Abstract

Existing web applications that generate lists of isotropy subgroups of crystallographic space groups are either not user friendly or are unnecessarily complicated. We developed a web application, ISOSUBGROUP, which we will show is simple and easy to use. The software development included the assembly of FORTRAN subroutines and an HTML user interface enhanced by CSS, resulting in a visually appealing environment.

Abstract

We studied the wurtzite to rock salt phase transition in gallium nitride ( GaN ). Using the mapping algorithm of COMSUBS we found 435 possible mechanisms for this transition. We then used FIREBALL to do density functional theory calculations and found enthalpy barrier heights for the transition pathway. We used this to determine the mechanisms that are the most favorable for GaN. The most favorable mechanisms for GaN are those that break no bonds during the phase transition. The bond-preserving mechanisms involve bilayer sliding of (010) hexagonal plane in the plus or minus [100] hexagonal direction.

Abstract

We investigate the mechanism responsible for the reconstructive fcc-hcp phase transition in Pb that occurs at about 13 GPa. The two most energetically favorable atomic pathways have enthalpy barriers much lower than any other pathway we considered. Both mechanisms involve only shifts of (111) planes in the fcc structure. We physically interpret these pathways and represent them graphically.

Anthony Dale Smith (Masters Thesis, April 2001,
Advisor: Harold Stokes
)

Abstract

A bush of modes is a set of one or more vibrational modes to which energy spreads, once energy is introduces into one or more modes of the set. Bushes are determined by symmetry principles alone, regardless of the kinds and strengths of nonlinear interactions present. Bushes can be separated into universality classes in which the bushes share a common dynamical solution, such that once a solution for one bush is found, that solution applies to any other bush in the same universality class. Using group theoretical methods on computer, we found all irreducible bushes (of any dimension) and all two-dimensional reducible bushes of vibrational modes at k points of symmetry in all possible crystalline structures. We then separated these bushes into universality classes listing an example for each class as well as the form, of the energy out to sixth degree. In all, we have determined that there are 17 classes of irreducible bushes and 8 classes of two-dimensional reducible bushes of modes at k points of symmetry.

Levi Barnes (Senior Thesis, May 2000,
Advisor: Harold Stokes
)

Abstract

Sam Carter (Senior Thesis, January 1999,
Advisor: Harold Stokes
)

Abstract

Xiaohua Yu (PhD Dissertation, December 1999,
Advisor: Harold Stokes
)

Abstract

The Self-Consistent Atomic Deformation model is a first principles method for calculating the energy per unit cell in ionic crystalline solids. It has a difficulty in calculating the lattice parameters of some materials like AgCl. The major suspect of the problem is the kinetic energy functional. In an effort to improve the functional, we have developed and implemented a gradient term expansion of the functional. In an effort to avoid the functional, we have developed and tested a Tight-Binding-Like approach. The results are improved for AgCl but are not satisfactory for other materials.

Marshall Bartlett (Senior Thesis, April 1997,
Advisor: Harold Stokes
)

Abstract

Branton Campbell (Senior Thesis, May 1992,
Advisor: Harold Stokes
)

Abstract

James D Wells (Masters Thesis, December 1990,
Advisor: Harold Stokes
)

Abstract

A computer program EXPAND has been written which calculates basis functions of space group physically irreducible representations. These basis functions are projected out from the basis functions of Wyckoff point group physically irreducible representations. Algorithms to determine independent sets if basis functions (or models) are developed for physically irreducible representations. Also, scalar and vector component representations of the Wyckoff point group physically irreducible representations are presented, along with their applications to order-disorder probabilities, atomic displacements, and molecular rotations in the Landau theory of phase transitions.