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The electronic and topological properties of heterovalent and heterocrystalline a-Sn/CdTe(001) quantum wells (QWs) are studied in dependence on the thickness of α-Sn by means of ab initio calculations. We calculate the topological Z2 invariants of the respective bulk crystals, which identify α-Sn as strong three-dimensional (3D) topological insulators (TIs), whereas CdTe is a trivial insulator. We predict the existence of two-dimensional (2D) topological interface states between both materials and show that a topological phase transition from a trivial insulating phase into the quantum spin Hall (QSH) phase in the QW structures occurs at much higher thicknesses than in the HgTe case. The QSH effect is characterized by the localization, dispersion, and spin polarization of the topological interface states. We address the distinction of the 3D and 2D TI characters of the studied QW structures, which is inevitable for an understanding of the underlying quantum state of matter. The 3D TI nature is characterized by two-dimensional topological interface states, while the 2D phase exhibits one-dimensional edge states. The two different state characteristics are often intermixed in the discussion of the topology of 2D QW structures, especially, the comparison of ab initio calculations and experimental transport studies. [ABSTRACT FROM AUTHOR]

We investigate structural and electronic properties of graphene-based one-dimensional (1D) superlattices by means of first-principles calculations. The 1D superlattices are modeled by three different chemical and structural modifications of the carbon lattice. We study partial hydrogenation of graphene, substitution of carbon-atom pairs by boron and nitrogen atoms, and incorporation of ribbons of the recently discovered silicene. Our investigations based on density functional theory provide accurate predictions of structural and electronic properties of all systems under consideration. In detail, the formation of 1D interfaces between two-dimensional honeycomb crystals is studied, including the construction of a coincidence lattice. The importance of the interface for the energy alignment and the Fermi-level position is proven. Finally, we discuss the results in light of the possible realization of a quasi-1D metal. [ABSTRACT FROM AUTHOR]

Generally, geothermal production and injection wells of a geothermal doublet have different performances. To maximize the energy production rate the well with higher performance should be used for production. One of the disadvantages of this option is the increase of costs for energy during production caused by the need to increase injection pressure over time. A potential method to enhance well performance by means of increasing the permeability of carbonate rocks is the injection of CO2- enriched water during heat and/or electrical power production. Geoenergie Bayern GmbH and the Department of Hydrogeology of the Technische Universität Bergakademie Freiberg in cooperation with Hydroisotop GmbH have started a research project "Long-term improvement and maintenance of reservoir-pathways in deep geothermal energy" [Langfristige Verbesserung und Erhaltung von Reservoir- Wegsamkeiten in der Tiefen Geothermie (LERWTG)] to investigate the potential and the opportunities for CO2 injection at the geothermal pilot project Kirchweidach. Reactive transport modeling was done using TOUGHREACT to simulate the use of CO2 for prevention of scaling and enhancement of permeability. Simulation results show that the injection of CO2 reduces the pH which in turn inhibits precipitation inside the well and also in the aquifer pathways. [ABSTRACT FROM AUTHOR]

The deposition of silicene on several metals is investigated. For fcc crystals the (111) surfaces while for hexagonal ones the (0001) surfaces are used. The Ca(111)1 × 1 substrate is found to be the most promising candidate. The silicene adsorption on Ca-functionalized Si(111)1 × 1 and 2 × 1 surfaces is also studied. The 1 × 1 substrates lead to overlayer silicene with hexagonal symmetry and Dirac cones. However, the Dirac points are below the Fermi level, and small energy gaps are opened. In the case of 2 × 1 surfaces, strong lattice relaxation occurs. Only rudiments of conical linear bands remain visible. [ABSTRACT FROM AUTHOR]

xThe α phase of tin is a zero-gap semiconductor with an inverted band structure with respect to other group-IV elements like Ge. The Γ6c states lie energetically below the Γ8v levels. How these unique electronic properties transform in nanostructures with spatial confinement has not been studied. We apply density-functional theory within the local density approximation to investigate the energetic, structural, and electronic properties of bulk α-Sn and its nanocrystals (NCs) up to a size of 363 Sn atoms. For NCs with larger diameters up to 14 nm the tight-binding method is applied for the electronic states. Spin-orbit coupling is taken into account. The clusters are modeled in such a way that the Td symmetry of the bulk system is conserved. Their surfaces are passivated with hydrogen. We show that the spatial confinement causes not only a decrease of the fundamental gap for increased NC size but also a topological transition where the ordering of s- and p-like highest-occupied molecular orbital and lowest-unoccupied molecular orbital states is interchanged. The influence of quasiparticle and excitonic effects on the lowest pair excitation energies is investigated within approximations based on the hybrid exchange-correlation functional by J. Heyd, G. E. Scuseria, and M. Ernzerhof [ J. Chem. Phys. 118 8207 (2003)] (HSE) and the ΔSCF method. [ABSTRACT FROM AUTHOR]