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.

A series of GW calculations for zinc-blende Zn, Cd, and Hg chalcogenides ͑S, Se, and Te͒ is presented. The resulting quasiparticle gaps are 0.3-0.6 eV smaller than in experiment. The cation semicore states remain similarly underbined as in previous GW calculations for II B -VI materials. It is shown that application of the plasmon-pole model for screening leads to systematic and qualitative errors: the band gaps result larger and the occupied band widths undergo expansion, instead of contraction. Several steps, like including off-diagonal matrix elements of the self-energy, updating eigenvalues in the Green's function and screening, and using the special vertex corrections, are examined. We also propose a scheme to treat core corrections in the case when Zn 2+ , Cd 2+ , or Hg 2+ pseudopotentials are used in a GW calculation.

The popular local-density approximation neglects long-range correlations which, in the presence of the rapid rate of change of the electron density at the surface, lead to observable effects. We evaluate the exchange-correlation potential V xc for the electron gas-vacuum interface from the knowledge of the electron self-energy Z xc in the GW approximation. The electron-electron correlations built into I xc automatically produce an imagelike surface barrier. Our result for V xc is the basis of a nonlocal densityfunctional calculation of the electronic structure of Al(lOO) which yields a Rydberg series of image states from first principles. PACS numbers: 73.20.-r, 71.10,+x The major advances witnessed in the last two decades in the quantitative computation of ground-state properties of condensed matter systems are to a large extent due to the development of density-functional theory [1] into a powerful tool for dealing with the complicated system of 10 23 interacting electrons. Now, in the implementation of the density-functional scheme for a metal surface one must in principle account for the fact that the very presence of the surface introduces a source of inhomogeneity on a microscopic scale. However, in the widely used (and, for many purposes, very successful [2]) local-density approximation (LDA) [1], this feature of the surface problem is simply ignored in the treatment of the crucial electron-electron interactions.Because of its neglect of long-range correlations, the LDA gives rise to a surface barrier with a qualitatively incorrect asymptotic behavior (exponential decay, rather than the expected inverse power [3]). This failure of the LDA is experimentally relevant; new surface-sensitive techniques have produced a wealth of data on observables and processes influenced by the image tail of the surface barrier, such as binding energies and lifetimes of image potential-bound surface states [4], tunneling currents in the scanning-tunneling microscope [5], resonant-tunneling rates for ion-surface collisions [6], etc.In this Letter we report a first-principles evaluation of the exchange-correlation potential (V xc ) for the electron gas-vacuum interface. We proceed by solving an exact integral equation relating V xc and the electron self-energy. The main physical ingredients of the self-energy, namely, its energy dependence and long-range correlations, and their interplay with the extreme inhomogeneity

The one-electron excitation spectrum of the prototype semiconductor Si has been obtained from a firstprinciples calculation of the spectral-weight function A(q ជ ,) of the interacting one-electron Green's function. The Dyson equation has been solved with the self-energy operator obtained in the GW approximation, where the bare propagator G and the -dependent screening matrix W, without ͑random-phase approximation͒ and with ͑time-dependent local density approximation͒ vertex corrections, have been computed within Kohn-Sham-local-density-approximation theory. Positions of quasiparticle peaks ͑i.e., the ''band structure''͒, their lifetimes, and satellite ͑plasmaron͒ spectral structures are extracted in a broad energy range.

We have measured the plasmon dispersion of diamond along the high-symmetry directions using electron energy-loss spectroscopy in transmission. We found the plasmon dispersion to be considerably anisotropic. A comparison of the experimental results to ab initio calculations that take local-field effects into account demonstrates the importance of local-field effects for the dielectric response of systems with strongly inhomogeneous electron distributions.