6,753 research outputs found
Resonant x-ray scattering study on multiferroic BiMnO3
Resonant x-ray scattering is performed near the Mn K-absorption edge for an
epitaxial thin film of BiMnO3. The azimuthal angle dependence of the resonant
(003) peak (in monoclinic indices) is measured with different photon
polarizations; for the channel a 3-fold symmetric oscillation
is observed in the intensity variation, while the scattering
intensity remains constant. These features are accounted for in terms of the
peculiar ordering of the manganese 3d orbitals in BiMnO3. It is demonstrated
that the resonant peak persists up to 770 K with an anomaly around 440 K; these
high and low temperatures coincide with the structural transition temperatures,
seen in bulk, with and without a symmetry change, respectively. A possible
relationship of the orbital order with the ferroelectricity of the system is
discussed.Comment: 14 pages, 4 figure
Thermodynamic potential with correct asymptotics for PNJL model
An attempt is made to resolve certain incongruities within the Nambu -
Jona-Lasinio (NJL) and Polyakov loop extended NJL models (PNJL) which currently
are used to extract the thermodynamic characteristics of the quark-gluon
system. It is argued that the most attractive resolution of these incongruities
is the possibility to obtain the thermodynamic potential directly from the
corresponding extremum conditions (gap equations) by integrating them, an
integration constant being fixed in accordance with the Stefan-Boltzmann law.
The advantage of the approach is that the regulator is kept finite both in
divergent and finite valued integrals at finite temperature and chemical
potential. The Pauli-Villars regularization is used, although a standard 3D
sharp cutoff can be applied as well.Comment: 16 pages, 5 figures, extended version, title change
Kinetics of macroion coagulation induced by multivalent counterions
Due to the strong correlations between multivalent counterions condensed on a
macroion, the net macroion charge changes sign at some critical counterion
concentration. This effect is known as the charge inversion. Near this critical
concentration the macroion net charge is small. Therefore, short range
attractive forces between macroions dominate Coulomb repulsion and lead to
their coagulation. The kinetics of macroion coagulation in this range of
counterion concentrations is studied. We calculate the Coulomb barrier between
two approaching like charged macroions at a given counterion concentration. Two
different macroion shapes (spherical and rod-like) are considered. A new
"self-regulated" regime of coagulation is found. As the size of aggregates
increases, their charge and Coulomb barrier also grow and diminish the sticking
probability of aggregates. This leads to a slow, logarithmic increase of the
aggregate size with time.Comment: Some formulas correcte
Scaling and Universality in the Counterion-Condensation Transition at Charged Cylinders
We address the critical and universal aspects of counterion-condensation
transition at a single charged cylinder in both two and three spatial
dimensions using numerical and analytical methods. By introducing a novel
Monte-Carlo sampling method in logarithmic radial scale, we are able to
numerically simulate the critical limit of infinite system size (corresponding
to infinite-dilution limit) within tractable equilibration times. The critical
exponents are determined for the inverse moments of the counterionic density
profile (which play the role of the order parameters and represent the inverse
localization length of counterions) both within mean-field theory and within
Monte-Carlo simulations. In three dimensions (3D), correlation effects
(neglected within mean-field theory) lead to an excessive accumulation of
counterions near the charged cylinder below the critical temperature
(condensation phase), while surprisingly, the critical region exhibits
universal critical exponents in accord with the mean-field theory. In two
dimensions (2D), we demonstrate, using both numerical and analytical
approaches, that the mean-field theory becomes exact at all temperatures
(Manning parameters), when number of counterions tends to infinity. For finite
particle number, however, the 2D problem displays a series of peculiar singular
points (with diverging heat capacity), which reflect successive de-localization
events of individual counterions from the central cylinder. In both 2D and 3D,
the heat capacity shows a universal jump at the critical point, and the energy
develops a pronounced peak. The asymptotic behavior of the energy peak location
is used to locate the critical temperature, which is also found to be universal
and in accordance with the mean-field prediction.Comment: 31 pages, 16 figure
Variáveis e modelos para estimativa da produtividade do cafeeiro a partir de índices de vegetação derivados de imagens Landsat.
