Native defects in the Co2_2TiZZ (Z=Z= Si, Ge, Sn) full Heusler alloys: formation and influence on the thermoelectric properties

We have performed first-principles investigations on the native defects in the full Heusler alloys Co$_2$Ti$Z$ ($Z$ one of the group IV elements Si, Ge, Sn), determining their formation energies and how they influence the transport properties. We find that Co vacancies (Vc) in all compounds and the Ti$_\text{Sn}$ anti-site exhibit negative formation energies. The smallest positive values occur for Co in excess on anti-sites (Co$_Z$ or Co$_\text{Ti}$) and for Ti$_Z$. The most abundant native defects were modeled as dilute alloys, treated with the coherent potential approximation in combination with the multiple-scattering theory Green function approach. The self-consistent potentials determined this way were used to calculate the residual resistivity via the Kubo-Greenwood formula and, based on its energy dependence, the Seebeck coefficient of the systems. The latter is shown to depend significantly on the type of defect, leading to variations that are related to subtle, spin-orbit coupling induced, changes in the electronic structure above the half-metallic gap. Two of the systems, Vc$_\text{Co}$ and Co$_Z$, are found to exhibit a negative Seebeck coefficient. This observation, together with their low formation energy, offers an explanation for the experimentally observed negative Seebeck coefficient of the Co$_2$Ti$Z$ compounds as being due to unintentionally created native defects

Spin caloric transport and related phenomena from first principles

The present dissertation gives an account of a first-principles description of linear response phenomena in the field of spin caloric transport or spincaloritronics and closely related areas of condensed matter physics and materials science. Embed- ded into the priority program SPP 1538 “Spin Caloric Transport (SpinCaT)” of the DFG, the major aim was the development and application of computational methods suitable for a parameter-free determination of material specific response coefficients in metals and alloys. The underlying band structure method, the Korringa-Kohn- Rostoker multiple scattering framework, provides an ideal basis for an accurate and versatile representation of the systems under study in terms of the single-electron Green function. Most importantly, this quantity serves as the fundamental variable in Kubo’s linear response formalism that is employed for the calculation of charge and spin conductivities as well as related electric-field-induced response coefficients. Using and extending an approach to temperature-induced phenomena based on the energy dependence of the electrical conductivity put forward by Mott, the diffusion contribution to thermoelectric and spincaloritronic response properties can be as- sessed. Making furthermore use of an efficient description of substitutional as well as thermally-induced disorder by means of the coherent potential approximation and the closely-related alloy analogy model for finite-temperature effects, a reliable description of charge-, heat-, and spin-related transport properties of dilute as well as concentrated alloys under realistic conditions can be achieved. The methodol- ogy just outlined has been applied to various thermoelectric and spincaloritronic phenomena such as the Seebeck effect and its magnetic anisotropy, the anoma- lous and spin Nernst effects as well as to the corresponding electric-field-induced material properties: anisotropic magnetoresistance, anomalous and spin Hall con- ductivity. In addition so-called spinorbitronic responses have been studied, namely the spin-orbit torque and the Edelstein or inverse spin-galvanic effect. Accompa- nying space-time symmetry considerations on the transformation properties of the respective operator-operator correlation functions in terms of the Kubo formula al- lowed the model-independent determination of symmetry-restricted response tensor shapes for direct and inverse effects, based on the crystallographic and magnetic structure. The predictions of the occurrence and relation of tensor elements were verified numerically by first-principles calculations in collinear, non-collinear as well as non-coplanar magnetic configurations, providing in addition reliable estimates of their magnitude.Die vorliegende Arbeit erstattet Bericht über eine ab initio-Beschreibung linearer Antwortphänomene im Feld des spinkalorischen Transports oder der Spinkaloritronik und eng verwandter Bereiche der Festkörperphysik und Materialwissenschaften. Eingebettet in das Schwerpunktprogramm SPP 1538 Spin Caloric Transport (Spin- ” CaT)“ der DFG, war das Hauptanliegen die Entwicklung und Anwendung rechner- gestützter Verfahren zur parameterfreien Bestimmung materialspezifischer Antwort- größen von Metallen und Legierungen. Das zugrundeliegende Bandstrukturver- fahren, die Korringa-Kohn-Rostoker-Vielfachstreumethode, stellt eine ideale Basis für die akkurate und vielseitige Darstellung der zu beschreibenden Systeme durch ihre Ein-Elektronen-Greensche Funktion dar. Insbesondere dient diese Größe als fundamentale Variable in Kubos linearem Antwortformalismus, der zur Berechnung elektrischer und spinpolarisierter Leitfähigkeiten sowie verwandter, ebenfalls durch ein elektrisches Feld hervorgerufene Prozesse beschreibende, Koeffizienten verwendet wird. Durch Anwendung und Erweiterung eines auf Mott zurückgehenden Ansatzes zur Beschreibung thermisch induzierter Phänomene mittels der Energieabängigkeit der elektrischen Leitfähigkeit, kann der diffusive Anteil thermoelektrischer und spin- kaloritronischer Antwortgrößen ermittelt werden. Des Weiteren kann unter Verwen- dung einer effizienten Beschreibung substitutioneller sowie thermisch induzierter Unordnung mittels der sogenannten Coherent Potential Approximation und des nahe verwandten Alloy Analogy Model für endliche Temperaturen eine verlässliche Beschreibung von Ladungs-, Wärme- und Spintransporteigenschaften in sowohl ver- dünnten als auch konzentrierten Legierungen unter realistischen Bedingungen er- reicht werden. Die soeben vorgestellte Methodik wurde auf eine Reihe thermoelek- trischer und spinkaloritronischer Phänomene wie den Seebeck-Effekt und seine ma- gnetische Anisotropie, den Anomalen und den Spin-Nernst-Effekt angewendet, sowie auf die entsprechenden elektrisch induzierten Materialeigenschaften, den anisotropen Magnetwiderstand, die anomale und die Spin-Hall-Leitfähigkeit. Zusätzlich wur- den sogenannte spinorbitronische Antwortprozesse untersucht, namentlich der Spin Orbit Torque und der Edelstein- oder inverse spingalvanische Effekt. Begleitende Überlegungen zur Raum-Zeit-Symmetrie der zugrundeliegenden Operator-Operator- Korrelationsfunktionen, ausgedrückt durch die Kubo-Formel, erlaubten eine modell- unabängige Bestimmung der symmetriekonformen Tensorstruktur für die direkten und inversen Effekte auf Basis der kristallographischen und magnetischen Struk- tur. Die Vorhersagen zum Auftreten von Tensorelementen und ihrer Beziehung wurden nummerisch durch ab initio-Berechnungen in kollinearen, nichtkollinearen, sowie nichtkoplanaren Anordnungen magnetischer Momente verifiziert. Gleichzeitig konnten hierdurch belastbare Aussagen über ihre Größe gemacht werden

