An approximate MO-LCAO-SCF method including overlap

Molecular orbital method based on atomic self consistent field functions and applicable where pi electron restrictions not fulfille

Electron-phonon interaction in Graphite Intercalation Compounds

Motivated by the recent discovery of superconductivity in Ca- and Yb-intercalated graphite (CaC$_{6}$ and YbC$_{6}$) and from the ongoing debate on the nature and role of the interlayer state in this class of compounds, in this work we critically study the electron-phonon properties of a simple model based on primitive graphite. We show that this model captures an essential feature of the electron-phonon properties of the Graphite Intercalation Compounds (GICs), namely, the existence of a strong dormant electron-phonon interaction between interlayer and $\pi ^{\ast}$ electrons, for which we provide a simple geometrical explanation in terms of NMTO Wannier-like functions. Our findings correct the oversimplified view that nearly-free-electron states cannot interact with the surrounding lattice, and explain the empirical correlation between the filling of the interlayer band and the occurrence of superconductivity in Graphite-Intercalation Compounds.Comment: 13 pages, 12 figures, submitted to Phys. Rev.

Development of Bond-Order Potentials for Body-Centered-Cubic Transition Metals and Their Application in atomistic Studies of Plastic Properties Mediated by Dislocations

ABSTRACT DEVELOPMENT OF BOND-ORDER POTENTIALS FOR BODY-CENTERED-CUBIC TRANSITION METALS AND THEIR APPLICATION IN ATOMISTIC STUDIES OF PLASTIC PROPERTIES MEDIATED BY DISLOCATIONS Yi-Shen Lin Professor Vaclav Vitek Bond-order potentials (BOPs), based on the tight binding (TB) approach for the evaluation of bonding, are an real-space method. They are eminently suitable for atomistic simulations of extended defects in transition metals in which the bonding is mixed nearly free electron and covalent. The latter requires a rigorous quantum mechanical treatment performed within the TB. In this Thesis, new BOPs were developed for non-magnetic BCC transition metals, V, Nb, Ta, Cr, Mo and W, as well as for the ferromagnetic Fe. In these BOPs, bond integrals used in the bond part of the cohesive energy were directly extracted from DFT calculations employing a projection formalism and a physically more transparent functional form was established for the repulsive part of the cohesive energy. In the ferromagnetic Fe, the magnetism was introduced through the Stoner’s model of the itinerant magnetism. In the bond part of the cohesive energy only d bonds are included explicitly but the screening of these bonds by the surrounding s electrons needs to be taken into account. This is particularly important when studying atomic arrangements in which the deviation from the ideal BCC lattice is very localized and inhomogeneous. The developed BOPs show an excellent transferability to various atomic environments which was tested by calculating energies of alternative crystal structures, vacancy formation energies, transformation paths, phonon spectra and -surfaces, all of which allow for direct comparisons with experiments and/or DFT based calculations. Moreover, we show that with slight variation of the number of d electrons used in BOPs, they are suitable for the atomistic studies involving self-interstitial atoms. This is essential if the BOPs are to be used in studies of the radiation damage. Employing these BOPs, the core structures and glide of ½\u3c111\u3e screw dislocations, which govern the plastic deformation in BCC metals, were investigated using a variety of applied stress tensors. These simulations reveal a breakdown of the Schmid law that has two aspects. First is the so-called twinning-antitwinning asymmetry of the critical resolved shear stress, which has been known for a long time and occurs even when the applied stress is the pure shear stress parallel to the Burgers vector. The second relates to core transformations induced by the shear stress components perpendicular to the Burgers vector. Our simulations suggest that the latter may explain the anomalous slip found in a number of BCC transition metals, which has been known for a long time but its full understanding is still elusive. Most importantly, the calculations employing BOPs suggest significantly different anomalous slip for group 5 (V, Nb and Ta) and group 6 (Mo and W) metals, which is observed but has never been explained based on the standard continuum theory of dislocations. Finally, a very simple formalism was proposed for the development of BOPs for binary homogeneous substitutional alloys that leads to a good agreement with DFT calculations of basic structural and mechanical properties. Studies of ½[111] screw dislocations in Ta-W alloy confirm the applicability of this formalism to investigation of the effect of substitutional alloying on the dislocation glide

Green function techniques in the treatment of quantum transport at the molecular scale

The theoretical investigation of charge (and spin) transport at nanometer length scales requires the use of advanced and powerful techniques able to deal with the dynamical properties of the relevant physical systems, to explicitly include out-of-equilibrium situations typical for electrical/heat transport as well as to take into account interaction effects in a systematic way. Equilibrium Green function techniques and their extension to non-equilibrium situations via the Keldysh formalism build one of the pillars of current state-of-the-art approaches to quantum transport which have been implemented in both model Hamiltonian formulations and first-principle methodologies. We offer a tutorial overview of the applications of Green functions to deal with some fundamental aspects of charge transport at the nanoscale, mainly focusing on applications to model Hamiltonian formulations.Comment: Tutorial review, LaTeX, 129 pages, 41 figures, 300 references, submitted to Springer series "Lecture Notes in Physics