Fate of the spin-\frac{1}{2} Kondo effect in the presence of temperature gradients

We consider a strongly interacting quantum dot connected to two leads held at quite different temperatures. Our aim is to study the behavior of the Kondo effect in the presence of large thermal biases. We use three different approaches, namely, a perturbation formalism based on the Kondo Hamiltonian, a slave-boson mean-field theory for the Anderson model at large charging energies and a truncated equation-of-motion approach beyond the Hartree-Fock approximation. The two former formalisms yield a suppression of the Kondo peak for thermal gradients above the Kondo temperature, showing a remarkably good agreement despite their different ranges of validity. The third technique allows us to analyze the full density of states within a wide range of energies. Additionally, we have investigated the quantum transport properties (electric current and thermocurrent) beyond linear response. In the voltage-driven case, we reproduce the split differential conductance due to the presence of different electrochemical potentials. In the temperature-driven case, we observe a strongly nonlinear thermocurrent as a function of the applied thermal gradient. Depending on the parameters, we can find nontrivial zeros in the electric current for finite values of the temperature bias. Importantly, these thermocurrent zeros yield direct access to the system's characteristic energy scales (Kondo temperature and charging energy).Comment: 14 pages, 11 figures, revised versio

Maize Production and Agricultural Policies in Central America and Mexico

This paper reviews trends in maize production and consumption in Central America and Mexico in the context of the political and economic changes taking place in the region since the 1970s. The authors focus on the effects of the structural adjustment programs in the 1980s and 1990s. The analysis begins by reviewing the economic context in which maize production occurs in the region and the main economic policy instruments affecting the maize economy. Next, trends in maize consumption and production are analyzed, along with the main factors influencing maize production, including trends in the public financing of maize research and extension. The authors find that several factors related to structural adjustment have defined--and are still defining--the course of agriculture, including maize production, in the countries of the region. The impact of these factors on maize production, consumption, and import trends has been different in Central America and in Mexico. In particular, the reduction or complete elimination of production incentives, the reduction of trade barriers, the liberalization of input and product prices, the deregulation of the currency exchange rate, the control of inflation, and the restructuring of agricultural research systems between the public and the private sectors have determined how basic grains are produced in the region and how they will be produced in the future. Furthermore, the visible and increasing deterioration of the natural resource base has raised great concern about the need to promote more sustainable, environmentally friendly uses of production systems and natural resources.Agricultural and Food Policy, Crop Production/Industries,

Quasi-Exactly Solvable Potentials on the Line and Orthogonal Polynomials

In this paper we show that a quasi-exactly solvable (normalizable or periodic) one-dimensional Hamiltonian satisfying very mild conditions defines a family of weakly orthogonal polynomials which obey a three-term recursion relation. In particular, we prove that (normalizable) exactly-solvable one-dimensional systems are characterized by the fact that their associated polynomials satisfy a two-term recursion relation. We study the properties of the family of weakly orthogonal polynomials defined by an arbitrary one-dimensional quasi-exactly solvable Hamiltonian, showing in particular that its associated Stieltjes measure is supported on a finite set. From this we deduce that the corresponding moment problem is determined, and that the $k$-th moment grows like the $k$-th power of a constant as $k$ tends to infinity. We also show that the moments satisfy a constant coefficient linear difference equation, and that this property actually characterizes weakly orthogonal polynomial systems.Comment: 22 pages, plain TeX. Please typeset only the file orth.te

The Maize Seed Industries of Brazil and Mexico: Past Performance, Current Issues, and Future Prospects

This paper describes results of a study of the main factors affecting the development of the maize seed industries in Brazil and Mexico (and, by extension, other developing countries). The authors develop a framework that researchers and policy makers can use to evaluate seed industry performance in developing countries. This framework is used to analyze the seed industries of Brazil and Mexico, where very different sets of circumstances influence seed industry development, efficiency, and structure. The analysis gives special attention to the different maize breeding strategies pursued by the public and private sectors, measures of industry competitiveness and efficiency, and the trade-offs involved in developing and producing different kinds of maize seed, particularly improved open-pollinated maize varieties versus different types of hybrids. The authors identify key seed industry issues for researchers, administrators of national maize programs, and agricultural policy makers in developing countries, especially issues related to the appropriate roles for public and private organizations in maize seed industries in the developing world.Crop Production/Industries,

Nonequilibrium Phase Transitions in Directed Small-World Networks

Many social, biological, and economic systems can be approached by complex networks of interacting units. The behaviour of several models on small-world networks has recently been studied. These models are expected to capture the essential features of the complex processes taking place on real networks like disease spreading, formation of public opinion, distribution of wealth, etc. In many of these systems relations are directed, in the sense that links only act in one direction (outwards or inwards). We investigate the effect of directed links on the behaviour of a simple spin-like model evolving on a small-world network. We show that directed networks may lead to a highly nontrivial phase diagram including first and second-order phase transitions out of equilibrium.Comment: 4 pages, RevTeX format, 4 postscript figs, uses eps

The Informational and Institutional Theories of Off-Label Promotion

This Article contends that there are two distinct theories of the offense of off-label promotion—the informational theory and the institutional theory. One is concerned with controlling the flow of medical knowledge and the other is concerned with protecting regulatory legitimacy. Different kinds of evidence are key under each theory. I argue that although the Federal Food, Drug, and Cosmetic Act (FD&C Act) and its accompanying regulations emphasize the informational theory, federal prosecutors rely more heavily on the legal arguments that underpin the institutional theory of enforcement. A corollary to this contention is that the informational theory of off-label promotion does most of the work in determining the evolution of FDA policy and guidance with respect to drug marketing and labeling. The institutional theory, on the other hand, drives the blunt force of government enforcement, meant to give the regulatory system it protects an extra measure of deterrent power. I briefly summarize these two theories below

Higher-order moments and overlaps of Cartesian beams

We introduce a closed-form expression for the overlap between two different Cartesian beams. In the course of obtaining this expression, we establish a linear relation between the overlap of circular beams with azimuthal symmetry and the overlap of Cartesian beams such that the knowledge of the former allows the latter to be calculated very easily. Our formalism can be easily applied to calculate relevant beam parameters such as the normalization constants, the M2 factors, the kurtosis parameters, the expansion coefficients of Cartesian beams, and therefore of all their relevant special cases, including the standard, elegant, and generalized Hermite–Gaussian beams, cosh-Gaussian beams, Lorentz beams, and Airy beams, among others