Thermoelectric transport in strained Si and Si/Ge heterostructures

The anisotropic thermoelectric transport properties of bulk silicon strained in [111]-direction were studied by detailed first-principles calculations focussing on a possible enhancement of the power factor. Electron as well as hole doping were examined in a broad doping and temperature range. At low temperature and low doping an enhancement of the power factor was obtained for compressive and tensile strain in the electron-doped case and for compressive strain in the hole-doped case. For the thermoelectrically more important high temperature and high doping regime a slight enhancement of the power factor was only found under small compressive strain with the power factor overall being robust against applied strain. To extend our findings the anisotropic thermoelectric transport of an [111]-oriented Si/Ge superlattice was investigated. Here, the cross-plane power factor under hole-doping was drastically suppressed due to quantum-well effects, while under electron-doping an enhanced power factor was found. With that, we state a figure of merit of ZT$=0.2$ and ZT$=1.4$ at T=\unit[300]{K} and T=\unit[900]{K} for the electron-doped [111]-oriented Si/Ge superlattice. All results are discussed in terms of band structure features

On calculating the Berry curvature of Bloch electrons using the KKR method

We propose and implement a particularly effective method for calculating the Berry curvature arising from adiabatic evolution of Bloch states in wave vector k space. The method exploits a unique feature of the Korringa-Kohn-Rostoker (KKR) approach to solve the Schr\"odinger or Dirac equations. Namely, it is based on the observation that in the KKR method k enters the calculation via the structure constants which depend only on the geometry of the lattice but not the crystal potential. For both the Abelian and non-Abelian Berry curvature we derive an analytic formula whose evaluation does not require any numerical differentiation with respect to k. We present explicit calculations for Al, Cu, Au, and Pt bulk crystals.Comment: 13 pages, 5 figure

Influence of strain on anisotropic thermoelectric transport of Bi2_2Te3_3 and Sb2_2Te3_3

On the basis of detailed first-principles calculations and semi-classical Boltzmann transport, the anisotropic thermoelectric transport properties of Bi$_2$Te$_3$ and Sb$_2$Te$_3$ under strain were investigated. It was found that due to compensation effects of the strain dependent thermopower and electrical conductivity, the related powerfactor will decrease under applied in-plane strain for Bi$_2$Te$_3, while being stable for Sb$_2$Te$_3$. A clear preference for thermoelectric transport under hole-doping, as well as for the in-plane transport direction was found for both tellurides. In contrast to the electrical conductivity anisotropy, the anisotropy of the thermopower was almost robust under applied strain. The assumption of an anisotropic relaxation time for Bi$_2$Te$_3$ suggests, that already in the single crystalline system strong anisotropic scattering effects should play a role

Mulheres na Política, a repressão, a masculinização e a Mídia: os casos de Angela Merkel, Dilma Rousseff e Park Geun-Hye

III Semana Acadêmica de Relações Internacionais foi realizada de 05 a 09 denovembro de 2017, na Universidade Federal da Integração Latino-Americana (UNILA)A atuação feminina no processo democrático e na política, apesar de ter evoluído nas últimas décadas, ainda é uma luta constante, pois apesar de haver-se conquistado o direito de votar e a atuar em pleitos eleitorais, todavia surgem outros problemas e formas de repressão advindas de uma cultura machista e patriarcal, cujo intuito é manter as mulheres sobre domínio masculino e na esfera privada. Uma forma de coibir, reprimir a participação, inserção das mulheres na política e a criação de estereótipos reforçados através da mídia. A mulher mais dura, com características de liderança associadas aos homens, desde as vestimentas até a forma de falar e de posicionar, e a mistura de características que remetem ao feminino com a política das mulheres é o que esse artigo se propõe a analisar, focando em três casos específicos, o de Angela Merkel na Alemanha, de Dilma Rousseff no Brasil, e de Park Geun-hye na Coréia do Sul La actuación femenina en el proceso democrático y en la política, a pesar de haber evolucionado en las últimas décadas, sigue siendo una lucha constante, pues a pesar de haber conquistado el derecho de votar y la actuación en pleitos electorales, todavía surgen otros problemas y formas de represión de una cultura machista y patriarcal, cuyo propósito es mantener a las mujeres sobre dominio masculino y en la esfera privada. Una forma de cohibir, reprimir la participación, inserción de las mujeres en la política y la creación de estereotipos reforzados a través de los medios de comunicación. La mujer más dura, con características de liderazgo asociadas a los hombres, desde las vestimentas hasta la forma de hablar y de posicionar, y la mezcla de características que remiten al femenino con la política de las mujeres es lo que ese artículo se propone analizar, enfocando en tres casos específicos, el de Angela Merkel en Alemania, de Dilma Rousseff en Brasil, y de Park Geun- hye en Corea del Su

Numerical Object Oriented Quantum Field Theory Calculations

The qft++ package is a library of C++ classes that facilitate numerical (not algebraic) quantum field theory calculations. Mathematical objects such as matrices, tensors, Dirac spinors, polarization and orbital angular momentum tensors, etc. are represented as C++ objects in qft++. The package permits construction of code which closely resembles quantum field theory expressions, allowing for quick and reliable calculations.Comment: 9 pages, 3 figures, submitted to Comp. Phys. Com

Fractional-Power-Law Level-Statistics due to Dynamical Tunneling

For systems with a mixed phase space we demonstrate that dynamical tunneling universally leads to a fractional power law of the level-spacing distribution P(s) over a wide range of small spacings s. Going beyond Berry-Robnik statistics, we take into account that dynamical tunneling rates between the regular and the chaotic region vary over many orders of magnitude. This results in a prediction of P(s) which excellently describes the spectral data of the standard map. Moreover, we show that the power-law exponent is proportional to the effective Planck constant h.Comment: 4 pages, 2 figure

Magnetoresistance of atomic-sized contacts: an ab-initio study

The magnetoresistance (MR) effect in metallic atomic-sized contacts is studied theoretically by means of first-principle electronic structure calculations. We consider three-atom chains formed from Co, Cu, Si, and Al atoms suspended between semi-infinite Co leads. We employ the screened Korringa-Kohn-Rostoker Green's function method for the electronic structure calculation and evaluate the conductance in the ballistic limit using the Landauer approach. The conductance through the constrictions reflects the spin-splitting of the Co bands and causes high MR ratios, up to 50%. The influence of the structural changes on the conductance is studied by considering different geometrical arrangements of atoms forming the chains. Our results show that the conductance through s-like states is robust against geometrical changes, whereas the transmission is strongly influenced by the atomic arrangement if p or d states contribute to the current.Comment: Revised version, presentation of results is improved, figure 2 is splitted to two figure

Gauge freedom for degenerate Bloch states

In nonmagnetic crystals with inversion symmetry the electronic bands are twofold degenerate. As a consequence, any orthonormalized linear combination of the two corresponding eigenfunctions can represent the electron wave function. A priori it is not obvious which superposition, gauge, should be chosen to calculate a quantity which is not gauge invariant within a certain approximation. Here we consider gauge options appropriate under particular physical conditions

Resistivity due to low-symmetrical defects in metals

The impurity resistivity, also known as the residual resistivity, is calculated ab initio using multiple-scattering theory. The mean-free path is calculated by solving the Boltzmann equation iteratively. The resistivity due to low-symmetrical defects, such as an impurity-vacancy pair, is calculated for the FCC host metals Al and Ag and the BCC transition metal V. Commonly, 1/f noise is attributed to the motion of such defects in a diffusion process.Comment: 24 pages in REVTEX-preprint format, 10 Postscript figures. Phys. Rev. B, vol. 57 (1998), accepted for publicatio