Large enhancement of the thermoelectric power factor in disordered materials through resonant scattering

In the search for more efficient thermoelectric materials, scientists have placed high hopes in the possibility of enhancing the power factor using resonant states. In this study, we investigate theoretically the effects of randomly distributed resonant impurities on the power factor. Using the Chebyshev Polynomial Green's Function method, we compute the electron transport properties for very large systems (10 million atoms) with an exact treatment of disorder. The introduction of resonant defects can lead to a large enhancement of the power factor together with a sign inversion in the Seebeck coefficient. This boost depends crucially on the position of the resonant peak, and on the interplay between elastic impurity scattering and inelastic processes. Strong electron-phonon or electron-electron scattering are found detrimental. Finally, the robustness of our results is examined in the case of anisotropic orbitals and two-dimensional confinement. Our findings are promising for the prospect of thermoelectric power generation.Comment: To appear in Phys. Rev.

A Poincar\'e formula for differential forms and applications

We prove a new general Poincar\'e-type inequality for differential forms on compact Riemannian manifolds with nonempty boundary. When the boundary is isometrically immersed in Euclidean space, we derive a new inequality involving mean and scalar curvatures of the boundary only and characterize its limiting case in codimension one. A new Ros-type inequality for differential forms is also derived assuming the existence of a nonzero parallel form on the manifold

Investigating the high-temperature thermoelectric properties of n-type rutile TiO2_2

Transition metal oxides are considered promising thermoelectric materials for harvesting high-temperature waste heat due to their stability, abundance and low toxicity. Despite their typically strong ionic character, they can exhibit surprisingly high power factors $\sigma S^2$, as in n-type SrTiO$_3$ for instance. Thus, it is worth examining other transition metal oxides that might surpass the performances of SrTiO$_3$. This theoretical paper investigates the thermoelectric properties of n-type rutile TiO$_2$, which is the most stable phase of titanium oxide up to 2000 K. The electronic structure is obtained through ab initio calculations, while the prominent features of strong electron-phonon interaction and defects states are modelled using a small number of parameters. The theoretical results are compared with a wealth of experimental data from the literature, yielding very good agreements over a wide range of carrier concentrations. This validates the hypothesis of band conduction in rutile TiO$_2$ and allows the prediction of the high-temperature thermoelectric properties

Causes of Multifunctionality: Externalities or Political Pressure?

The EU has argued that some agricultural subsidies are needed to provide the optimal amount of externalities (both positive and negative) produced by agriculture. The argument is that agriculture is "multifunctional" and externalities such as rural development and landscape would be underproduced, while some forms of pollution (such as nitrogen runoff) would be overproduced without government intervention. Meanwhile, the United States has raised the concern that multifunctionality is primarily an argument to transfer income to producers. In this paper, we discuss the motivation for the EU agri-environmental measures and empirically test for those underlying causes. We find that the programs are not targeted at those regions with the highest environmental need, but neither are they purely a substitute for traditional forms of agricultural subsidies. Demand for general environmental expenditure does influence agri-environmental expenditure as well, as does political structure.Environmental Economics and Policy,