Photoelectron-diffraction studies of Nb(001) have been performed to determine the first-interlayer contraction using Mg K alpha radiation (h nu = 1253.6 eV) as an excitation source. Photoemission intensities of the 3d(5/2) core level were measured as a function of the polar angle along several azimuths on the single-crystal surface. The 202.3-eV (205.0-eV) binding energy for the 3d(5/2) (3d(3/2)) core level was well resolved in the photoemission spectra, where the peak intensity could be easily evaluated by curve-fitting processes. Large oscillations of the 3d(5/2) intensity as a function of the polar angle due to forward-focusing were observed. Based on multiple-scattering calculations for several first-interlayer spacings ranging from the bulk value to 16% contraction, the best agreement with experiment was obtained for a (13 +/- 5)% contraction of the first-interlayer spacing.

When disilane (Si2H6) is used in the homoepitaxial growth of Si by chemical vapor deposition (CVD), the fragment SiH2 is believed to be the basic unit adsorbed on the surface. The bonding site of SiH2 on Si(100) has been proposed in the literature to be either on top of a dimer (the on-dimer site) or between two dimers in the same row (the intrarow site). Since the pathway of SiH2 combination is dependent on the adsorption site, a first-principles calculation will shed light on the underlying process. We have performed self-consistent pseudopotential density-functional calculations within the local-density approximation. On the bare Si(100) surface, the on-dimer site is found to be more stable than the intrarow site, even though the former has unfavorable Si-Si bond angles. This is ascribed to the extra dangling bond created in the latter geometry when the weak dimer a bonds are broken. However, the presence of hydrogen adatoms eliminates this difference and makes the intrarow site more favorable than the on-dimer site. It is therefore revealed in this theoretical study that hydrogen, an impurity unavoidable in the CVD process, plays an important role in determining the stable configuration of adsorbed SiH2 on Si(100) and hence affects the growth mechanism. [S0163-1829(98)52544-1].

Using classical mechanical and quantum Monte Carlo methods we trace the ground-state behavior with an applied magnetic field of localized electron pair states in a quantum dot. By developing a method to treat nonconserved paramagnetic interactions using variational and diffusion quantum Monte Carlo techniques we find (i) a single-triplet transition at very small magnetic field strengths, (ii) enhanced localization of the two electrons with increasing magnetic field, and (iii) a mechanism for pair breakup that is different from that proposed recently by Wan et al. [Phys. Rev. Lett. 75, 2879 (1995)]. [S0163-1829(98)04016-8].

We have evaluated the surface free energies of hydrogen-covered (100), (111), and (110) surfaces of diamond and silicon as a function of the hydrogen chemical potential using first-principles methods. The change in surface-energy anisotropy and equilibrium crystal shape due to hydrogen adsorption is examined. The three low-index facets are affected differently by the presence of hydrogen and unexpected differences are found between diamond and silicon. Taking into account possible formation of local facets on the hydrogen-covered (100) surfaces, we find that the dihydride phase is not stable on both C(100) and Si(100). nor is the 3x1 phase on C(100).

A combination of the coupling constant integration technique and the quantum Monte Carlo method is used to investigate the most relevant quantities in Kohn-Sham density-functional theory. Variational quantum Monte Carlo is used to construct realistic many-body wave functions for diamond-structure silicon at different values of the Coulomb coupling constant. The exchange-correlation energy density along with the coupling constant dependence and the coupling-constant-integrated form of the pair-correlation function, the exchange-correlation hole, and the exchange-correlation energy are presented. Comparisons of these functions an mode with results obtained from the local-density approximation, the average density approximation, the weighted density approximation, and the generalized gradient approximation. We discuss reasons for the success of the local-density approximation. The insights provided by this approach will make it possible to carry out stringent tests of the effectiveness of exchange-correlation functionals and in the long term aid in the search for better functionals. [S0163-1829(98)02115-8].

A model interaction is introduced for quantum many-body simulations of Coulomb systems using periodic I boundary conditions. The interaction gives much smaller finite size effects than the standard Ewald interaction and is also much faster to compute. Variational quantum Monte Carlo simulations of diamond-structure silicon with up to 1000 electrons demonstrate the effectiveness of our method.

The structural bases on the metal/semiconductor interfaces, such as gold trimers on the Au/Si (111)(root 3 x root 3)R30 degrees surface and antimony trimers on the Sb/Si(111)(root 3 x root 3)R30 degrees surface, can be imaged directly with a simple inversion of low-energy (<600 eV) Kikuchi-electron patterns (Kikuchi-electron holography-KEH). The relative positions of the building blocks (trimers) on the adsorbates to the substrate atoms are also determined. This short-range-order KEH tool, which provides the 3D Patterson function, can be viewed as a twin of grazing-incidence X-ray diffraction. Using direct structural information obtained by KEH, one can greatly reduce the tested models in a complete trial-and-error structural-determination process.

This paper gives a brief overview of the capability of modern electron-structure calculations, the widely used density-functional theory, and the challenge to search for the exact nonlocal exchange-correlation functional. The study of the thermal properties of silicon is used as an example to illustrate the accuracy accomplished by the state-of-the-art first-principles calculations. A recent attempt to extract quantities of central importance in density function theory via computational many-body techniques is also discussed.

Realistic many-body wave functions for diamond-structure silicon are constructed for different values of the Coulomb coupling constant. The coupling-constant-integrated pair correlation function, the exchange-correlation hole, and the exchange-correlation energy density are calculated and compared with those obtained from the local density and average density approximations. We draw conclusions about the reasons for the success of the local density approximation and suggest a method for testing the effectiveness of exchange-correlation functionals.

The bare and hydrogen-covered diamond (100) surfaces were investigated through pseudopotential density-functional calculations within the local-density approximation. Different hydrogen coverages, ranging from one to two, were considered. These corresponded to different structures (1x1, 2x1, and 3x1) and different hydrogen-carbon arrangements (monohydride, dihydride, and configurations in between). Assuming the system was in equilibrium with a hydrogen reservoir, the formation energy of each phase was expressed as a function of hydrogen chemical potential. As the chemical potential increased, the stable phase successively changed from bare 2x1 to (2x1):H, to (3x1):1.33H, and finally to the canted (1x1):2H. Setting the chemical potential at the energy per hydrogen in H-2 and in a free atom gave the (3x1):1.33H and the canted (1x1):2H phase as the most stable one, respectively. However, after comparing with the formation energy of CH4, only the (2x1):H and (3x1):1.33H phases were stable against spontaneous formation of CH4. The former existed over a chemical potential range ten times wider than the latter, which may explain why the latter, despite having a low energy, has not been observed so far. Finally, the vibrational energies of the C-H stretch mode were calculated for the (2x1):H phase.