Anomalous Hall Effect in three ferromagnets: EuFe4Sb12, Yb14MnSb11, and Eu8Ga16Ge30

The Hall resistivity (Rho_xy), resistivity (Rho_xx), and magnetization of three metallic ferromagnets are investigated as a function of magnetic field and temperature. The three ferromagnets, EuFe4Sb12 (Tc = 84 K), Yb14MnSb11 (Tc = 53 K), and Eu8Ga16Ge30 (Tc = 36 K) are Zintl compounds with carrier concentrations between 1 x 10^21 cm^-3 and 3.5 x 10^21 cm^-3. The relative decrease in Rho_xx below Tc [Rho_xx(Tc)/Rho_xx(2 K)] is 28, 6.5, and 1.3 for EuFe4Sb12, Yb14MnSb11, and Eu8Ga16Ge30 respectively. The low carrier concentrations coupled with low magnetic anisotropies allow a relatively clean separation between the anomalous (Rho_'xy), and normal contributions to the measured Hall resistivity. For each compound the anomalous contribution in the zero field limit is fit to alpha Rho_xx + sigma_xy rho_xx^2 for temperatures T < Tc. The anomalous Hall conductivity, sigma_xy, is -220 +- 5 (Ohm^-1 cm^-1), -14.7 +- 1 (Ohm^-1 cm^-1), and 28 +- 3 (Ohm^-1 cm^-1) for EuFe4Sb12, Yb14MnSb11, and Eu8Ga16Ge30 respectively and is independent of temperature for T < Tc if the change in spontaneous magnetization (order parameter) with temperature is taken into account. These data are consistent with recent theories of the anomalous Hall effect that suggest that even for stochiometric ferromagnetic crystals, such as those studied in this article, the intrinsic Hall conductivity is finite at T = 0, and is a ground state property that can be calculated from the electronic structure.Comment: 22 pages, 13 figures Submitted to PR

Surveillance on the light-front gauge fixing Lagrangians

In this work we propose two Lagrange multipliers with distinct coefficients for the light-front gauge that leads to the complete (non-reduced) propagator. This is accomplished via $(n\cdot A)^{2}+(\partial \cdot A)^{2}$ terms in the Lagrangian density. These lead to a well-defined and exact though Lorentz non invariant light front propagator.Comment: 7 pages. This is an improved version of hep-th/030406

Chemical abundances and kinematics of barium stars

In this paper we present an homogeneous analysis of photospheric abundances based on high-resolution spectroscopy of a sample of 182 barium stars and candidates. We determined atmospheric parameters, spectroscopic distances, stellar masses, ages, luminosities and scale height, radial velocities, abundances of the Na, Al, $alpha$-elements, iron-peak elements, and s-process elements Y, Zr, La, Ce, and Nd. We employed the local-thermodynamic-equilibrium model atmospheres of Kurucz and the spectral analysis code {\sc moog}. We found that the metallicities, the temperatures and the surface gravities for barium stars can not be represented by a single gaussian distribution. The abundances of $alpha$-elements and iron peak elements are similar to those of field giants with the same metallicity. Sodium presents some degree of enrichment in more evolved stars that could be attributed to the NeNa cycle. As expected, the barium stars show overabundance of the elements created by the s-process. By measuring the mean heavy-element abundance pattern as given by the ratio [s/Fe], we found that the barium stars present several degrees of enrichment. We also obtained the [hs/ls] ratio by measuring the photospheric abundances of the Ba-peak and the Zr-peak elements. Our results indicated that the [s/Fe] and the [hs/ls] ratios are strongly anti-correlated with the metallicity. Our kinematical analysis showed that 90% of the barium stars belong to the thin disk population. Based on their luminosities, none of the barium stars are luminous enough to be an AGB star, nor to become self-enriched in the s-process elements. Finally, we determined that the barium stars also follow an age-metallicity relation.Comment: 30 pages, 26 figures, 18 tables, accepted for publication in MNRA

Coupling Between An Optical Phonon and the Kondo Effect

We explore the ultra-fast optical response of Yb_{14}MnSb_{11}, providing further evidence that this Zintl compound is the first ferromagnetic, under-screened Kondo lattice. These experiments also provide the first demonstration of coupling between an optical phonon mode and the Kondo effect.Comment: 4 Pages, 3 Figures, submitted to Phys. Rev. Let

Modificações na fertilidade do solo após cinco anos do cultivo de espécies florestais em sistema agrossilvipastoril no Nordeste do Pará.

A pecuária no nordeste do Pará gerou um passivo ambiental de milhares de hectares de pastagens degradadas. A inclusão da agricultura e silvicultura em áreas de pastagens degradadas é uma forma de viabilizar economicamente a recuperação e diminuir a pressão sobre as áreas naturais. O sistema de integração Lavoura-Pecuária-Floresta (iLPF) possibilita a recuperação dessas áreas de forma sustentável e com uma maior produção. O trabalho tem como objetivo avaliar as modificações na fertilidade do solo após cinco anos de cultivo de espécies florestais em sistema Integração Lavoura-Pecuária-Floresta (iLPF) para recuperação de áreas de pastagens degradadas. O estudo foi desenvolvido na fazenda Vitória, no município de Paragominas ? PA. Segundo a classificação de Koppen, o clima é Aw. A precipitação média é de 1743 mm. O solo foi classificado como Latossolo amarelo textura argilosa. Utilizaram-se as espécies florestais mogno africano (Khaya ivorensis) e paricá (Schizolobium amazonicum), para forragem usou-se a Brachiaria ruziziensis e para o cultivo de grãos foi utilizado o milho (BRS 1030). O experimento foi composto por um cultivo de milho intercalado com paricá e mogno africano (iLPF) em áreas de 4,05 e 4,05 ha, respectivamente. Utilizou-se 5 ha para o cultivo do milho no sistema Santa Fé. Foram coletadas amostras de solo em quatro locais da unidade experimental, retirando-se amostras nas profundidades de 0-10, 10-20, 20-30 e 30-40 cm. Na avaliação do solo após os cinco anos de iLPF houve recuperação e manutenção da capacidade produtiva do solo, redução da erosão dos solos, redução de carbono, matéria orgânica, teor de fósforo, saturação por bases e aumento no teor de alumínio com o aumento da profundidade do solo. Essas peculiaridades do iLPF implicam em diferentes estratégias de manejo da fertilidade do solo. Para tanto, as recomendações devem ser mais bem estudadas, respeitando as situações em particular