Andrzej Fleszar

A self-consistent surface-Green-function (SSGF) method

Abstract We describe the basic aspects of a new, self-consistent Green-function method which allows to calculate the density of states, electron density, and related quantities for a localized perturbation (e.g. an isolated adsorbate or an intrinsic surface defect) at a crystal surface. The method is based on the density-functional theory and combines several ideas from recent theoretical developments, as, for example, from the layer-KKR Green-function method, from ab-initio pseudopotential theory, and from the self-consistent defect-Green-function method. Two applications of the method are presented (S on Pd(1 0 0) and Na on Al(1 0 0)) in order to demonstrate its efficiency and to address a recent controversial discussion concerning the nature of the bonding of alkali adsorbates on metals at very low coverage (Θ → 0).

Elemental topological insulator with tunable Fermi level: strained α-Sn on InSb(001).

We report on the epitaxial fabrication and electronic properties of a topological phase in strained α-Sn on InSb. The topological surface state forms in the presence of an unusual band order not based on direct spin-orbit coupling, as shown in density functional and GW slab-layer calculations. Angle-resolved photoemission including spin detection probes experimentally how the topological spin-polarized state emerges from the second bulk valence band. Moreover, we demonstrate the precise control of the Fermi level by dopants.

Self-consistent Green-function method for the calculation of electronic properties of localized defects at surfaces and in the bulk

The authors present a self-consistent Green-function method which enables parameter-free calculations of the charge density, the density of states, and related quantities in electronic systems where the three- or two-dimensional translational symmetry is broken by a perturbation which is localized in real space. In particular, the method is suited to study point defects in the interior of a metallic or semiconducting crystal or at a crystal surface. The self-consistent Green operator describes an infinitely extended system. The only restrictive assumption is that the self-consistent electronic structure of the unperturbed bulk material is well reproduced by a muffin-tin (pseudo) potential. In the perturbed region, however, no significant constraint is imposed either on the shape of the potential or on the charge density.

Double band inversion in α -Sn: Appearance of topological surface states and the role of orbital composition

The electronic structure of \graySn(001) thin films strained compressively in-plane was studied both experimentally and theoretically. A new topological surface state (TSS) located entirely within the gapless projected bulk bands is revealed by \textit{ab initio}-based tight-binding calculations as well as directly accessed by soft X-ray angle-resolved photoemission. The topological character of this state, which is a surface resonance, is confirmed by unravelling the band inversion and by calculating the topological invariants. In agreement with experiment, electronic structure calculations show the maximum density of states in the subsurface region, while the already established TSS near the Fermi level is strongly localized at the surface. Such varied behavior is explained by the differences in orbital composition between the specific TSS and its associated bulk states, respectively. This provides an orbital protection mechanism for topological states against mixing with the background of bulk bands.