4,216 research outputs found
Crystal Growth in Fluid Flow: Nonlinear Response Effects
We investigate crystal-growth kinetics in the presence of strong shear flow
in the liquid, using molecular-dynamics simulations of a binary-alloy model.
Close to the equilibrium melting point, shear flow always suppresses the growth
of the crystal-liquid interface. For lower temperatures, we find that the
growth velocity of the crystal depends non-monotonically on the shear rate.
Slow enough flow enhances the crystal growth, due to an increased particle
mobility in the liquid. Stronger flow causes a growth regime that is nearly
temperature-independent, in striking contrast to what one expects from the
thermodynamic and equilibrium kinetic properties of the system, which both
depend strongly on temperature. We rationalize these effects of flow on crystal
growth as resulting from the nonlinear response of the fluid to strong shearing
forces.Comment: to appear in Phys. Rev. Material
Factors That Influence The Size Of The Repair Patch In The Dna Of Cultured Human Cells
The buoyant density shift method was employed in the study of the enzymology and its regulation of DNA repair in cultured human cells. Measurements of the size of the DNA repair patch after treatments with UV-light or methylating agents revealed that about 30 nucleotides are replaced at each damage site, and that the extent of replacement is independent of the DNA-damaging agents employed.;In the examination of the role of poly(ADP-ribosyl)ation in DNA repair, the size of the repair patch was not increased by the presence of 3-aminobenzamide (3-AB), an inhibitor of poly(ADP-ribose) polymerase, as had been predicted by other investigators.;To elucidate the role of polymerase (alpha) in excision repair, aphidicolin was used to inhibit this enzyme specifically during the repair of DNA. Although there was no difference in the extent of repair-incorporation of the label between cells that performed DNA repair in the absence or in the presence of aphidicolin following UV-irradiation, the presence of the inhibitor produced repair patches that were at least twice that of normal. However, when DMS was used as the DNA-damaging agent, aphidicolin caused a decrease in the extent of repair replication and an accompanying increase in repair patch size.;On the basis of the study done with aphidicolin, several conclusions were drawn. Firstly, DNA polymerase (alpha) is involved in the repair of DNA. Secondly, the degree of incorporation of label into DNA following exposure to DNA-damaging agents may not always be a reliable way to quantitate repair events in the cell. Finally, it is proposed that the size of the repair patch is regulated by the relative rates of excision and polymerization in the cell.;It is further proposed, based on the known characteristics of (alpha) and (beta) polymerases, that the size of the single-stranded gap produced by the exonuclease determines the polymerase which fills the gap. A small gap of about 10 nucleotides in length, produced when the dose of UV-light is low, is filled in primarily by polymerase (beta). When the dose of UV-light is high, and the level of activity of the (beta) enzyme is insufficient to fill in the gaps, a larger gap of 30 nucleotides or more is produced. This is filled in partially by polymerase (alpha), and completed by the (beta) enzyme
M\"ossbauer, nuclear inelastic scattering and density functional studies on the second metastable state of Na2[Fe(CN)5NO]2H2O
The structure of the light-induced metastable state SII of
Na2[Fe(CN)5NO]2H2O 14 was investigated by transmission M\"ossbauer
spectroscopy (TMS) in the temperature range 15 between 85 and 135 K, nuclear
inelastic scattering (NIS) at 98 K using synchrotron 16 radiation and density
functional theory (DFT) calculations. The DFT and TMS results 17 strongly
support the view that the NO group in SII takes a side-on molecular orientation
18 and, further, is dynamically displaced from one eclipsed, via a staggered,
to a second 19 eclipsed orientation. The population conditions for generating
SII are optimal for 20 measurements by TMS, yet they are modest for
accumulating NIS spectra. Optimization 21 of population conditions for NIS
measurements is discussed and new NIS experiments on 22 SII are proposed
Growth of (110) Diamond using pure Dicarbon
We use a density-functional based tight-binding method to study diamond
growth steps by depositing dicarbon species onto a hydrogen-free diamond (110)
surface. Subsequent C_2 molecules are deposited on an initially clean surface,
in the vicinity of a growing adsorbate cluster, and finally, near vacancies
just before completion of a full new monolayer. The preferred growth stages
