A aproximação de campo médio de Bethe-Peierls

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Electrically detected and conventional magnetic resonance investigation of surface and bulk states in polyaniline thin films

Electrically detected magnetic resonance (EDMR) and electron paramagnetic resonance (EPR) were used to investigate emeraldine base polyaniline films. The magnetic susceptibility presented a Curie (localized spins)-Pauli (delocalized spins) transition at 240 K, when we also observed a transition in the dependence of the g factor with temperature (T). Peak-to-peak linewidth decreases with increasing temperature, reflecting that motional narrowing limits the hyperfine and dipolar broadening in this polymer. EDMR spectra could only be observed above 250 K in accordance to EPR results. Surface and bulk transport could be separated and their analysis reflected the effect of magnetic interaction with oxygen. (c) 2007 American Institute of Physics.

The energy landscape for solvent dynamics in electron transfer reactions: A minimalist model

Energy fluctuations of a solute molecule embedded in a polar solvent are investigated to depict the energy landscape for solvation dynamics. The system is modeled by a charged molecule surrounded by two layers of solvent dipolar molecules with simple rotational dynamics. Individual solvent molecules are treated as simple dipoles that can point toward or away from the central charge (Ising spins). Single-spin-flip Monte Carlo kinetics simulations are carried out in a two-dimensional lattice for different central charges, radii of outer shell, and temperatures. By analyzing the density of states as a function of energy and temperatures, we have determined the existence of multiple freezing transitions. Each of them can be associated with the freezing of a different layer of the solvent. (C) 2002 American Institute of Physics.

Multimethod study of the degree of humification of humic substances extracted from different tropical soil profiles in Brazil's Amazonian region

This work consisted of determining the degree of humification of humic substances (HS) extracted from six different Amazonian soils collected from flooded and unflooded regions at different depths (0-10, 10-20, 20-40, and 60 cm). The humic substances were extracted according to procedures recommended by the International Humic Substances Society and characterized using elemental analysis, electron paramagnetic resonance (EPR), and fluorescence spectroscopy. The findings on semiquinone-type free radical concentrations in HS showed variations of 0.10-7.55x10(18) spins g(-1) of carbon (g C)(-1), indicating considerable differences between the humification levels of HS extracted from Amazonian soils. The results showed an average of 1.71 +/- 0.04 x 10(18) spins (g C)(-1), which is congruent with other data reported in the literature on Tropical soils. It was found that, on average, HS extracted from flooded soil contained higher semiquinone-type free radical concentrations than HS extracted from unflooded soils, indicating the influence of humidity in the humification process of organic matter. The humification process varies according to the profile, and the 10-20- and 0-10-cm profiles generally showed more humified HS. The degree of humification of the HS studied here displayed a similar behavior when exposed to fluorescence (excitation at 465 nm) and EPR (R=0.85). However, the low correlation between the C/H, C/O, and C/N atomic ratios and the semiquinone-type free radical concentration/fluorescence intensities indicated that data obtained by these techniques with regard to the degree of humification of HS may lead to different conclusions. (c) 2004 Elsevier B.V. All rights reserved.

Characterization of humic acids from a Brazilian Oxisol under different tillage systems by EPR, C-13 NMR, FTIR and fluorescence spectroscopy