Combining and Steganography of 3D Face Textures

One of the serious issues in communication between people is hiding information from others, and the best way for this, is deceiving them. Since nowadays face images are mostly used in three dimensional format, in this paper we are going to steganography 3D face images, detecting which by curious people will be impossible. As in detecting face only its texture is important, we separate texture from shape matrices, for eliminating half of the extra information, steganography is done only for face texture, and for reconstructing 3D face, we can use any other shape. Moreover, we will indicate that, by using two textures, how two 3D faces can be combined. For a complete description of the process, first, 2D faces are used as an input for building 3D faces, and then 3D textures are hidden within other images.Comment: 6 pages, 10 figures, 16 equations, 5 section

Tunable high-index photonic glasses

Materials with extreme photonic properties such as maximum diffuse reflectance, high albedo, or tunable band gaps are essential in many current and future photonic devices and coatings. While photonic crystals, periodic anisotropic structures, are well established, their disordered counterparts, photonic glasses (PGs), are less understood despite their most interesting isotropic photonic properties. Here, we introduce a controlled high index model PG system. It is made of monodisperse spherical TiO$_2$ colloids to exploit strongly resonant Mie scattering for optimal turbidity. We report spectrally resolved combined measurements of turbidity and light energy velocity from large monolithic crack-free samples. This material class reveals pronounced resonances enabled by the possibility to tune both the refractive index of the extremely low polydisperse constituents and their radius. All our results are rationalized by a model based on the energy coherent potential approximation, which is free of any fitting parameter. Surprisingly good quantitative agreement is found even at high index and elevated packing fraction. This class of PGs may be the key to optimized tunable photonic materials and also central to understand fundamental questions such as isotropic structural colors, random lasing or strong light localization in 3D.Comment: Main text: 8 pages, 4 figures; Supporting Information: 5 pages, 5 figure