369 research outputs found
Light and Kno3 on Tridax procumbens seed germination at constant and alternating temperatures
Poucos são os estudos relacionados à germinação de espécies de plantas daninhas tropicais, incluindo-se a de Tridax procumbens, apesar de sua importância como infestante em áreas de lavoura. Este trabalho teve como objetivo avaliar o efeito de luz e KNO3 sobre a germinação de sementes de T. procumbens em temperatura constante e alternada. Quatro subamostras de 75 sementes foram submetidas ao teste de germinação utilizando-se uma combinação fatorial de luz (escuro; fotoperíodo de 12 horas diárias de luz) e umedecimento do substrato com solução de KNO3 (0% de KNO3; 0,2% de KNO3) para os ensaios na temperatura de 25 °C constante e alternada de 15 ºC/35 ºC, em delineamento experimental inteiramente casualizado. Efetuou-se a contagem diária da germinação pela emissão da raiz primária, bem como as análises de porcentagem de germinação acumulada, velocidade de germinação e curva de germinação acumulada. Em temperatura constante, a luz contribuiu para aumentar a porcentagem e a velocidade de germinação, enquanto em temperaturas alternadas houve aumento na porcentagem e velocidade de germinação com a aplicação de KNO3, independentemente da presença ou ausência de luz. As curvas de germinação acumulada se ajustaram ao modelo logístico tanto a 25 ºC quanto a 15 ºC/35 ºC, demonstrando assincronia na germinação de sementes no tempo.There are few studies available on germination of tropical weed species, Tridax procumbens included, despite its importance as a crop weed. This study was carried out at the Laboratory of Plant Biophysics of Embrapa Cerrados, Planaltina, DF, to evaluate the effect of light and KNO3 on T. procumbens seed germination at constant and alternating temperatures. Four sub-samples of 75 seeds were submitted to a factorial combination of light (darkness; 12 daily hours of light photoperiod) and substrate moistened with KNO3 solution (0% of KNO3; 0.2% of KNO3) for the assays at 25 °C constant and 15 ºC/35 ºC alternating temperatures, arranged in a completely randomized experimental design. Seed germination was daily counted based on emission of primary roots, with accumulated germination percentage, germination speed and accumulated germination curve being evaluated. Light contributed to increase germination percentage and speed at constant temperature, while at alternating temperatures, increase of germination percentage and speed occurred with KNO3 application, regardless of the presence or absence of light. The accumulated germination curves at 25 ºC and 15 ºC/35 ºC were adjusted to the logistic model, showing seed germination asynchrony along time
On the harmonic measure of stable processes
Using three hypergeometric identities, we evaluate the harmonic measure of a
finite interval and of its complementary for a strictly stable real L{\'e}vy
process. This gives a simple and unified proof of several results in the
literature, old and recent. We also provide a full description of the
corresponding Green functions. As a by-product, we compute the hitting
probabilities of points and describe the non-negative harmonic functions for
the stable process killed outside a finite interval
Non-existence of a dilaton gravity action for the exact string black hole
We prove that no local diffeomorphism invariant two-dimensional theory of the
metric and the dilaton without higher derivatives can describe the exact string
black hole solution found a decade ago by Dijkgraaf, Verlinde and Verlinde. One
of the key points in this proof is the concept of dilaton-shift invariance. We
present and solve (classically) all dilaton-shift invariant theories of
two-dimensional dilaton gravity. Two such models, resembling the exact string
black hole and generalizing the CGHS model, are discussed explicitly.Comment: 24 pages, 3 eps-figures, revised version (more references, clarified
some of the discussion
Friedmann Robertson-Walker model in generalised metric space-time with weak anisotropy
A generalized model of space-time is given, taking into consideration the
anisotropic structure of fields which are depended on the position and the
direction (velocity).In this framework a generalized FRW-metric the
Raychaudhouri and Friedmann equations are studied.A long range vector field of
cosmological origin is considered in relation to the physical geometry of
space-time in which Cartan connection has a fundamental role.The generalised
Friedmann equations are produced including anisotropic terms.The variation of
anisotropy is expressed in terms of the Cartan torsion tensor of the
Finslerian space-time.A possible estimation of the anisotropic parameter
can be achieved with the aid of the de-Sitter model of the empty flat universe
with weak anisotropy. Finally a physical generalisation for the model of
inflation is also studied.Comment: 21 pages- to appear in GR
Positive specific heat of the quantum corrected dilaton black hole
Path integral quantization of dilaton gravity in two dimensions is applied to
the CGHS model to the first nontrivial order in matter loops. Our approach is
background independent as geometry is integrated out exactly. The result is an
effective shift of the Killing norm: the apparent horizon becomes smaller. The
Hawking temperature which is constant to leading order receives a quantum