The importance of soil organic matter functions is well known, but structural information, chemical composition and changes induced by anthropogenic factors such as tillage practices are still being researched. In the present paper were characterized Brazilian humic acids (HAs) from an Oxisol under different treatments: conventional tillage/maize-bare fallow (CT1); conventional tillage/maize rotation with soybean-bare fallow (CT2)-, no-till/maize-bare fallow (NT1); no-till/maize rotation with soybean-bare fallow (NT2); no-till/maize-cajanus (NT3) and no cultivated soil under natural vegetation (NC). Soil HA samples were analyzed by electron paramagnetic resonance (EPR), solid-state C-13 nuclear magnetic resonance (C-13 NMR), Fourier transform intra-red (FTIR) and UV-Vis fluorescence spectroscopies and elemental analysis (CHNS). The FTIR spectra of the HAs were similar for all treatments. The level of semiquinone-type free radical determined from the EPR spectra was lower for treatments no-till/maize-cajanus (NT3) and noncultivated soil (1.74 X 10(17) and 1.02 x 10(17) spins g(-1) HA, respectively), compared with 2.3 X 10(17) spins g(-1) HA for other soils under cultivation. The percentage of aromatic carbons determined by C-13 NMR also decreases for noncultivated soil to 24%, being around 30% for samples of the other treatments. The solid-state C-13 NMR and EPR spectroscopies showed small differences in chemical composition of the HA from soils where incorporation of vegetal residues was higher, showing that organic matter (OM) formed in this cases is less aromatic. The fluorescence intensities were in agreement with the percentage of aromatic carbons, determined by NMR (r = 0.97 P < 0.01) and with semiquinone content, determined by EPR (r = 0.97 P < 0.01). No important effect due to tillage system was observed in these areas after 5 years of cultivation. Probably, the studied Oxisol has a high clay content that offers protection to the clay-Fe-OM complex against strong structural alterations. (C) 2003 Elsevier B.V. All rights reserved.

Absence of true long-range orbital order in a two-leg Kondo ladder

We investigate, through the density-matrix renormalization group and the Lanczos technique, the possibility of a two-leg Kondo ladder presenting an incommensurate orbital order. Our results indicate staggered short-range orbital order at half-filling. Away from half-filling our data are consistent with incommensurate quasi-long-range orbital order. We also observed that an interaction between the localized spins enhances the rung-rung current correlations.

PHENOMENOLOGICAL APPLICATION OF THE PROJECTION-OPERATOR METHOD AND ITS CONNECTION WITH CRITICAL-DYNAMICS

We consider the dynamics of a system of interacting spins described by the Ginzburg-Landau Hamiltonian. The method used is Zwanzig's version of the projection-operator method, in contrast to previous derivations in which we used Mori's version of this method. It is proved that both methods produce the same answer for the Green's function. We also make contact between the projection-operator method and critical dynamics.

Nuclear magnetic relaxation of F-19 in fluoride glasses

Temperature and frequency dependence of the F-19 nuclear spin relaxation of the fluoroindate glass, 40InF(3)-20ZnF(2)- 20SrF(2)-2GaF(3)-2NaF-16BaF(2) and the fluorozirconate glass, 50ZrF(4)-20BaF(2)-21LiF-5LaF(3)-4AlF(3); are reported. Measurements were undertaken on pure and Gd3+ doped samples, in the temperature range of 185-1000 K, covering the region below and above the glass transition temperature, T-g. The temperature and frequency dependence of the spin-lattice relaxation rate, T-1(-1), measured in the glassy state at temperature <300 K, is less than the observed dependence at higher temperatures. At temperatures >T-g, the fluorine mobility increases, leading to a more efficient spins lattice relaxation process. Activation energies, for F- motion, are 0.8 eV for the fluoroindate glass and 1 eV for the fluorozirconate glass. The addition of Gd3+ paramagnetic impurities;at 0.1-wt%, does not alter the temperature and frequency dependence of T-1(-1), but increases its magnitude more than one order of magnitude. At temperatures <400 K, the spin-spin relaxation time, T-2(-1), measured for all samples, is determined by the rigid-lattice nuclear dipole-dipole coupling, and it is temperature independent within the accuracy of the measurements. Results obtained for the pure glass, at temperatures >400 K, show that T-2(-1) decreases monotonically as the temperature increases. This decrease is explained as a consequence of the motional narrowing effect caused by the onset of the diffusive motion of the F- ions, with an activation energy around 0.8 eV. For the doped samples, the hyperfine interaction with the paramagnetic impurities is most effective in the relaxation of the nuclear spin, causing an increase in the T(2)(-1)s observed at temperatures >600 K. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Charge-orbital ordering and phase separation in the two-orbital model for manganites: Roles of Jahn-Teller phononic and Coulombic interactions

