476 research outputs found
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 and ZT 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
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
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
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
Influence of strain on anisotropic thermoelectric transport of BiTe and SbTe
On the basis of detailed first-principles calculations and semi-classical
Boltzmann transport, the anisotropic thermoelectric transport properties of
BiTe and SbTe 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 BiTe_2_3_2_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
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
Automatic one-loop calculation of MSSM processes with GRACE
We have developed the system for the automatic computation of cross-sections,
{\tt GRACE/SUSY}, including the one-loop calculations for processes of the
minimal supersymmetric extension of the the standard model. For an application,
we investigate the process .Comment: 4 pages, 1 figure, Talk presented by Jimbo, M. at ACAT-0
Searching for Si-based spintronics by first principles calculations
Density functional theory (DFT) calculations are used to study the epitaxial growth and the magnetic properties of thin films of MnSi on the Si(001) surface. For adsorption of a single Mn atom, we find that binding at the subsurface site below the Si surface dimers is the most stable adsorption site. There is an energy barrier of only 0.3 eV for adsorbed Mn to go subsurface, and an energy barrier of 1.3 eV for penetration to deeper layers. From the calculated potential-energy surface for the Mn adatom we conclude that the most stable site on the surface corresponds to the hollow site where Mn is placed between two Si surface dimers. Despite Si(001) geometrically being an anisotropic surface, the on-surface diffusion for both directions along and perpendicular to the Si dimer rows has almost the same diffusion barrier of 0.65 eV. For coverage above 1 ML, the lowest energy structure is a pure Mn subsurface layer, capped by a layer of Si adatoms. We conclude that the Mn-silicide films stabilize in an epitaxially CsCl-like (B2) crystal structure. Such MnSi films are found to have sizable magnetic moments at the Mn atoms near the surface and interface, and ferromagnetic coupling of the Mn clarify within the layers. Layer-resolved electronic densities-of-states are presented that show a high degree of spin polarization at the Fermi level, up to 30 and 50% for films with one or two MnSi films, respectively. In order to clarify the stability of ferromagnetism at finite temperatures we estimate the Curie temperature (Tc) of MnSi films using a multiple-sublattice Heisenberg model with first- and second-nearest neighbor interactions determined from DFT calculations for various collinear spin configurations. The Curie temperature is calculated both in the mean-field approximation (MFA) and in the random-phase approximation (RPA). In the latter case, we find a weak logarithmic dependence of Tc on the magnetic anisotropy parameter, which was calculated to be 0.4 meV. Large Curie temperatures of above 200K for a monolayer MnSi film, and above 300K for a 2ML MnSi film are obtained within the RPA, and even higher values in MFA. Complementary calculations are performed for non-collinear spin structures to study the limitations of the mapping of the system onto a Heisenberg model. We demonstrate that biquadratic interatomic exchange interactions and longitudinal fluctuations of atomic moments give important contributions to the energetics of the system
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