correction. As a consequence, the specific heat becomes positive and
proportional to the square of the black hole mass.Comment: 18 pages, JHEP style, 1 eps figure, v2: extended the discussion,
added new formulas for mass change, added three new references (in particular
[35]
Conserved Quasilocal Quantities and General Covariant Theories in Two Dimensions
General matterless--theories in 1+1 dimensions include dilaton gravity,
Yang--Mills theory as well as non--Einsteinian gravity with dynamical torsion
and higher power gravity, and even models of spherically symmetric d = 4
General Relativity. Their recent identification as special cases of
'Poisson--sigma--models' with simple general solution in an arbitrary gauge,
allows a comprehensive discussion of the relation between the known absolutely
conserved quantities in all those cases and Noether charges, resp. notions of
quasilocal 'energy--momentum'. In contrast to Noether like quantities,
quasilocal energy definitions require some sort of 'asymptotics' to allow an
interpretation as a (gauge--independent) observable. Dilaton gravitation,
although a little different in detail, shares this property with the other
cases. We also present a simple generalization of the absolute conservation law
for the case of interactions with matter of any type.Comment: 21 pages, LaTeX-fil
An action for the exact string black hole
A local action is constructed describing the exact string black hole
discovered by Dijkgraaf, Verlinde and Verlinde in 1992. It turns out to be a
special 2D Maxwell-dilaton gravity theory, linear in curvature and field
strength. Two constants of motion exist: mass M>1, determined by the level k,
and U(1)-charge Q>0, determined by the value of the dilaton at the origin. ADM
mass, Hawking temperature T_H \propto \sqrt{1-1/M} and Bekenstein-Hawking
entropy are derived and studied in detail. Winding/momentum mode duality
implies the existence of a similar action, arising from a branch ambiguity,
which describes the exact string naked singularity. In the strong coupling
limit the solution dual to AdS_2 is found to be the 5D Schwarzschild black
hole. Some applications to black hole thermodynamics and 2D string theory are
discussed and generalizations - supersymmetric extension, coupling to matter
and critical collapse, quantization - are pointed out.Comment: 41 pages, 2 eps figures, dedicated to Wolfgang Kummer on occasion of
his Emeritierung; v2: added ref; v3: extended discussion in sections 3.2, 3.3
and at the end of 5.3 by adding 2 pages of clarifying text; updated refs;
corrected typo
Interatomic potentials for atomistic simulations of the Ti-Al system
Semi-empirical interatomic potentials have been developed for Al, alpha-Ti,
and gamma-TiAl within the embedded atomic method (EAM) by fitting to a large
database of experimental as well as ab-initio data. The ab-initio calculations
were performed by the linear augmented plane wave (LAPW) method within the
density functional theory to obtain the equations of state for a number of
crystal structures of the Ti-Al system. Some of the calculated LAPW energies
were used for fitting the potentials while others for examining their quality.
The potentials correctly predict the equilibrium crystal structures of the
phases and accurately reproduce their basic lattice properties. The potentials
are applied to calculate the energies of point defects, surfaces, planar faults
in the equilibrium structures. Unlike earlier EAM potentials for the Ti-Al
system, the proposed potentials provide reasonable description of the lattice
thermal expansion, demonstrating their usefulness in the molecular dynamics or
Monte Carlo studies at high temperatures. The energy along the tetragonal
deformation path (Bain transformation) in gamma-TiAl calculated with the EAM
potential is in a fairly good agreement with LAPW calculations. Equilibrium
point defect concentrations in gamma-TiAl are studied using the EAM potential.
It is found that antisite defects strongly dominate over vacancies at all
compositions around stoichiometry, indicating that gamm-TiAl is an antisite
disorder compound in agreement with experimental data.Comment: 46 pages, 6 figures (Physical Review B, in press
Colossal dielectric constants in transition-metal oxides
Many transition-metal oxides show very large ("colossal") magnitudes of the
dielectric constant and thus have immense potential for applications in modern
microelectronics and for the development of new capacitance-based
energy-storage devices. In the present work, we thoroughly discuss the
mechanisms that can lead to colossal values of the dielectric constant,
especially emphasising effects generated by external and internal interfaces,
including electronic phase separation. In addition, we provide a detailed
overview and discussion of the dielectric properties of CaCu3Ti4O12 and related
systems, which is today's most investigated material with colossal dielectric
constant. Also a variety of further transition-metal oxides with large
dielectric constants are treated in detail, among them the system La2-xSrxNiO4
where electronic phase separation may play a role in the generation of a
colossal dielectric constant.Comment: 31 pages, 18 figures, submitted to Eur. Phys. J. for publication in
the Special Topics volume "Cooperative Phenomena in Solids: Metal-Insulator
Transitions and Ordering of Microscopic Degrees of Freedom
- …