The main properties of realistic models for manganites are studied using analytic mean-field approximations and computational numerical methods, focusing on the two-orbital model with electrons interacting through Jahn-Teller (JT) phonons and/or Coulombic repulsions. Analyzing the model including both interactions by the combination of the mean-field approximation and the exact diagonalization method, it is argued that the spin-charge-orbital structure in the insulating phase of the purely JT-phononic model with a large Hund couphng J(H) is not qualitatively changed by the inclusion of the Coulomb interactions. As an important application of the present mean-held approximation, the CE-type antiferromagnetic state, the charge-stacked structure along the z axis, and (3x(2) - r(2))/(3y(2) - r(2))-type orbital ordering are successfully reproduced based on the JT-phononic model with large JH for the half-doped manganite, in agreement with recent Monte Carlo simulation results. Topological arguments and the relevance of the Heisenberg exchange among localized t(2g) spins explains why the inclusion of the nearest-neighbor Coulomb interaction does not destroy the charge stacking structure. It is also verified that the phase-separation tendency is observed both in purely JT-phononic (large JH) and purely Coulombic models in the vicinity of the hole undoped region, as long as realistic hopping matrices are used. This highlights the qualitative similarities of both approaches and the relevance of mixed-phase tendencies in the context of manganites. In addition, the rich and complex phase diagram of the two-orbital Coulombic model in one dimension is presented. Our results provide robust evidence that Coulombic and JT-phononic approaches to manganites are not qualitatively different ways to carry out theoretical calculations, but they share a variety of common features.

The Yang-Lee zeros of the 1D Blume-Capel model on connected and non-connected rings

We carry out a numerical and analytic analysis of the Yang-Lee zeros of the ID Blume-Capel model with periodic boundary conditions and its generalization on Feynman diagrams for which we include sums over all connected and nonconnected rings for a given number of spins. In both cases, for a specific range of the parameters, the zeros originally on the unit circle are shown to depart from it as we increase the temperature beyond some limit. The curve of zeros can bifurcate- and become two disjoint arcs as in the 2D case. We also show that in the thermodynamic limit the zeros of both Blume-Capel models on the static (connected ring) and on the dynamical (Feynman diagrams) lattice tend to overlap. In the special case of the 1D Ising model on Feynman diagrams we can prove for arbitrary number of spins that the Yang-Lee zeros must be on the unit circle. The proof is based on a property of the zeros of Legendre polynomials.

The ferromagnetic behaviour of conducting polymers revisited

The magnetic properties of doped pellets of poly(3-methylthiophene) showing room temperature ferromagnetic behaviour have been discussed in a previous article. The magnetic behaviour was attributed to a weak ferromagnetic phase, due to the superexchange interaction of polarons via the dopant anions. The Dzialoshinsky-Morya interaction among canted spins was proposed to explain the ferromagnetism. In this article the main conclusions of that work concerning the magnetic behaviour are revised. The basic assumption now is that the magnetic moments are spin 1/2 polarons that can interact antiferromagnetically and/or ferromagnetically. In the small crystalline regions of the polymer, which are identified with the polymer portion that remains ferromagnetic at room temperature, the interaction gives rise to S = 0 and 1 bipolarons and the S = 1 triplet state is lower in energy. In the disordered region, disorder will prevent the complete S = 1 and 0 coupling and bands of polarons ferromagnetically and antiferromagnetically coupled will appear. Using this approach, all the magnetization data can be qualitatively explained, as well as the electron spin resonance data.