arise from C_2n clusters in near ideal lattice positions forming zigzag chains
running along the [-110] direction parallel to the surface. The adsorption
energies are consistently exothermic by 8--10 eV per C_2, depending on the size
of the cluster. The deposition barriers for these processes are in the range of
0.0--0.6 eV. For deposition sites above C_2n clusters the adsorption energies
are smaller by 3 eV, but diffusion to more stable positions is feasible. We
also perform simulations of the diffusion of C_2 molecules on the surface in
the vicinity of existing adsorbate clusters using an augmented Lagrangian
penalty method. We find migration barriers in excess of 3 eV on the clean
surface, and 0.6--1.0 eV on top of graphene-like adsorbates. The barrier
heights and pathways indicate that the growth from gaseous dicarbons proceeds
either by direct adsorption onto clean sites or after migration on top of the
existing C_2n chains.Comment: 8 Pages, 7 figure
Teleportation-based realization of an optical quantum two-qubit entangling gate
In recent years, there has been heightened interest in quantum teleportation,
which allows for the transfer of unknown quantum states over arbitrary
distances. Quantum teleportation not only serves as an essential ingredient in
long-distance quantum communication, but also provides enabling technologies
for practical quantum computation. Of particular interest is the scheme
proposed by Gottesman and Chuang [Nature \textbf{402}, 390 (1999)], showing
that quantum gates can be implemented by teleporting qubits with the help of
some special entangled states. Therefore, the construction of a quantum
computer can be simply based on some multi-particle entangled states, Bell
state measurements and single-qubit operations. The feasibility of this scheme
relaxes experimental constraints on realizing universal quantum computation.
Using two different methods we demonstrate the smallest non-trivial module in
such a scheme---a teleportation-based quantum entangling gate for two different
photonic qubits. One uses a high-fidelity six-photon interferometer to realize
controlled-NOT gates and the other uses four-photon hyper-entanglement to
realize controlled-Phase gates. The results clearly demonstrate the working
principles and the entangling capability of the gates. Our experiment
represents an important step towards the realization of practical quantum
computers and could lead to many further applications in linear optics quantum
information processing.Comment: 10 pages, 6 figure
Fabrication of Highly Ordered Polymeric Nanodot and Nanowire Arrays Templated by Supramolecular Assembly Block Copolymer Nanoporous Thin Films
Realizing the vast technological potential of patternable block copolymers requires both the precise controlling of the orientation and long-range ordering, which is still a challenging topic so far. Recently, we have demonstrated that ordered nanoporous thin film can be fabricated from a simple supramolecular assembly approach. Here we will extend this approach and provide a general route to fabricate large areas of highly ordered polymeric nanodot and nanowire arrays. We revealed that under a mixture solvent annealing atmosphere, a near-defect-free nanoporous thin film over large areas can be achieved. Under the direction of interpolymer hydrogen bonding and capillary action of nanopores, this ordered porous nanotemplate can be properly filled with phenolic resin precursor, followed by curation and pyrolysis at middle temperature to remove the nanotemplate, a perfect ordered polymer nanodot arrays replication was obtained. The orientation of the supramolecular assembly thin films can be readily re-aligned parallel to the substrate upon exposure to chloroform vapor, so this facile nanotemplate replica method can be further extend to generate large areas of polymeric nanowire arrays. Thus, we achieved a successful sub-30 nm patterns nanotemplates transfer methodology for fabricating polymeric nanopattern arrays with highly ordered structure and tunable morphologies
An Over-Massive Black Hole in the Compact Lenticular Galaxy NGC1277
All massive galaxies likely have supermassive black holes at their centers,
and the masses of the black holes are known to correlate with properties of the
host galaxy bulge component. Several explanations have been proposed for the
existence of these locally-established empirical relationships; they include
the non-causal, statistical process of galaxy-galaxy merging, direct feedback
between the black hole and its host galaxy, or galaxy-galaxy merging and the
subsequent violent relaxation and dissipation. The empirical scaling relations
