Research Objectives
To develop first-principles quantum mechanical methods for addressing materials problems microscopically, especially the relationship between technical magnetic properties and microstructure. Major problems associated with this goal involve microstructure (independent of magnetism), magnetism (independent of microstructure), giant magneto-resistance, and thermal properties.
Computational Approach
A number of different first-principles techniques, including tight-binding molecular dynamics (TBMD), an iterative pseudopotential (IP) method, and the locally self-consistent multiple scattering (LSMS) method, are used to perform fundamental studies of the atomistic, electronic, and magnetic structure of microstructural defects in metals and semiconductors that involve the interactions between large numbers of atoms (TBMD 20,000 atoms, IP > 200, LSMS 250 to 3000 atoms). In addition we are developing spin dynamics based on both model Hamilitonians and local spin density calculations as a fundamental theory of the magnetic properties of metals and alloys.
Accomplishments
During this year our codes have been ported to the Cray T3E. All of the codes are specifically designed to take advantage of the massively parallel architecture and one of our codes (LSMS) achives the highest Mflop performance of any code currently running on the machine.
The fully relaxed structure of a clean Si(111) surface has been well established. However, the electronic structure of the surface states and phase transitions between the 7x7 and 1x1 phases have not been studied due to the large unit cell needed for the ab initio calculation. The enormous computing power of the Cray T3E has allowed us to attack these problems using ab initio plane-wave psuedopotential methods and Car-Parinello molecular dynamics.
Significance
The availability of powerful and accurate first-principles techniques permits the study of quantum interatomic interactions on a length scale not previously accessible, opening up the possiblity of relating these fundamental interatomic intertactions to the strength, ductility, transport and magnetic properties of materials. Applied to magnetic materials, these techniques should help establish the foundations for understanding the relationship between the technical magnetic properties (permeability, coercivity, remenance) of magnets and microstructure.
Publications Chetty, N., and M. Weinert. 1997. Stacking faults in magnesium. Phys. Rev. B 56:10844-10851.
Faulkner, J. S., Y. Wang, and G. M. Stocks. 1997. Coulomb energies in alloys. Phys. Rev. B 55:7492.
M. Alatalo, M. Weinert, and R. E. Watson. N.d. Stability of Zr-Al alloys. Phys. Rev. B, in press.
As computer chip designers put more and more components onto a single chip, they need
to understand the microscopic electronic structure of the chip surface, which can only be
obtained accurately through large-scale quantum simulations on supercomputers. This plot
shows the isovalue surface for the total charge density of the DAS(7x7) reconstruction of
the Silicon(111) surface. The different colors show the different gradients of the surface.
This 498-atom simulation was performed on 128- and 256-processor runs on NERSC's
T3E-900. (Zhong-Yi Lu, Dave Turner, Cai-Zhuang Wang, Kai-Ming Ho, Iowa State
University/Ames Laboratory; Andrew Canning, NERSC; Malcolm Stocks, Oak Ridge
National Laboratory)