Surface energies for different GaAs surface orientations have been calculated as a function of the chemical potential. We use an energy density formalism within the first-principles pseudopotential density-functional approach. The equilibrium crystal shape has been derived from the surface energies for the ͑110͒, ͑100͒, ͑111͒, and ͑1 1 1͒ orientations. Under As-rich conditions all four considered surface orientations exist in thermodynamic equilibrium, in agreement with experimental observations. Moreover, our calculations allow us to decide on previous contradictory theoretical values for the surface energies of the ͑111͒ and ͑1 1 1͒ facets.

Typically surface core-level shifts (SCLS) of clean surfaces are explained in the initial-state model, thus ignoring the screening of the photon-induced hole. We will show that this approach is not valid for the (001) surfaces of Si and Ge. Using ab initio density-functional theory we calculate the SCLS from differences of total energies of slabs containing excited atoms at different positions at the surface and in the bulk. Comparison with initial-state results reveals an enhanced screening at the surface, which is even remarkably different for the two atoms forming the surface dimer.

Low-temperature scanning tunneling spectroscopy of magnetic and non-magnetic metal atoms on Ag(111) and on Cu(111) surfaces reveals the existence of a common electronic resonance at an energy below the binding energies of the surface states. Using an extended Newns-Anderson model, we assign this resonance to an adsorbate-induced bound state, split off from the bottom of the surface-state band, and broadened by the interaction with bulk states. A lineshape analysis of the bound state indicates that native adatoms decrease the surface-state lifetime, while a cobalt adatom causes no significant change. PACS numbers: 73.20.Fz, 68.37.Ef, 72.15.Qm The unique ability of scanning tunneling microscopy and spectroscopy (STS) to access locally the density of states of single adsorbed atoms and clusters has been recently used to investigate magnetic adatoms which -owing to the Kondo effect -exhibit a sharp spectroscopic structure close to the Fermi energy E F [1,2,3,4,5,6]. Surprisingly, only few studies of metal surfaces have been reported for non-magnetic adatoms and for a wider energy range around the Fermi level [4,7,8,9,10]. There is a good reason to explore the physics beyond a narrow range around E F . For instance, it is known that a localized attractive perturbation of a two-dimensional electron gas should result in the appearance of a bound state, split off from the bottom of the continuum, and with a wave function localized around the perturbation [11]. Surface and image-potential states represent an opportunity to investigate this scenario with STS. In fact, it has been predicted that bound states should appear around single alkali atoms on metal surfaces as a consequence of their attractive perturbation on these twodimensional electron gases [12].In this Letter, we present a comparative lowtemperature STS study of a set of adsorbates on the Ag(111) and the Cu(111) surfaces. We show that the density of states (DOS) of single silver and cobalt atoms adsorbed on Ag(111), as well as single copper and cobalt atoms adsorbed on Cu(111), exhibit a resonance below the binding energy E 0 of the surface states of these substrates. Within the framework of a Newns-Anderson model extended to a two-band interaction, we assign this resonance to a bound state split-off from the bottom edge of the surface-state band. A lineshape analysis suggests that the scattering of the surface state at the adsorbate affects its lifetime, depending on the adatom nature. This appears to be a general property of atoms interacting with a two-dimensional electron gas. The measurements were performed in a homebuilt ultrahigh vacuum scanning tunneling microscope at a working temperature of T = 4.6 K. The Ag(111) and the Cu(111) surfaces were cleaned by Ar + sputter/anneal cycles. The single copper and silver adatoms were created by controlled tip-sample contact, whereas the single cobalt atoms were evaporated onto the cold substrates by heating a degassed cobalt wire wound around a pure W wire (> 99.95 %). The evaporation, through an opening of ...

The equilibrium shapes of InAs quantum dots (i.e., dislocation-free, strained islands with sizes ≥ 10, 000 atoms) grown on a GaAs (001) substrate are studied using a hybrid approach which combines density functional theory (DFT) calculations of microscopic parameters, surface energies, and surface stresses with elasticity theory for the long-range strain fields and strain relaxations. In particular we report DFT calculations of the surface stresses and analyze the influence of the strain on the surface energies of the various facets of the quantum dot. The surface stresses have been neglected in previous studies. Furthermore, the influence of edge energies on the island shapes is briefly discussed. From the knowledge of the equilibrium shape of these islands, we address the question whether experimentally observed quantum dots correspond to thermal equilibrium structures or if they are a result of the growth kinetics.

In ab initio pseudopotential calculations within density-functional theory the nonlinear exchangecorrelation interaction between valence and core electrons is often treated linearly through the pseudopotential. We discuss the accuracy and limitations of this approximation regarding a comparison of the local density approximation (LDA) and generalized gradient approximations (GGA), which we find to describe core-valence exchange-correlation markedly different. (1) Evaluating the binding properties of a number of typical solids we demonstrate that the pseudopotential approach and namely the linearization of core-valence exchange-correlation are both accurate and limited in the same way in GGA as in LDA. (2) Examining the practice to carry out GGA calculations using pseudopotentials derived within LDA we show that the ensuing results differ significantly from those obtained using pseudopotentials derived within GGA. As principal source of these differences we identify the distinct behavior of core-valence exchange-correlation in LDA and GGA which, accordingly, contributes substantially to the GGA induced changes of calculated binding properties.