also for certain larger loops
of spins on the Kagom\'{e} lattice. For small, integer spins, our results
suggest that tunneling strongly competes with \mbox{in-plane}
order-from-disorder selection effects; it constitutes a disordering mechanism
that might drive the system into a partially disordered ground state, related
to a spin nematic.Comment: 38 pages (RevTex), 8 figures upon request PRB921
Numerical Estimation of the Asymptotic Behaviour of Solid Partitions of an Integer
The number of solid partitions of a positive integer is an unsolved problem
in combinatorial number theory. In this paper, solid partitions are studied
numerically by the method of exact enumeration for integers up to 50 and by
Monte Carlo simulations using Wang-Landau sampling method for integers up to
8000. It is shown that, for large n, ln[p(n)]/n^(3/4) = 1.79 \pm 0.01, where
p(n) is the number of solid partitions of the integer n. This result strongly
suggests that the MacMahon conjecture for solid partitions, though not exact,
could still give the correct leading asymptotic behaviour.Comment: 6 pages, 4 figures, revtex
Feasibility of free space quantum key distribution with coherent polarization states
We demonstrate for the first time the feasibility of free space quantum key
distribution with continuous variables under real atmospheric conditions. More
specifically, we transmit coherent polarization states over a 100m free space
channel on the roof of our institute's building. In our scheme, signal and
local oscillator are combined in a single spatial mode which auto-compensates
atmospheric fluctuations and results in an excellent interference. Furthermore,
the local oscillator acts as spatial and spectral filter thus allowing
unrestrained daylight operation.Comment: 12 pages, 8 figures, extensions in sections 2, 3.1, 3.2 and 4. This
is an author-created, un-copyedited version of an article accepted for
publication in New Journal of Physics (Special Issue on Quantum Cryptography:
Theory and Practice). IOP Publishing Ltd is not responsible for any errors or
omissions in this version of the manuscript or any version derived from i
RNA:protein ratio of the unicellular organism as a characteristic of phosphorous and nitrogen stoichiometry and of the cellular requirement of ribosomes for protein synthesis
Background
Mean phosphorous:nitrogen (P:N) ratios and relationships of P:N ratios with the growth rate of organisms indicate a surprising similarity among and within microbial species, plants, and insect herbivores. To reveal the cellular mechanisms underling this similarity, the macromolecular composition of seven microorganisms and the effect of specific growth rate (SGR) on RNA:protein ratio, the number of ribosomes, and peptide elongation rate (PER) were analyzed under different conditions of exponential growth.
Results
It was found that P:N ratios calculated from RNA and protein contents in these particular organisms were in the same range as the mean ratios reported for diverse organisms and had similar positive relationships with growth rate, consistent with the growth-rate hypothesis. The efficiency of protein synthesis in microorganisms is estimated as the number of active ribosomes required for the incorporation of one amino acid into the synthesized protein. This parameter is calculated as the SGR:PER ratio. Experimental and theoretical evidence indicated that the requirement of ribosomes for protein synthesis is proportional to the RNA:protein ratio. The constant of proportionality had the same values for all organisms, and was derived mechanistically from the characteristics of the protein-synthesis machinery of the cell (the number of nucleotides per ribosome, the average masses of nucleotides and amino acids, the fraction of ribosomal RNA in the total RNA, and the fraction of active ribosomes). Impairment of the growth conditions decreased the RNA:protein ratio and increased the overall efficiency of protein synthesis in the microorganisms.
Conclusion
Our results suggest that the decrease in RNA:protein and estimated P:N ratios with decrease in the growth rate of the microorganism is a consequence of an increased overall efficiency of protein synthesis in the cell resulting from activation of the general stress response and increased transcription of cellular maintenance genes at the expense of growth related genes. The strong link between P:N stoichiometry, RNA:protein ratio, ribosomal requirement for protein synthesis, and growth rate of microorganisms indicated by the study could be used to characterize the N and P economy of complex ecosystems such as soils and the oceans
The local atomic quasicrystal structure of the icosahedral Mg25Y11Zn64 alloy
A local and medium range atomic structure model for the face centred
icosahedral (fci) Mg25Y11Zn64 alloy has been established in a sphere of r = 27
A. The model was refined by least squares techniques using the atomic pair
distribution (PDF) function obtained from synchrotron powder diffraction. Three
hierarchies of the atomic arrangement can be found: (i) five types of local
coordination polyhedra for the single atoms, four of which are of Frank-Kasper
type. In turn, they (ii) form a three-shell (Bergman) cluster containing 104
atoms, which is condensed sharing its outer shell with its neighbouring
clusters and (iii) a cluster connecting scheme corresponding to a
three-dimensional tiling leaving space for few glue atoms. Inside adjacent
clusters, Y8-cubes are tilted with respect to each other and thus allow for
overall icosahedral symmetry. It is shown that the title compound is
essentially isomorphic to its holmium analogue. Therefore fci-Mg-Y-Zn can be
seen as the representative structure type for the other rare earth analogues
fci-Mg-Zn-RE (RE = Dy, Er, Ho, Tb) reported in the literature.Comment: 12 pages, 8 figures, 2 table
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