Quantum Theory of Advanced Materials

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].

Real-time in situ x-ray studies of continuous Pb deposition on Si(111)-(7x7) at 180 K reveal an unusual growth behavior. A wetting layer forms first to cover the entire surface. Then islands of a fairly uniform height of about five monolayers form on top of the wetting layer and grow to fill the surface. The growth then switches to a layer-by-layer mode upon further deposition. This behavior of alternating layer and island growth can be attributed to spontaneous quantum phase separation based on a first-principles calculation of the system energy.

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.

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).

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.

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].

We have performed calculations of adsorption energetics on the graphene surface using the state-of-the-art diffusion quantum Monte Carlo method. Two types of configurations are considered in this work: the adsorption of a single O, F, or H atom on the graphene surface and the H-saturated graphene system (graphane). The adsorption energies are compared with those obtained from density functional theory with various exchange-correlation functionals. The results indicate that the approximate exchange-correlation functionals significantly overestimate the binding of O and F atoms on graphene, although the preferred adsorption sites are consistent. The energy errors are much less for atomic hydrogen adsorbed on the surface. We also find that a single O or H atom on graphene has a higher energy than in the molecular state, while the adsorption of a single F atom is preferred over the gas phase. In addition, the energetics of graphane is reported. The calculated equilibrium lattice constant turns out to be larger than that of graphene, at variance with a recent experimental suggestion.

We have used the combination of the coupling-constant integration procedure and the variational quantum Monte Carlo method to study the exchange-correlation (XC) interaction in small molecules: Si-2, C2H2, C2H4, and C2H6. In this paper we report the calculated XC energy density, a central quantity in density functional theory, as deduced from the interaction between the electron and its XC hole integrated over the interaction strength. Comparing these "exact" XC energy densities with results using the local-density approximation (LDA), one can analyze the errors in this widely used approximation. Since the XC energy is an integrated quantity, error cancellation among the XC energy density in different regions is possible. Indeed we find a general error cancellation between the high-density and low-density regions. Moreover, the error distribution of the exchange contribution is out of phase with the error distribution of the correlation contribution. Similar to what is found for bulk silicon and an isolated silicon atom, the spatial variation of the errors of the LDA XC energy density in these molecules largely follows the sign and shape of the Laplacian of the electron density. Some noticeable deviations are found in Si-2 in which the Laplacian peaks between the atoms, while the LDA error peaks in the regions "behind" atoms where a good portion of the charge density originates from an occupied 1 sigma(u) antibonding orbital. Our results indicate that, although the functional form could be quite complex, an XC energy functional containing the Laplacian of the energy is a promising possibility for improving LDA.

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Using first-principles calculations within density functional theory, we have investigated the electronic properties of H-passivated Si nanowires (SiNWs) oriented along the 112 direction, with the atomic geometries retrieved via global search using genetic algorithm. We show that [112] SiNWs have an indirect band gap in the ultrathin diameter regime, whereas the energy difference between the direct and indirect fundamental band gaps progressively decreases as the wire size increases, indicating that larger [112] SiNWs could have a quasi-direct band gap. We further show that this quasi-direct gap feature can be enhanced when applying uniaxial compressive stress along the wire axis. Moreover, our calculated results also reveal that the electronic band structure is sensitive to the change of the aspect ratio of the cross sections.

Angle-resolved photoemission is employed to measure the band structure of TiSe2 in order to clarify the nature of the (2x2x2 ) charge density wave transition. The results show a very small indirect gap in the normal phase transforming into a larger indirect gap at a different location in the Brillouin zone. Fermi surface topology is irrelevant in this case. Instead, electron-hole coupling together with a novel indirect Jahn-Teller effect drives the transition.

Angle-resolved photoemission has been utilized to study the surface electronic structure of 1/3 monolayer of Sn on Ge(lll) in both the room-temperature (root3 x root3)R30 degrees phase and the low-temperature (3 x 3) charge-density-wave phase. The results reveal a gap opening around the (3 x 3) Brillouin zone boundary, suggesting a Peierls-like transition despite the well-documented lack of Fermi nesting, a highly sensitive electronic response to doping by intrinsic surface defects is the cause for this unusual behavior, and a detailed calculation illustrates the origin of the (3 x 3) symmetry.

In this paper, we present the direct observation of quantum size effects (QSE) on the work function in ultrathin Pb films. By using scanning tunneling microscopy and spectroscopy, we show that the very existence of quantum well states (QWS) in these ultrathin films profoundly affects the measured tunneling decay constant kappa, resulting in a very rich phenomenon of "quantum oscillations" in kappa as a function of thickness, L, and bias voltage, V(s). More specifically, we find that the phase of the quantum oscillations in kappa vs. L depends sensitively upon the bias voltage, which often results in a total phase reversal at different biases. On the other hand, at very low sample bias (vertical bar V(s)vertical bar < 0.03 V) the measurement of kappa vs. L accurately reflects the quantum size effect on the work function. In particular, the minima in the quantum oscillations of kappa vs. L occur at the locations where QWS cross the Fermi energy, thus directly unraveling the QSE on the work function in ultrathin films, which was predicted more than three decades ago. This further clarifies several contradictions regarding the relationship between the QWS locations and the work function.

We studied the hydrogen-induced reconstruction of the W(110) surface using the pseudopotential plane wave approach. The calculations for a full monolayer of hydrogen coverage showed that the quasithreefold hollow site (distorted bridge) has the lowest energy, and that for this geometry a surface reconstruction, consisting of a small uniform shift of the W top layer in the [1(1) over bar0$] direction, is energetically favorable. We also studied the surface states for clean and H-covered W(110) and investigated the effect of the reconstruction on electronic structure.

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].

Both the size and the magnetic-field dependences of low-lying spectra of two-dimensional (2D) graphene based magnetic dot and ring in perpendicular inhomogeneous magnetic fields, where the magnetic field is zero inside the dot and ring, and constant elsewhere, are studied by the massless Dirac-Weyl equation. Numerical results obtained from direct diagonalization with 2D harmonic basis show that, under nonuniform magnetic fields, the higher Landau levels (N >= 1) for such massless Dirac electron interacting system in general become nondegenerate and split into discrete angular momentum states with level crossings with the lowest one (N=0) being an exception. (C) 2010 American Institute of Physics. [doi:10.1063/1.3435478]