Mark Mostoller

Evidence for a new class of solids. First-principles study of K(Al13)

Abstract The stability of the crystalline phase of a cluster-assembled solid K(Al 13 ) has been investigated using first-principles total energy calculations. We find that K(Al 13 ) may form in the CsCl structure with a lattice constant of 6.52 A. Unlike the gas phase, in which the ground state of the Al 13 cluster is icosahedral, the Al 13 becomes cuboctahedral in the solid phase due to crystal field effects. The system is metallic and is stable against lattice distortions. The calculations suggest that a new metastable solid could be made from two immiscible elements through specially designed synthesis processes.

Temperature dependence of the elastic constants of Ni: reliability of EAM in predicting thermal properties

The temperature dependence of the elastic constants of Ni is calculated using molecular dynamics (MD) simulations in conjunction with the embedded atom method (EAM). The Parrinello - Rahman version of molecular dynamics is employed along with the fluctuation formulae in the and EhN ensembles at various temperatures from 0 K to somewhat below the melting point (experimental value 1725 K). The calculated results for the elastic constants, compressibility, linear coefficient of thermal expansion, specific heat and the melting temperature compare reasonably well to experiment.

Ordering of As impurities in a Si dislocation core

We demonstrate by ab initio calculations that segregation of As in a dislocation core in Si occurs in the form of an ordered chain of As atoms running along the dislocation pipe. All As atoms in the chain achieve threefold coordination and the segregation energy is close to 1 eV per As atom.

A spin polaron model of high-Tc superconductivity

Abstract Spin-polaron model has been studies as a basis for high- T c superconductivity. Parameterized antiferromagnetic band calculations for a CuO 2 plane show how a Mott-Hubbard (M-H) gap opens as a consequence of an electron repulsion energy U at the Cu sites. The parameters are chosen to give good agreement with measured density of states and the magnetic moment per Cu site. Variation of the hole concentration by Sr doping in La 2− x Sr x CuO 4 and O depletion in YBa 2 Cu 3 O 7− x moves the Fermi level relative to the M-H band edge. The holes induce spin deviations on the neighboring Cu sites to form small spin polarons, with effective masses several times the band masses. Two spin polarons have an attractive interaction because the spin deviations are partially “healed” when the polarons approach one another. The proximity of the Fermi level to the M-H band edge and the interplay of O 2pσ and 2pπ bands can provide fits to the variations of T c with x in La 2− x Sr x CuO 4 and YBa 2 Cu 3 O 7− x . A discussion of various aspects and implications of the model is given.

Pair potentials at simple metal surfaces

Abstract Screened pair potentials at simple metal surfaces are treated within linear response theory. Quantum interference effects and terms that do not depend on the shape of the surface potential energy barrier are separated in a clear fashion.

Triangular step instability and 2D/3D transition during the growth of strained Ge films on Si(100)

We show that an activation energy barrier exists to the formation of wavy step edges due to stress-driven 2D instability. The barrier height and the barrier width depend sensitively on the surface stress anisotropy and step free energy. The large misfit strain of Ge films significantly reduces the barrier by lowering the S{sub B} step energy, inducing S{sub A} steps to undergo a triangular instability even during low temperature growth of Ge on Si(100). The step instability results in a novel arrangement of stress domains, and the interaction between the domains causes a spatial variation of surface strain with a surprisingly large influence on the energy barrier for island nucleation. Calculations indicate a dramatic enhancement in the nucleation of 3D islands at the apex regions of triangular steps, in good agreement with our experimental measurements.

Large scale simulations of melting in two dimensional Lennard-Jones systems: Evidence for a metastable hexatic phase

Large scale molecular dynamics simulations have been performed for two-dimensional Lennard-Jones systems on massively parallel computers. The calculations were done in the NPT ensemble as recently reformulated by Melchionna et al., avoiding problems associated with mixed phases and artificial treatment of vacancies and interstitials. In the largest systems studied (36864 and 102400 atoms), a metastable hexatic phase was found between solid and liquid, in agreement with the predictions of the theory of melting in two dimensions developed by Kosterlitz and Thouless, Halperin and Nelson, and Young. The hexatic phase was not seen in smaller samples, calling into question the conclusions of many previous simulations performed on smaller systems and shorter time scales.

Simulations of the Dislocation Array at Ge/Si Interfaces

When Ge is grown epitaxially on Si(001), the 4% mismatch between the lattice parameters of Ge and Si can produce a regular two-dimensional grid of (a/2) [1,{plus_minus}1,0] edge dislocations at the interface, a checkerboard with a spacing of {approximately} 100 {Angstrom}. We have performed classical molecular dynamical simulations of this checkboard in large microcrystals. Results show the expected 5-fold plus 7-fold ring structure at the cores of the individual dislocations, and a new closed symmetric structure of 18 atoms at their intersections. Tetrahedral coordination is everywhere retained, with relatively small changes in the bond lengths of less than 10 and in the bond angles of less than 25%. The energetics and dislocation offset of the system are explored for the Stillinger-Weber and Tersoff potentials.