10,981 research outputs found
Not Just a TheoryâThe Utility of Mathematical Models in Evolutionary Biology
Models have made numerous contributions to evolutionary biology, but misunderstandings persist regarding their purpose. By formally testing the logic of verbal hypotheses, proof-of-concept models clarify thinking, uncover hidden assumptions, and spur new directions of study. thumbnail image credit: modified from the Biodiversity Heritage Librar
Spatial and frequency domain effects of defects in 1D photonic crystal
The aim of this paper is to present the analysis of influence of defects in
1D photonic crystal (PC) on the density of states and simultaneously
spontaneous emission, in both spatial and frequency domains. In our
investigations we use an analytic model of 1D PC with defects. Our analysis
reveals how presence of a defect causes a defect mode to appear. We show that a
defect in 1D PC has local character, being negligible in regions of PC situated
far from the defected elementary cell. We also analyze the effect of multiple
defects, which lead to photonic band gap splitting.Comment: presented at International Workshop on Physics of Photonic Crystals
and Metamaterials, Brussels, Belgium, 12-13.06.200
Robustness of One-Dimensional Photonic Bandgaps Under Random Variations of Geometrical Parameters
The supercell method is used to study the variation of the photonic bandgaps
in one-dimensional photonic crystals under random perturbations to thicknesses
of the layers. The results of both plane wave and analytical band structure and
density of states calculations are presented along with the transmission
cofficient as the level of randomness and the supercell size is increased. It
is found that higher bandgaps disappear first as the randomness is gradually
increased. The lowest bandgap is found to persist up to a randomness level of
55 percent.Comment: Submitted to Physical Review B on April 8 200
Model elastomeric networks prepared by selectively crossâlinking polydimethylsiloxane chains having known amounts of reactive side chains
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70335/2/JCPSA6-70-11-5324-1.pd
Mass And Heat Transfer Relations In Evaporation Through Porous Membranes
This study concerns rates of evaporation and mass transfer of water vapor from a heated salt solution through a water repellent porous membrane to a cooled water condensate. This transfer is a result of temperature differences and corresponding vapor pressure differences across the membrane. Three groups of experiments were carried out which indicate that the major factor influencing the rates of transfer is diffusion through a stagnant gas in the membrane pores. However, an equation considering film heat transfer coefficients, membrane thermal conductivity, and an empirical correction based on temperature driving force appears to be necessary for representing all the data. The empirical correction appears to be related to internal condensation and possibly diffusion along surfaces. Copyright © 1969 American Institute of Chemical Engineer
Nucleocytoplasmic Shuttling Modulates Activity and Ubiquitination-Dependent Turnover of SUMO-Specific Protease 2
Small ubiquitin-related modifier (SUMO) proteins are conjugated to numerous polypeptides in cells, and attachment of SUMO plays important roles in regulating the activity, stability, and subcellular localization of modified proteins. SUMO modification of proteins is a dynamic and reversible process. A family of SUMO-specific proteases catalyzes the deconjugation of SUMO-modified proteins. Members of the Sentrin (also known as SUMO)-specific protease (SENP) family have been characterized with unique subcellular localizations. However, little is known about the functional significance of or the regulatory mechanism derived from the specific localizations of the SENPs. Here we identify a bipartite nuclear localization signal (NLS) and a CRM1-dependent nuclear export signal (NES) in the SUMO protease SENP2. Both the NLS and the NES are located in the nonhomologous domains of SENP2 and are not conserved among other members of the SENP family. Using a series of SENP2 mutants and a heterokaryon assay, we demonstrate that SENP2 shuttles between the nucleus and the cytoplasm and that the shuttling is blocked by mutations in the NES or by treating cells with leptomycin B. We show that SENP2 can be polyubiquitinated in vivo and degraded through proteolysis. Restricting SENP2 in the nucleus by mutations in the NES impairs its polyubiquitination, whereas a cytoplasm-localized SENP2 made by introducing mutations in the NLS can be efficiently polyubiquitinated, suggesting that SENP2 is ubiquitinated in the cytoplasm. Finally, treating cells with MG132 leads to accumulation of polyubiquitinated SENP2, indicating that SENP2 is degraded through the 26S proteolysis pathway. Thus, the function of SENP2 is regulated by both nucleocytoplasmic shuttling and polyubiquitin-mediated degradation
Local and macroscopic tunneling spectroscopy of Y(1-x)CaxBa2Cu3O(7-d) films: evidence for a doping dependent is or idxy component in the order parameter
Tunneling spectroscopy of epitaxial (110) Y1-xCaxBa2Cu3O7-d films reveals a
doping dependent transition from pure d(x2-y2) to d(x2-y2)+is or d(x2-y2)+idxy
order parameter. The subdominant (is or idxy) component manifests itself in a
splitting of the zero bias conductance peak and the appearance of subgap
structures. The splitting is seen in the overdoped samples, increases
systematically with doping, and is found to be an inherent property of the
overdoped films. It was observed in both local tunnel junctions, using scanning
tunneling microscopy (STM), and in macroscopic planar junctions, for films
prepared by either RF sputtering or laser ablation. The STM measurements
exhibit fairly uniform splitting size in [110] oriented areas on the order of
10 nm2 but vary from area to area, indicating some doping inhomogeneity. U and
V-shaped gaps were also observed, with good correspondence to the local
faceting, a manifestation of the dominant d-wave order parameter
An experimental study on (2) modular symmetry in the quantum Hall system with a small spin-splitting
Magnetic-field-induced phase transitions were studied with a two-dimensional
electron AlGaAs/GaAs system. The temperature-driven flow diagram shows the
features of the (2) modular symmetry, which includes distorted
flowlines and shiftted critical point. The deviation of the critical
conductivities is attributed to a small but resolved spin splitting, which
reduces the symmetry in Landau quantization. [B. P. Dolan, Phys. Rev. B 62,
10278.] Universal scaling is found under the reduction of the modular symmetry.
It is also shown that the Hall conductivity could still be governed by the
scaling law when the semicircle law and the scaling on the longitudinal
conductivity are invalid. *corresponding author:[email protected]: The revised manuscript has been published in J. Phys.: Condens.
Matte
Surface effects on nanowire transport: numerical investigation using the Boltzmann equation
A direct numerical solution of the steady-state Boltzmann equation in a
cylindrical geometry is reported. Finite-size effects are investigated in large
semiconducting nanowires using the relaxation-time approximation. A nanowire is
modelled as a combination of an interior with local transport parameters
identical to those in the bulk, and a finite surface region across whose width
the carrier density decays radially to zero. The roughness of the surface is
incorporated by using lower relaxation-times there than in the interior.
An argument supported by our numerical results challenges a commonly used
zero-width parametrization of the surface layer. In the non-degenerate limit,
appropriate for moderately doped semiconductors, a finite surface width model
does produce a positive longitudinal magneto-conductance, in agreement with
existing theory. However, the effect is seen to be quite small (a few per cent)
for realistic values of the wire parameters even at the highest practical
magnetic fields. Physical insights emerging from the results are discussed.Comment: 15 pages, 7 figure
Branching ratios of decays in perturbative QCD approach
We study the rare decays , which can occur only via
annihilation type diagrams in the standard model. We calculate all of the four
modes, , in the framework of perturbative QCD approach
and give the branching ratios of the order about .Comment: 18 pages, 1 figure, Revte
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