3,930 research outputs found
Effect of grain shape on the agglomeration of polycrystalline thin films
Grain-boundary grooving is a general phenomenon occurring in all
polycrystalline materials at the intersection between the grain-boundary and
the interface or free surface. It has been studied theoretically for some time.
Grain-boundary grooving in the context of faceted interfaces in particular has
attracted some attention. However, these works did not consider the case of
thin films and the consequences on agglomeration of the shape of the interface.
In this Letter, we compare the agglomeration of thin films with rounded and
faceted interfaces. The shape of the grains can dramatically affect the
agglomeration of polycrystalline thin films by grain-boundary grooving.
Anisotropy plays a central role in the stability against agglomeration of
faceted films. Even a small difference between the interface energies of the
facets can destabilize faceted grains or, on the contrary, it can make them
perfectly stable at any thickness. keywords: grain-boundary grooving, dihedral
angle, faceting, energy, silicide, theory, model.Comment: 3 pages, 3 figure
On the properties of surface reconstructed silicon nanowires
We study by means of density-functional calculations the role of lateral
surface reconstructions in determining the electrical properties of
silicon nanowires. The different lateral reconstructions are explored by
relaxing all the nanowires with crystalline bulk silicon structure and all
possible ideal facets that correspond to an average diameter of 1.5 nm. We show
that the reconstruction induces the formation of ubiquitous surface states that
turn the wires into semi-metallic or metallic
Xanthomonas campestris pv. campestris race 1 is the main causal agent of black rot of Brassicas in Southern Mozambique
Severe outbreaks of bacterial black rot caused by Xanthomonas campestris pv. campestris (Xcc) were observed in Brassica production fields of Southern Mozambique. The causal agent of the disease in the Mahotas and Chòkwé districts was identified and characterised. In total, 83 Xanthomonas-like strains were isolated from seed samples and leaves of cabbage and tronchuda cole with typical symptoms of the disease. Forty-six out of the 83 strains were found to be putative Xcc in at least one of the tests used: Classical biochemical assays, enzyme-linked immunosorbent assay (ELISA) with monoclonal antibodies, Biolog identification system, polymerase chain reaction (PCR) with specific primers and pathogenicity tests. The ELISA tests were positive for 43 strains. Biolog identified 43 strains as Xanthomonas, but only 32 as Xcc. PCR tests with primers targeting a fragment of the hrpF gene were positive for all 46 strains tested. Three strains were not pathogenic or weakly pathogenic and all other strains caused typical black rot symptoms in brassicas. Race type differentiation tests revealed the Xcc strains from Mozambique as members of race 1. The prevalence of this pathogenic race of the Xcc pathogen in Mozambique should be considered when black rot resistant cultivars are evaluated or introduced into the production regions of this country
Sensitivity of Antarctic Urospora penicilliformis (Ulotrichales, Chlorophyta) to ultraviolet radiation is life stage dependent
The sensitivity of different life stages of the eulittoral green alga Urospora penicilliformis (Roth) Aresch. to ultraviolet radiation (UVR) was examinedin the laboratory. Gametophytic filaments and propagules (zoospores and gametes) released from filaments were separately exposed to different fluence of radiation treatments consisting of PAR (P = 400700 nm), PAR + ultraviolet A (UVA) (PA, UVA = 320400 nm), and PAR + UVA + ultraviolet B (UVB) (PAB, UVB = 280320 nm). Photophysiological indices (ETRmax, Ek, and a) derived from rapid light curves were measured in controls, while photosynthetic efficiency and amount of DNA lesions in terms of cyclobutane pyrimidine dimers (CPDs) were measured after exposure to radiation treatments and after recovery in low PAR; pigments of propagules were quantified after exposure treatment only. The photosynthetic conversion efficiency (a) and photosynthetic capacity (rETRmax) were higher in gametophytes compared with the propagules. The propagules were slightly more sensitive to UVB-induced DNA damage; however, both life stages of the eulittoral inhabiting turf alga were not severely affected by the negative impacts of UVR. Exposure to a maximum of 8 h UVR caused mild effects on the photochemical efficiency of PSII and induced minimal DNA lesions in both the gametophytes and propagules. Pigment concentrations were not significantly different between PAR-exposed and PAR + UVRexposed propagules. Our data showed that U. penicilliformis from the Antarctic is ratherinsensitive to the applied UVR. This amphi-equatorial species possesses different protective mechanisms that can cope with high UVR in coldtemperatewaters of both hemispheres and in polar regions under conditions of increasing UVR as a consequence of further reduction of stratosphericozone
