Thermoelectric transport of mesoscopic conductors coupled to voltage and thermal probes

We investigate basic properties of the thermopower (Seebeck coefficient) of phase-coherent conductors under the influence of dephasing and inelastic processes. Transport across the system is caused by a voltage bias or a thermal gradient applied between two terminals. Inelastic scattering is modeled with the aid of an additional probe acting as an ideal potentiometer and thermometer. We find that inelastic scattering reduces the conductor's thermopower and, more unexpectedly, generates a magnetic-field asymmetry in the Seebeck coefficient. The latter effect is shown to be a higher-order effect in the Sommerfeld expansion. We discuss our result using two illustrative examples. First, we consider a generic mesoscopic system described within random matrix theory and demonstrate that thermopower fluctuations disappear quickly as the number of probe modes increases. Second, the asymmetry is explicitly calculated in the quantum limit of a ballistic microjunction. We find that asymmetric scattering strongly enhances the effect and discuss its dependence on temperature and Fermi energy.Comment: 4+ pages, 2 figures and supplementary material. Published versio

Price volatility in ethanol markets

Our paper looks at how price volatility in the Brazilian ethanol industry changes over time and across markets by using a new methodological approach suggested by Seo (2007). The main advantage of Seo’s proposal over previously existing methods is that it allows to jointly estimate the cointegration relationship between the price series investigated and the multivariate GARCH process. Our results suggest that crude oil prices not only influence ethanol price levels, but also their volatility. Increased volatility in crude oil markets results in increased volatility in ethanol markets. Ethanol prices, on the other hand, influence sugar price levels and an increase in their volatility levels also impacts, though less strongly, on sugar markets.volatility, ethanol, GARCH, cointegration, Demand and Price Analysis, Resource /Energy Economics and Policy, Risk and Uncertainty, Q11, C32,

Evanescent states in quantum wires with Rashba spin-orbit coupling

We discuss the calculation of evanescent states in quasi-one-dimensional quantum wires in the presence of Rashba spin-orbit interaction. We suggest a computational algorithm devised for cases in which longitudinal and transverse motions are coupled. The dispersion relations are given for some selected cases, illustrating the feasibility of the proposed computational method. As a practical application, we discuss the solutions for a wire containing a potential step.Comment: 8 pages, 8 figure

Strongly modulated transmission of a spin-split quantum wire with local Rashba interaction

We investigate the transport properties of ballistic quantum wires in the presence of Zeeman spin splittings and a spatially inhomogeneous Rashba interaction. The Zeeman interaction is extended along the wire and produces gaps in the energy spectrum which allow electron propagation only for spinors lying along a certain direction. For spins in the opposite direction the waves are evanescent far away from the Rashba region, which plays the role of the scattering center. The most interesting case occurs when the magnetic field is perpendicular to the Rashba field. Then, the spins of the asymptotic wavefunctions are not eigenfunctions of the Rashba Hamiltonian and the resulting coupling between spins in the Rashba region gives rise to sudden changes of the transmission probability when the Fermi energy is swept along the gap. After briefly examining the energy spectrum and eigenfunctions of a wire with extended Rashba coupling, we analyze the transmission through a region of localized Rashba interaction, in which a double interface separates a region of constant Rashba interaction from wire leads free from spin-orbit coupling. For energies slightly above the propagation threshold, we find the ubiquitous occurrence of transmission zeros (antiresonances) which are analyzed by matching methods in the one-dimensional limit. We find that a a minimal tight-binding model yields analytical transmission lineshapes of Fano antiresonance type. More general angular dependences of the external magnetic field is treated within projected Schroedinger equations with Hamiltonian matrix elements mixing wavefunction components. Finally, we consider a realistic quantum wire where the energy subbands are coupled via the Rashba intersubband coupling term and discuss its effect on the transmission zeros.Comment: 11 pages, 12 figure

Farms' technical inefficiencies in the presence of government programs

We focus on determining the impacts of government programs on farms’ technical inefficiency levels. We use Kumbhakar’s stochastic frontier model that accounts for both production risks and risk preferences. Our theoretical framework shows that decoupled government transfers are likely to increase (decrease) DARA (IARA) farmers’ production inefficiencies if variable inputs are risk decreasing. However, the impacts of decoupled payments cannot be anticipated if variable inputs are risk increasing. We use farm-level data collected in Kansas to illustrate the model.Just and Pope production function, Agricultural and Food Policy, Farm Management,

Differential Uncertainties and Risk Attitudes between Conventional and Organic Producers: The Case of Spanish COP Farmers

The growing importance of economic factors in farmers decision to go organic has raised interest in characterizing the economic behavior of organic versus conventional farms. Published analyses so far have not considered differential uncertainties and farmers risk preferences between conventional and organic practices when comparing these techniques. Our article attempts to assess this issue. We use a model of farmer decision under risk to analyze the differential values between Spanish COP organic and conventional farms and to assess the incentives for adoption of organic practices. Results show that organic and conventional farms do have different abilities to control production risk as well as different risk preferences. Organic price premiums and subsidies are found to be powerful instruments to motivate adoption of organic techniques.Crop Production/Industries, Risk and Uncertainty,

Multichannel effects in Rashba quantum wires

We investigate intersubband mixing effects in multichannel quantum wires in the presence of Rashba spin-orbit coupling and attached to two terminals. When the contacts are ferromagnetic and their magnetization direction is perpendicular to the Rashba field, the spin-transistor current is expected to depend in a oscillatory way on the Rashba coupling strength due to spin coherent oscillations of the travelling electrons. Nevertheless, we find that the presence of many propagating modes strongly influences the spin precession effect, leading to (i) a quenching of the oscillations and (ii) strongly irregular curves for high values of the Rashba coupling. We also observe that in the case of leads' magnetization parallel to the Rashba field, the conductance departs from a uniform value as the Rashba strength increases. We also discuss the Rashba interaction induced current polarization effects when the contacts are not magnetic and investigate how this mechanism is affected by the presence of several propagating channels.Comment: 8 pages, 7 figure

An Analysis of incentives mechanisms and evaluation on BitTorrent

Since the first peer-to-peer communities appeared, their number of users has increased considerably owing to the benefits they offer compared to their alternative architectures in the sharing and distribution of multimedia content. However, due to its distributed nature, they can suffer an important problem of misuse: free-riding. Free-riding consists on users consuming resources without contributing to the system. Such behaviour not only is not fair for the rest of the users, but also threatens the success of this type of nets. With the motivation to avoid free-riding, the mechanisms of incentives were born. They provide the system with a method to motivate the nodes and make them share their resources with the other users. In one word, they provide the net with the needed fairness to achieve a good performance for all users. This thesis is organised in two main parts. In the first part there is a comprehensive study of the state of the art regarding the incentive mechanisms, resulting in a classification depending on the characteristics of the studied algorithms. That study provides the reader with a first sight of the strengths and weaknesses of each algorithm. In the second part there is a test scenario based in the virtualization of machines that was useful to evaluate empirically some of the studied algorithms. Finally, a series of experiments were carried out in order to compare some characteristics of these algorithms and thus verify or deny the conclusions resulted in the study of the state of the art