Magnetic field dependent magnetization of a conducting plasticized poly(aniline) film

We report magnetic data of free standing films of poly( aniline) (PANI) protonated with a plasticizing di-ester of succinic acid. The data have been obtained using the electron spin resonance (ESR) technique at two different frequencies, X-band (9.4 GHz) and Q-band ( 34 GHz), on one hand, and by magnetization measurements in broad ranges of temperatures and magnetic fields on the other hand. All the data can be explained assuming a transition as a function of temperature from delocalized magnetic moments in the valence band to localized positive polarons in several antiferromagnetically correlated bands. By increasing the magnetic field, the magnetic properties are affected in several ways. An intra-band admixture of states occurs; it contributes to increase the spins' localization and finally promotes an antiferromagnetic-metamagnetic transition.

Electric and magnetic properties of polymer electrolyte/carbon black composites

Measurements of 1H Nuclear Magnetic Resonance (NMR) relaxation times, Electron Paramagnetic Resonance (EPR) and AC Impedance Spectroscopy (IS) are reported for composites based on PEO8:LiClO4 and carbon black (CB), prepared by two methods: solvent and fusion processing. Three nuclear relaxation processes were identified for 1H nuclei: (i) belonging to the polymer chains in the amorphous phase, loosely bound to the CB particles, whose dynamics is almost the same as for unfilled polymer, (ii) belonging to the polymer chains which are tightly attached to the CB particles, and (iii) belonging to the crystalline phase in the loose polymer chain. The paramagnetic electronic susceptibility of the composite samples, measured by EPR, was interpreted by assuming a contribution of localized spin states that follow a Curie law, and a Pauli-like contribution of delocalized spins. A significant change of the EPR linewidth was observed at 40 K, which is the temperature where the Curie and Pauli susceptibilities equally contribute to the paramagnetic electronic susceptibility. The electrical properties are very sensitive to the preparation methods of the composites, which conditions the interaction between carbon particle-carbon particle and carbon particle-polymer chain. Classical statistic models to describe the conductivity in these media were not satisfactory. © 1998 Published by Elsevier Science B.V. All rights reserved.

Influence of nearest-neighbor Coulomb interactions on the phase diagram of the ferromagnetic Kondo model

The influence of a nearest-neighbor Coulomb repulsion of strength V on the properties of the ferromagnetic Kondo model is analyzed using computational techniques. The Hamiltonian studied here is defined on a chain using localized S = 1/2 spins, and one orbital per site. Special emphasis is given to the influence of the Coulomb repulsion on the regions of phase separation recently discovered in this family of models, as well as on the double-exchange-induced ferromagnetic ground state. When phase separation dominates at V= 0, the Coulomb interaction breaks the large domains of the two competing phases into small islands of one phase embedded into the other. This is in agreement with several experimental results, as discussed in the text. Vestiges of the original phase separation regime are found in the spin structure factor as incommensurate peaks, even at large values of V. In the ferromagnetic regime close to density n = 0.5, the Coulomb interaction induces tendencies to charge ordering without altering the fully polarized character of the state. This regime of charge-ordered ferromagnetism may be related with experimental observations of a similar phase by Chen and Cheong [Phys. Rev. Lett. 76, 4042 (1996)]. Our results reinforce the recently introduced notion [see, e.g., S. Yunoki et al., Phys. Rev. Lett. 80, 845 (1998)] that in realistic models for manganites analyzed with unbiased many-body techniques, the ground state properties arise from a competition between ferromagnetism and phase-separation - charge-ordering tendencies. ©1999 The American Physical Society.

Magnetic behavior of poly(3-methylthiophene): Metamagnetism and room-temperature weak ferromagnetism

A weak ferromagnetic phase is shown in pressed pellets of partially doped poly(3-methylthiophene) (P3MT) in the whole range from 1.8 to 300 K in magnetic measurements. Thermoremanence data have been used to estimate the suppression of this phase to be around 815 K. We also show that instead of the classical antiferromagnetism for the first-order interaction that gives weak ferromagnetism as a second-order effect, metamagnetic behavior is observed. X-band electron spin resonance (ESR) measurements and magnetization measurements allowed us to estimate that 8.1% of the total number of spins contributes to the weak ferromagnetism at room temperature. The doping level obtained from the ESR data is in good agreement with that estimated from electron dispersive spectroscopy measurements.