Fermionic van Hemmen Spin Glass Model with a Transverse Field

In the present work it is studied the fermionic van Hemmen model for the spin glass (SG) with a transverse magnetic field $\Gamma$. In this model, the spin operators are written as a bilinear combination of fermionic operators, which allows the analysis of the interplay between charge and spin fluctuations in the presence of a quantum spin flipping mechanism given by $\Gamma$. The problem is expressed in the fermionic path integral formalism. As results, magnetic phase diagrams of temperature versus the ferromagnetic interaction are obtained for several values of chemical potential $\mu$ and $\Gamma$. The $\Gamma$ field suppresses the magnetic orders. The increase of $\mu$ alters the average occupation per site that affects the magnetic phases. For instance, the SG and the mixed SG+ferromagnetic phases are also suppressed by $\mu$. In addition, $\mu$ can change the nature of the phase boundaries introducing a first order transition.Comment: 9 pages, 4 figures, accepted for publication in Phys. Lett.

The organizational implementation of information systems: towards a new theory

This dissertation provides fundamental conceptual building blocks for a new theory of IS implementation. The main argument presented is for a new perspective to supplement and complement the main existing perspectives on IS implementation: the technological determinist perspective, the managerial imperative perspective, and the socio-technical interactionist perspective. in this the research seeks to overcomes identified shortcomings of these existing approaches to understanding IS implementation. The research methodology emphasizes multidisciplinary theory-building, based on the resource-based approach to strategy, using autopoiesis as the key organizational epistemology. The research explores the concept of organizational climate dimensions as the shapers of organizational contexts, and relates these to information systems research in implementation. The new perspective developed emphasizes managerial action and organizational contexts as a mid-level approach, bridging the gap between top-down, rationalist methods and bottom-up, emergent approaches. Based on this conceptual framework, the context for IS corporate governance is operationalized and presented as a causal model with five independent variables - IS Intent, Discipline, Trust, Support and Structural IS-related factors and one independent variable. IS-Organizational Learning. Data collection is carried out in large Portuguese companies by means of a postal questionnaire. The empirical data is supplemented by five short case studies. The key conclusions of the thesis are: (1) The duality managerial action - organizational contexts opens up whole new possibilities for research and practice of IS implementation. (2) The use of the notion of organizational contexts dimensions as a research tool allows the analysis to go deeper than the vague generalization about organizations found in most current literature. (3) The use of quantitative methods to investigate IS-related organizational contexts is not suitable, except for descriptive purposes; semi-structured interviews and in-depth case studies are recommended. (4) Two specific dimensions of IS-related contexts are suggested as topics for further investigation: IS Intent and IS structural factors

Challenges in Signal Analysis of Resonant-Mass Gravitational Wave Detectors

An overview of the main points related to data analysis in resonant-mass gravitational wave detectors will be presented. Recent developments on the data analysis system for the Brazilian detector SCHENBERG will be emphasized.Comment: 6 pages, 3 figures, presented at the international conference "100 Years of Relativity" (Sao Paulo, Brazil, 22-24 August 2005), to appear in Brazilian Journal of Physics (Dec. 2005

The spin glass-antiferromagnetism competition in Kondo-lattice systems in the presence of a transverse applied magnetic field

A theory is proposed to describe the competition among antiferromagnetism (AF), spin glass (SG) and Kondo effect. The model describes two Kondo sublattices with an intrasite Kondo interaction strength $J_{K}$ and a random Gaussian interlattice interaction in the presence of a transverse field $\Gamma$. The $\Gamma$ field is introduced as a quantum mechanism to produce spin flipping and the random coupling has average $-2J_0/N$ and variance $32 J^{2}/N$. The path integral formalism with Grassmann fields is used to study this fermionic problem, in which the disorder is treated within the framework of the replica trick. The free energy and the order parameters are obtained using the static ansatz. In this many parameters problem, we choose $J_0/J \approx (J_{K}/J)^{2}$ and $\Gamma/J \approx (J_{K}/J)^{2}$ to allow a better comparison with the experimental findings. The obtained phase diagram has not only the same sequence as the experimental one for $Ce_{2}Au_{1-x}Co_{x}Si_{3}$, but mainly, it also shows a qualitative agreement concerning the behavior of the freezing temperature and the Neel temperature which decreases until a Quantum Critical Point (QCP).Comment: 4 pages, 1 figure, accepted for publication in Physica

