High temperature limit in static backgrounds

We prove that the hard thermal loop contribution to static thermal amplitudes can be obtained by setting all the external four-momenta to zero before performing the Matsubara sums and loop integrals. At the one-loop order we do an iterative procedure for all the 1PI one-loop diagrams and at the two-loop order we consider the self-energy. Our approach is sufficiently general to the extent that it includes theories with any kind of interaction vertices, such as gravity in the weak field approximation, for $d$ space-time dimensions. This result is valid whenever the external fields are all bosonic.Comment: 15 pages, 11 figures. To be published in Physical Review

Thermal Effective Lagrangian of Static Gravitational Fields

We compute the effective Lagrangian of static gravitational fields interacting with thermal fields. Our approach employs the usual imaginary time formalism as well as the equivalence between the static and space-time independent external gravitational fields. This allows to obtain a closed form expression for the thermal effective Lagrangian in $d$ space-time dimensions.Comment: Accepted for publication in the Physical Review

Possible chiral phase transition in two-dimensional solid 3^3He

We study a spin system with two- and four-spin exchange interactions on the triangular lattice, which is a possible model for the nuclear magnetism of solid $^3$He layers. It is found that a novel spin structure with scalar chiral order appears if the four-spin interaction is dominant. Ground-state properties are studied using the spin-wave approximation. A phase transition concerning the scalar chirality occurs at a finite temperature, even though the dimensionality of the system is two and the interaction has isotropic spin symmetry. Critical properties of this transition are studied with Monte Carlo simulations in the classical limit.Comment: 4 pages, Revtex, 4 figures, to appear in Phys.Rev.Let

Magnetization plateau in a two-dimensional multiple-spin exchange model

We study a multiple-spin exchange model on a triangular lattice, which is a possible model for low-density solid 3He films. Due to strong competitions between ferromagnetic three-spin exchange and antiferromagnetic four-spin one, the ground states are highly degenerate in the classical limit. At least 2^{L/2}-fold degeneracy exists on the L*L triangular lattice except for the SO(3) symmetry. In the magnetization process, we found a plateau at m/m_{sat}=1/2, in which the ground state is "uuud state" (a collinear state with four sublattices). The 1/2-plateau appears due to the strong four-spin exchange interaction. This plateau survives against both quantum and thermal fluctuations. Under a magnetic field which realizes the "uuud" ordered state, a phase transition occurs at a finite temperature. We predict that low-density solid 3He thin films may show the 1/2-plateau in the magnetization process. Experimental observation of the plateau will verify strength of the four-spin exchange. It is also discussed that this magnetization plateau can be understood as an insulating-conducting transition in a particle picture.Comment: 10 pages, RevTeX, 12 figures, added a reference and corrected typos, to be published in Phys.Rev.B (01 APR 99

Spin-dependent beating patterns in thermoelectric properties: Filtering the carriers of the heat flux in a Kondo adatom system

We theoretically investigate the thermoelectric properties of a spin-polarized two-dimensional electron gas hosting a Kondo adatom hybridized with an STM tip. Such a setup is treated within the single-impurity Anderson model in combination with the atomic approach for the Green's functions. Due to the spin dependence of the Fermi wavenumbers the electrical and thermal conductances, together with thermopower and Lorenz number reveal beating patterns as function of the STM tip position in the Kondo regime. In particular, by tuning the lateral displacement of the tip with respect to the adatom vicinity, the temperature and the position of the adatom level, one can change the sign of the Seebeck coefficient through charge and spin. This opens a possibility of the microscopic control of the heat flux analogously to that established for the electrical current

On the ergodic theory of impulsive semiflows

We consider impulsive semiflows and establish sufficient conditions to the existence of invariant measures. Namely, the impulsive set and its image are both submanifolds of codimension one that are transversal to the flow direction. Moreover, we show that under the same conditions a Variational Principle holds

Dimensionality effects in the LDOS of ferromagnetic hosts probed via STM: spin-polarized quantum beats and spin filtering

We theoretically investigate the local density of states (LDOS) probed by a STM tip of ferromagnetic metals hosting a single adatom and a subsurface impurity. We model the system via the two-impurity Anderson Hamiltonian. By using the equation of motion with the relevant Green functions, we derive analytical expressions for the LDOS of two host types: a surface and a quantum wire. The LDOS reveals Friedel-like oscillations and Fano interference as a function of the STM tip position. These oscillations strongly depend on the host dimension. Interestingly, we find that the spin-dependent Fermi wave numbers of the hosts give rise to spin-polarized quantum beats in the LDOS. While the LDOS for the metallic surface shows a damped beating pattern, it exhibits an opposite behavior in the quantum wire. Due to this absence of damping, the wire operates as a spatially resolved spin filter with a high efficiency.Comment: revised tex

Estruturação de base cartográfica digital para o extremo Sul do RS.

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A experiência da metodologia camponês a camponês em territórios de identidade rural no Nordeste do Brasil.

A pesquisa teve por objetivo a promoção da transição agroeocológica em territórios de identidade rural por meio do ajuste da metodologia ?campesino a campesino? visando à promoção do desenvolvimento sustentável e solidário nas condições do Nordeste do Brasil. Foram identificadas famílias agricultoras camponesas com perfil para inserção em redes de transição agroecológica, potenciais promotores de intercâmbios entre elas para a troca de experiências e saberes. O trabalho foi realizado no Território de identidade rural Sul Sergipano localizado nos tabuleiros costeiros do estado de Sergipe. A realização das ações resultou de uma parceria efetiva entre Colegiado Territorial, Empresa Brasileira de Pesquisa Agropecuária (Embrapa), Instituto Nacional de Colonização e Reforma Agrária (INCRA-SE), Movimento dos Trabalhadores Sem Terra (MST), Universidade Federal de Sergipe (UFS), Universidade Federal de Viçosa (UFV) e Centro de Formação e Assistência Comunitária (CFAC). Para a operacionalização do trabalho e ajuste da metodologia foram potencializadas as redes sociais de agroecologia existentes na região. Os resultados referem-se à identificação das principais experiências de transição agroecológica da região, a consolidação das redes existentes e a criação de formas inovadoras de intercâmbio de experiências potencializadas pela metodologia ajustada, então denominada ?camponês a camponês?. Os aprendizados referem-se à forma de construção coletiva necessária para a introdução de conhecimento agroecológico e, principalmente, de conceitos de maior complexidade, como agrofloresta sucessional e, a perspectiva de construção de uma nova abordagem para a extensão rural em agricultura familiar e camponesa em territórios de identidade rural