are thus important for distinguishing between various theoretical models of
galaxy evolution, and they further form the basis for all black hole mass
measurements at large distances. In particular, observations have shown that
the mass of the black hole is typically 0.1% of the stellar bulge mass of the
galaxy. The small galaxy NGC4486B currently has the largest published fraction
of its mass in a black hole at 11%. Here we report observations of the stellar
kinematics of NGC 1277, which is a compact, disky galaxy with a mass of 1.2 x
10^11 Msun. From the data, we determine that the mass of the central black hole
is 1.7 x 10^10 Msun, or 59% its bulge mass. Five other compact galaxies have
properties similar to NGC 1277 and therefore may also contain over-sized black
holes. It is not yet known if these galaxies represent a tail of a
distribution, or if disk-dominated galaxies fail to follow the normal black
hole mass scaling relations.Comment: 7 pages. 6 figures. Nature. Animation at
http://www.mpia.de/~bosch/blackholes.htm
Cell voltage versus electrode potential range in aqueous supercapacitors
Supercapacitors with aqueous electrolytes and nanostructured composite electrodes are attractive because of their high charging-discharging speed, long cycle life, low environmental impact and wide commercial affordability. However, the energy capacity of aqueous supercapacitors is limited by the electrochemical window of water. In this paper, a recently reported engineering strategy is further developed and demonstrated to correlate the maximum charging voltage of a supercapacitor with the capacitive potential ranges and the capacitance ratio of the two electrodes. Beyond the maximum charging voltage, a supercapacitor may still operate, but at the expense of a reduced cycle life. In addition, it is shown that the supercapacitor performance is strongly affected by the initial and zero charge potentials of the electrodes. Further, the differences are highlighted and elaborated between freshly prepared, aged under open circuit conditions, and cycled electrodes of composites of conducting polymers and carbon nanotubes. The first voltammetric charging-discharging cycle has an electrode conditioning effect to change the electrodes from their initial potentials to the potential of zero voltage, and reduce the irreversibility
Brownian bridges to submanifolds
We introduce and study Brownian bridges to submanifolds. Our method involves
proving a general formula for the integral over a submanifold of the minimal
heat kernel on a complete Riemannian manifold. We use the formula to derive
lower bounds, an asymptotic relation and derivative estimates. We also see a
connection to hypersurface local time. This work is motivated by the desire to
extend the analysis of path and loop spaces to measures on paths which
terminate on a submanifold
The Unified Model of Active Galactic Nuclei: I. Non-hidden Broad Line Region Seyfert 2 and Narrow Line Seyfert 1 Galaxies
The unified model of Seyfert galaxies suggests that there are hidden
broad-line regions (HBLRs) in Seyfert 2 galaxies (S2s). However, there is
increasing evidence for the appearance of a subclass of S2s lacking of HBLR
(non-HBLR S2s). An interesting issue arises as to relations of non-HBLR S2s
with other types of Seyfert galaxies and whether or not they can be included in
the unified model. We assemble two sub-samples consisting of 42 non-HBLR S2s
and 44 narrow-line Seyfert 1s (NLS1s) with redshift from published
literatures to explore this issue. We compare black hole masses in the galactic
centers, accretion rates, infrared color ratio () as a potential indicator of the dusty torus orientation, \oiii , radio and far infrared luminosities. We find that non-HBLR S2s and NLS1s
have: 1) similar distributions of the black hole masses (10^6-3\times
10^7\sunm) and the Eddington ratios (); 2)
significantly different distributions of
ratios; 3) similar distributions of bulge magnitudes and luminosities of [O
{\sc iii}], radio, far infrared emission. The similarities and differences can
be understood naturally if they are intrinsically same but non-HBLR S2s are
viewed at larger angles of observer's sight than NLS1s. We thus suggest that
non-HBLR S2s only have "narrower" broad line regions and they are the
counterparts of NLS1s viewed at high inclination angles. The absence of the
polarized emission line in non-HBLR S2s is caused by the less massive black
holes and high accretion rate similar to NLS1s. The implications of the
unification scheme of non-HBLR S2s and NLS1s are discussed.Comment: 13 page in emulateapj.sty, ApJ in pres
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