Abstract. Coffee fields present a specific pattern of productivity resulting in high and low production in alternated years. Branches grown the first phenological year will produce coffee beans the second phenological year. In high-production years a plant works mostly to grain-filling to the detriment of new branches which will be responsible for production the following year. In low-production years the plant works rather to grow new branches which will produce beans the subsequent year. This feature can be related to the foliar biomass, which can be estimated through remote sensing derived vegetation indices. Several studies report this feature must be incorporated in modeling coffee yield coupled with agrometeorogical models. In this paper we derived Landsat vegetation indices related to coffee plots in order to obtain relationships to yield of the same coffee plots. Biophisical variables and yield data were colected in interviews with farmers from four locations in the whole largest Brazilian coffee-exporting province. Vegetation indices and biophysical variables were selected through stepwise regression in order to obtain the best regression models to estimate coffee yield. Outcomes showed that general models and specific models obtained for Mundo Novo variety presented Pearson's correlation coeficients (r) from 0,64 to 0,71 while models for Catuaí variety showed better results (r = 0,85). Although coffee yield cannot be estimated exclusively from these models, they can be usefull coupled with agrometeorogical models for estimating coffee yield
Elastic and magnetic effects on the infrared phonon spectra of MnF2
We measured the temperature dependent infrared reflectivity spectra of MnF2
between 4 K and room temperature. We show that the phonon spectrum undergoes a
strong renormalization at TN. The ab-initio calculation we performed on this
compound accurately predict the magnitude and the direction of the phonon
parameters changes across the antiferromagnetic transition, showing that they
are mainly induced by the magnetic order. In this material, we found that the
dielectric constant is mostly from phonon origin. The large change in the
lattice parameters with temperature seen by X-ray diffraction as well as the
A2u phonon softening below TN indicate that magnetic order induced distortions
in MnF2 are compatible with the ferroelectric instabilities observed in TiO2,
FeF2 and other rutile-type fluorides. This study also shows the anomalous
temperature evolution of the lower energy Eu mode in the paramagnetic phase,
which can be compared to that of the B1g one seen by Raman spectroscopy in many
isostructural materials. This was interpreted as being a precursor of a phase
transition from rutile to CaCl2 structure which was observed under pressure in
ZnF2.Comment: 8 pages, 8 figures, updated version accepted in PR
Counterion Condensation and Fluctuation-Induced Attraction
We consider an overall neutral system consisting of two similarly charged
plates and their oppositely charged counterions and analyze the electrostatic
interaction between the two surfaces beyond the mean-field Poisson-Boltzmann
approximation. Our physical picture is based on the fluctuation-driven
counterion condensation model, in which a fraction of the counterions is
allowed to ``condense'' onto the charged plates. In addition, an expression for
the pressure is derived, which includes fluctuation contributions of the whole
system. We find that for sufficiently high surface charges, the distance at
which the attraction, arising from charge fluctuations, starts to dominate can
be large compared to the Gouy-Chapmann length. We also demonstrate that
depending on the valency, the system may exhibit a novel first-order binding
transition at short distances.Comment: 15 pages, 8 figures, to appear in PR
Conformational Instability of Rodlike Polyelectrolytes due to Counterion Fluctuations
The effective elasticity of highly charged stiff polyelectrolytes is studied
in the presence of counterions, with and without added salt. The rigid polymer
conformations may become unstable due to an effective attraction induced by
counterion density fluctuations. Instabilities at the longest, or intermediate
length scales may signal collapse to globule, or necklace states, respectively.
In the presence of added-salt, a generalized electrostatic persistence length
is obtained, which has a nontrivial dependence on the Debye screening length.
It is also found that the onset of conformational instability is a re-entrant
phenomenon as a function of polyelectrolyte length for the unscreened case, and
the Debye length or salt concentration for the screened case. This may be
relevant in understanding the experimentally observed re-entrant condensation
of DNA.Comment: 8 pages, 4 figure
Charge Fluctuations and Counterion Condensation
We predict a condensation phenomenon in an overall neutral system, consisting
of a single charged plate and its oppositely charged counterions. Based on the
``two-fluid'' model, in which the counterions are divided into a ``free'' and a
``condensed'' fraction, we argue that for high surface charge, fluctuations can
lead to a phase transition in which a large fraction of counterions is
condensed. Furthermore, we show that depending on the valence, the condensation
is either a first-order or a smooth transition.Comment: 16 pages, 1 figure, accepted to be published in PR
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