14,823 research outputs found
On the nature of the spin-polarized hole states in a quasi-two-dimensional GaMnAs ferromagnetic layer
A self-consistent calculation of the density of states and the spectral
density function is performed in a two-dimensional spin-polarized hole system
based on a multiple-scattering approximation. Using parameters corresponding to
GaMnAs thin layers, a wide range of Mn concentrations and hole densities have
been explored to understand the nature, localized or extended, of the
spin-polarized holes at the Fermi level for several values of the average
magnetization of the Mn ystem. We show that, for a certain interval of Mn and
hole densities, an increase on the magnetic order of the Mn ions come together
with a change of the nature of the states at the Fermi level. This fact
provides a delocalization of spin-polarized extended states anti-aligned to the
average Mn magnetization, and a higher spin-polarization of the hole gas. These
results are consistent with the occurrence of ferromagnetism with relatively
high transition temperatures observed in some thin film samples and
multilayered structures of this material.Comment: 3 page
Forest vintages and carbon sequestration
In the current paper we examine the role of forest carbon sequestration benefits in optimal forest management. When carbon benefits are considered not only the forested area is relevant, but also the flow of carbon between land and the atmosphere through the carbon cycle. To account for all these impacts a multi-vintage forest setting is used, following Salo and Tahvonen (2004). The model is extended to three different carbon accounting methods to measure the benefits form carbon sequestration: carbon flow regime, tonne-year crediting and average storage. In the case of the carbon flow regime, the impact on the optimal management and allocation of land will depend upon the amount of carbon released when the forest is harvested. Under the other two accounting systems optimal steady state forest area will be increased, and in cases where optimal management imply cyclical harvesting, considering carbon benefits will always increase cycles dimension.
Urban Deforestation and Urban Development
This paper has developed a model of a single forest owner operating with perfect foresight in a dynamic open-city environment that allows for switching between alternative competing land uses (forest and urban use) at some point in the future. The model also incorporates external values of an even-aged standing forest in addition to the value of timber when it is harvested. Timber is exploited based on a multiple rotation model a la Faustmann with clear-cut harvesting. In contrast to previous models, our alternative land use to forest land is endogenous. Within this framework, we study the problem of the private owner as well as that of the social planner, when choosing the time to harvest, the time to convert land and the intensity of development. We also examine the extent to which the two-way linkage between urban development and forest management practices (timber production and provision of forest amenities) contributes to economic efficiency and improvements in non-market forest benefits. Finally, we consider policy options available to a regulator seeking to achieve improvements in efficiency including anti-sprawl policies (impact fees and density controls) and forest policies such a yield tax. Numerical simulations illustrate our analytical results. JEL codes:Deforestation, Urban Development, Forest Management Practices, Anti-Sprawl Policies, Yield Taxes
A semiquantitative approach to the impurity-band-related transport properties of GaMnAs nanolayers
We investigate the spin-polarized transport of GaMnAs nanolayers in which a
ferromagnetic order exists below a certain transition temperature. Our
calculation for the self-averaged resistivity takes into account the existence
of an impurity band determining the extended ("metallic" transport) or
localized (hopping by thermal excitation) nature of the states at and near the
Fermi level. Magnetic order and resistivity are inter-related due to the
influence of the spin polarization of the impurity band and the effect of the
Zeeman splitting on the mobility edge. We obtain, for a given range of Mn
concentration and carrier density, a "metallic" behavior in which the transport
by extended carriers dominates at low temperature, and is dominated by the
thermally excited localized carriers near and above the transition temperature.
This gives rise to a conspicuous hump of the resistivity which has been
experimentally observed and brings light onto the relationship between
transport and magnetic properties of this material
Technology Adoption in Nonrenewable Resource Management
Nonrenewable resource scarcity has been a traditional concern when designing optimal growth models. Technological change has played an important role in those models, since its presence is assumed to mitigate the depletion effect on extraction paths over time. We formalize the general problem of a competitive nonrenewable resource extracting firm to analyze optimal extraction behavior and technology adoption when adoption is costly, both in a deterministic and a stochastic environment, when the firm either anticipates adoption or not. Based on a quadratic extraction cost function, our results do not support the traditional view according to which the firm will only incur in an adoption cost when the stock is depleted enough.nonrenewable resources; technology adoption; depletion effect; cost of adoption.
Generalised verification of the observer property in discrete event systems
The observer property is an important condition to be satisfied by abstractions of Discrete Event Systems (DES) models. This paper presents a generalised version of a previous algorithm which tests if an abstraction of a DES obtained through natural projection has the observer property. The procedure called OP-verifier II overcomes the limitations of the previously proposed verifier while keeping its computational complexity. Results are illustrated by a case study of a transfer line system
Generalised verification of the observer property in discrete event systems
The observer property is an important condition to be satisfied by abstractions of Discrete Event Systems (DES) models. This paper presents a generalised version of a previous algorithm which tests if an abstraction of a DES obtained through natural projection has the observer property. The procedure called OP-verifier II overcomes the limitations of the previously proposed verifier while keeping its computational complexity. Results are illustrated by a case study of a transfer line system
Spin-polarized transport in ferromagnetic multilayered semiconductor nanostructures
The occurrence of inhomogeneous spin-density distribution in multilayered
ferromagnetic diluted magnetic semiconductor nanostructures leads to strong
dependence of the spin-polarized transport properties on these systems. The
spin-dependent mobility, conductivity and resistivity in
(Ga,Mn)As/GaAs,(Ga,Mn)N/GaN, and (Si,Mn)/Si multilayers are calculated as a
function of temperature, scaled by the average magnetization of the diluted
magnetic semiconductor layers. An increase of the resistivity near the
transition temperature is obtained. We observed that the spin-polarized
transport properties changes strongly among the three materials.Comment: 3 pages, 4 figure
Relação de longo prazo entre a produtividade do arroz irrigado produzido no Rio Grande do Sul e de Santa Catarina.
O objetivo geral deste trabalho foi analisar a relação entre a produtividade do arroz irrigado no Rio Grande do Sul e de Santa Catarina. A análise de co-integração de Engle e Granger (1987) foi utilizada com forma de corroborar com a ideia de relação de longo prazo entres as produtividades dos arrozais
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