A novel method for evaluating the critical nucleus and the surface tension in systems with first order phase transition
We introduce a novel method for calculating the size of the critical nucleus
and the value of the surface tension in systems with first order phase
transition. The method is based on classical nucleation theory, and it consists
in studying the thermodynamics of a sphere of given radius embedded in a frozen
metastable surrounding. The frozen configuration creates a pinning field on the
surface of the free sphere. The pinning field forces the sphere to stay in the
metastable phase as long as its size is smaller than the critical nucleus. We
test our method in two first-order systems, both on a two-dimensional lattice:
a system where the parameter tuning the transition is the magnetic field, and a
second system where the tuning parameter is the temperature. In both cases the
results are satisfying. Unlike previous techniques, our method does not require
an infinite volume limit to compute the surface tension, and it therefore gives
reliable estimates even by using relatively small systems. However, our method
cannot be used at, or close to, the critical point, i.e. at coexistence, where
the critical nucleus becomes infinitely large.Comment: 12 pages, 15 figure
Texture and shape of two-dimensional domains of nematic liquid crystal
We present a generalized approach to compute the shape and internal structure
of two-dimensional nematic domains. By using conformal mappings, we are able to
compute the director field for a given domain shape that we choose from a rich
class, which includes drops with large and small aspect ratios, and sharp
domain tips as well as smooth ones. Results are assembled in a phase diagram
that for given domain size, surface tension, anchoring strength, and elastic
constant shows the transitions from a homogeneous to a bipolar director field,
from circular to elongated droplets, and from sharp to smooth domain tips. We
find a previously unaccounted regime, where the drop is nearly circular, the
director field bipolar and the tip rounded. We also find that bicircular
director fields, with foci that lie outside the domain, provide a remarkably
accurate description of the optimal director field for a large range of values
of the various shape parameters.Comment: 12 pages, 10 figure
Geometric variational problems of statistical mechanics and of combinatorics
We present the geometric solutions of the various extremal problems of
statistical mechanics and combinatorics. Together with the Wulff construction,
which predicts the shape of the crystals, we discuss the construction which
exhibits the shape of a typical Young diagram and of a typical skyscraper.Comment: 10 page
Chromosome contribution to andean polyploid species of Senecio (Asteraceae), from Argentina
Fil: López, Mariana G.. Lab. de Citogenética y Evol.; Depto. de Ciencias Biológicas; Universidad de Buenos AiresFil: Wulff, Arturo F.. Lab. de Citogenética y Evol.; Depto. de Ciencias Biológicas; Universidad de Buenos AiresFil: Xifreda, Cecilia Carmen. Laboratorio de Etnobotánica y Botánica Aplicada (LEBA); Facultad de Ciencias Naturales y Museo; Universidad Nacional de La Plat
Theoretical Characterization of the Interface in a Nonequilibrium Lattice System
The influence of nonequilibrium bulk conditions on the properties of the
interfaces exhibited by a kinetic Ising--like model system with nonequilibrium
steady states is studied. The system is maintained out of equilibrium by
perturbing the familiar spin--flip dynamics at temperature T with
completely--random flips; one may interpret these as ideally simulating some
(dynamic) impurities. We find evidence that, in the present case, the
nonequilibrium mechanism adds to the basic thermal one resulting on a
renormalization of microscopic parameters such as the probability of
interfacial broken bonds. On this assumption, we develop theory for the
nonequilibrium "surface tension", which happens to show a non--monotonous
behavior with a maximum at some finite T. It ensues, in full agreement with
Monte Carlo simulations, that interface fluctuations differ qualitatively from
the equilibrium case, e.g., the interface remains rough at zero--T. We discuss
on some consequences of these facts for nucleation theory, and make some
explicit predictions concerning the nonequilibrium droplet structure.Comment: 10 pages, 7 figures, submitted to Phys. Re
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