Computational Studies
Work focuses on large scale nonequilibrium molecular-dynamics (NEMD) simulations of stress-induced phase transformations and shock phenomena. the study of properties of materials at high pressures, in particular the dynamical mechanisms responsible for phase transformations have been until recently experimentally difficult because of the high pressures and/or temperatures involved. Recent advances in high pressure experimental techniques, especially in the areas of diamond anvil (DAC) pressure cells and ultrafast x-ray diffraction and photoelectron spectroscopy, have dramatically increased the study of pressure induced phase transitions within the last decade from both theoretical and experimental camps.
The three fields of large-scale nonequilibrium molecular-dynamics (NEMD) simulations, computationally intensive quantum-mechanical electronic structure calculations (referred to as "ab initio"), and shockwave and experiments have produced a timely convergence that allows us to study condensed-matter phase transformations under stress in far greater detail than ever before possible. For example, since shockwave phenomena occur on extremely short time and distance scales, NEMD shockwave simulations, which require sub-picosecond computational time steps and sub-micrometer sample sizes, are particularly relevant for studying plasticity and solid-solid phase transition -- provided that they occur within the allowable, practicable computational window. Ab initio calculations are limited to much smaller sizes, typically 1000 atoms (rather than the multi-million NEMD sample sizes). When quantum mechanics for the electrons is combined with classical mechanics for the nuclei -- so-called "ab initio NEMD" -- the computational limitations make the time and distance scales too severe to obeserve dynamical crystal sturcture changes under shock conditions.
However, the newly developed "uniaxial Hugoniostat" (a joint collaboration of UTEP, Los Alamos and the European Center for Atomic Molecular Computations (CECAM)) takes an initial state in a cold crystal, instantaneously compresses it uniaxially and homogenously, without the inhomogenous passage of a shock wave (as is done so laboriously in NEMD simulations, so as to achieve a steadily propagating wave), and then relaxes the system to the final normal stress, strain, and temperature on the shock Hugoniot by time-reversible integral feedback. Because the method focuses on the damaged final state, rather than the full propagation of the shock, longer times for the structural relaxation can be followed using either semi-empirical or ab initio descriptions of the interatomic interactions.
The Department of Physics also participates in the Ph.D. program in Materials Science and Engineering. |