Atomistic simulation highlights (2002)

This research is aimed at understanding the roles that surfaces and interfaces have on the properties of materials. Primarily, this work is separated into two areas: calculations of semiconductor surfaces and metal-semiconductor interfaces, and calculations of bulk interfaces such as twin boundaries and grain boundaries. Most of the computational work on semiconductor surfaces and metal-semiconductor interfaces will be carried out in collaboration with the experimental group (Tringides's group) at Ames laboratory. The second area is in collaboration with Man Yoo and Chong-Long Fu at Oak Ridge National Laboratory.

(a) First-principles density functional calculation of Pb/Si(111)

M. Hupalo, T. L. Chan, C. Z. Wang, K. M. Ho, and M. C. Tringides, Phys. Rev. B66, 161410(2002).

Upon deposition of Pb on Si(111) surface, variety of ordered structures have been observed by Michael Tringides's experimental group at Ames Lab. In collaboration with the experimental group, we have performed first-principles density functional calculations to study the energetics and electronic structures of Pb/Si(111) at different Pb coverage. Our calculation help experiment to identify the atomic model and domain wall arrangement for the controversial dense (coverage about 4/3 ML) Pb/Si(111)-R3xR3 phase.

(b) First-principles density functional calculation of mixed PbSi dimer chains on Si(001) surface

T. L. Chan, C. Z. Wang, Z. Y. Lu, and K. M. Ho, submitted

The Pb/Si(001) received considerable attention recently as a prototype system for atomic engineering nanoscale structures on semiconductor surfaces. Dimer chain structures have been observed by STM experiments when small amount of Pb deposited onto Si(001). It has been a common belief that the dimer-chain consists of pure Pb atoms. However, using first-principles density functional calculations, we show that the dimer-chain that consists of mixed PbSi dimers and runs perpendicular to the substrate dimer rows is energetically more stable than the pure Pb dimer-chains. We also show that the mixed dimer chain will appear buckled in the STM experiments while pure Pb dimer-chain will have symmetric STM image because the barrier for the "rocking" motion of the pure Pb chain is small (less than 0.31 eV).

(c) Tight-binding calculations of screw dislocation in Mo

J. Li, S. Yip and collaborators at MIT + C. Z. Wang and K. M. Ho (Ames)

We have recently initiated a collaboration with the group of Ju Li and Sidney Yip at MIT to study the structure and mobility of the screw dislocation in bcc transition metal (i.e., Mo). We have calculate the dislocation core structure in Mo. Comparison with available LDA calculation results indicates that the core structure predicted from our tight-binding calculations is consistent with that from LDA calculations. We will further examine the Peierls stress associated with the dislocation in order to understand the mechanisms of motion of the dislocation cores in this material.

(d) Molecular dynamic simulation of a homogeneous bcc to hcp phase

J. R. Morris and K. M. Ho, Phys. Rev. B63, 224116(2001); B. L. Zhang, C. Z. Wang, K. M. Ho, D. Turner, Y. Y. Ye, Phys. Rev. Lett. 74, 1375(1995).

We have performed molecular dynamic simulations of a martensitic bcc to hcp transformation in a homoge-neous system. The system evolves into three martensitic variants, sharing a common nearest-neighbor vector along a bcc <111> direction, plus an fcc region. Nucleation occurs locally, followed by subsequent growth. We monitor the time-dependent scattering S(Q,t) during the transformation, and find anomalous, Brillouin-zone-dependent scattering similar to that observed experimentally in a number of systems above the transformation temperature. This scattering is shown to be related to the elastic strain associated with the transformation and is not directly related to the phonon response.