Fermionic Ising Glasses with BCS Pairing Interaction. Tricritical Behaviour

We have examined the role of the BCS pairing mechanism in the formation of the magnetic moment and henceforth a spin glass (SG) phase by studying a fermionic Sherrington-Kirkpatrick model with a local BCS coupling between the fermions. This model is obtained by using perturbation theory to trace out the conduction electrons degrees of freedom in conventional superconducting alloys. The model is formulated in the path integral formalism where the spin operators are represented by bilinear combinations of Grassmann fields and it reduces to a single site problem that can be solved within the static approximation with a replica symmetric Ansatz. We argue that this is a valid procedure for values of temperature above the de Almeida-Thouless instability line. The phase diagram in the T-g plane, where g is the strength of the pairing interaction, for fixed variance J^2/N of the random couplings J_{ij}, exhibits three regions: a normal paramagnetic (NP) phase, a spin glass (SG) phase and a pairing (PAIR) phase where there is formation of local pairs.The NP and PAIR phases are separated by a second order transition line g=g_{c}(T) that ends at a tricritical point T_{3}=0.9807J, g_{3}=5,8843J, from where it becomes a first order transition line that meets the line of second order transitions at T_{c}=0.9570J that separates the NP and the SG phases. For T<T_{c} the SG phase is separated from the PAIR phase by a line of first order transitions. These results agree qualitatively with experimental data in Gd_{x}Th_{1-x}RU_{2}.Comment: 26 pages, 5 figures, to appear in The European Physical Journal

Role of the transverse field in inverse freezing in the fermionic Ising spin-glass model

We investigate the inverse freezing in the fermionic Ising spin-glass (FISG) model in a transverse field $\Gamma$. The grand canonical potential is calculated in the static approximation, replica symmetry and one-step replica symmetry breaking Parisi scheme. It is argued that the average occupation per site $n$ is strongly affected by $\Gamma$. As consequence, the boundary phase is modified and, therefore, the reentrance associated with the inverse freezing is modified too.Comment: 6 pages, 3 figures, accepted for publication in PR

Productivity convergence in Brazil: The case of grain production

"In recent years, Brazil has become a considerable player in agricultural markets for a number of commodities. Such agricultural growth in Brazil has largely been the result of gains in productivity over the last several decades. Still, there remain some sub-national regions and states that lag behind in both agricultural productivity and levels of per capita income. In this paper, we investigate whether technological spillovers in agriculture have reached the poorer or less productive regions with focus on the evolution and patterns of land productivity. To assess such spillovers, we examine three cereal crops: maize, rice and wheat, as these crops are grown by commercial and subsistence farmers throughout the country. We first apply a generalized entropy (GE) method to assess whether inequality in productivity has changed over time. The entropy analysis indicates that the trends for overall entropy did not increase over time for all three crops. Moreover, declining trends in between-group inequality were observed for maize and wheat and remained constant for rice. This result suggests that yields in less productive micro-regions, indeed, have grown faster than yields in more productive micro-regions, at least in the case of maize and wheat. Next, two types of econometric estimations are used to measure whether convergence has occurred in yields of the three crops. The econometric findings are consistent with the GE results and suggest that conditional convergence has occurred in all three crops, which demonstrates that yields in less productive regions converge to those in productive regions, given the control of other factors. However, the process has been rather slow." from authors' abstractproductivity, Convergence, Spillovers, Development strategies,