Matthew Zacate
Associate Professor of Physics

Research

My main research interests are synthesis of computational methods for study of diffusion, point defects, and highly disordered materials; adapting computational methods used in materials research to other areas of science; and perturbed angular correlation spectroscopy (research in Solid State Physics through detection of nuclear radiation) and related hyperfine methods.

Funded projects

National Science Foundation grant DMR 06-06006 (Metals Program)

RUI: Simulation and Improved Analysis of Data from PAC and other Hyperfine Methods for Studying Local Atomic Jumps and Long Range Diffusion in Intermetallic Compounds

In collaboration with William E. Evenson (co-PI) and Phil Matheson (faculty researcher) at Utah Valley University.

The overall aim of this project is to involve undergraduate students in research that will enable effective application of hyperfine methods for measuring atomic jump rates and discerning diffusion mechanisms. The goals are to provide software tools and, where needed, additional theoretical development, to use the software to identify conditions under which diffusion mechanisms can be determined, and to analyze experimental data. The emphasis of this work is on diffusion processes in intermetallic compounds, but the results will be applicable to ceramics, semiconductors, and nanoparticles. Perturbed angular correlation spectroscopy (PAC), nuclear magnetic resonance (NMR), and Mössbauer effect (ME) are examples of hyperfine techniques that have been used to detect atomic motion. In most cases studied in the past, analysis of experimental data has been empirical, so the purpose of this study is to provide a framework for analyzing data obtained from hyperfine methods, based for example on the work of Dattagupta [i], that will allow rapid development of diffusion models uniformly across hyperfine methods.

Student participants at NKU: Mike Lape, Harun Muhammed, and Justin Williams.
Student participants at UVU: Austin Bunker, Jeffery A. Hodges, Carlos Moreno, Tyler Park, Heidi Pope, and Michael Stufflebeam.

Software projects

Stochastic Hyperfine Interactions Modeling Library (SHIML)
The Adjustable Parameter Package (TAPP)
PolyPacFit - A least-squares program to fit PAC spectra obtained from polycrystalline samples. It uses SHIML to allow fits to stochastic models.
Programs to aid in development of stocahstic models: (1) Defect Configuration Generator and (2) Defect Configuration to Stochastic Model Converter
zGraphViewer

Published results

Simulations of PAC spectra for simple vacancy diffusion in L1₂-structured compounds using a developmental version of SHIML [34]
Clarification of the theoretical expression for PAC spectra when the initial distribution of probes is not in equilibrium [35]
Application of a developmental version of PolyPacFit to study Cd jump rates in rare earth tri-gallides [36]

Works nearing publication

A re-examination of N-state symmetric EFG models
Application of a developmental version of PolyPacFit to study Cd jump rates in Pd₃Ga₇

Other areas

Investigation of inhomogeneous broadening effects that arise from randomly distributed charged defects
Simulations of PAC spectra for more complex diffusion mechanisms in L1₂-structured compounds using SHIML
Extension of SHIML to include methods such as Mössbauer effect that measure hyperfine interactions on two nuclear spin states
Using PolyPacFit to PAC spectra from other systems as they become available in collaboration principally with Gary S. Collins at Washington State University

Center for Integrative Natural Science and Mathematics grant 2006-R7

Development of Software Utilities to Aid Simulation of Highly Disordered Ceramics

The first utility, which now is called GenCEMMC, is a generalized version of the combined energy minimization – Monte Carlo (CEMMC) method for simulating large degrees of disorder. Earlier applications of the CEMMC method [10, 16, 31] have been limited to the case of antisite disorder and was executed in conjunction with the program CASCADE [ii]. The present generalized version supports any mode of disorder and is designed to be used with the more widely available and more flexible General Utility Lattice Program (GULP) [iii]. The second application, called GAP4GULP (a Genetic Algorithm for the General Utility Lattice Program), provides a genetic algorithm extension for GULP in order to search for optimal parameters in empirical models of cohesion in solids.

Student participants: Billy Hartmann and David Heitz

Software projects

A Generalized Implementation of the Combined Energy Minimization – Monte Carlo Method (GenCEMMC)
A Genetic Algorithm Program for the General Untility Lattice Program (GAP4GULP)
The Adjustable Parameter Package (TAPP)

Other projects

(To be updated when they are closer to being ready for publication.)

Older research

I have used both experimental and computational techniques to study point defects and other atomic scale phenomena in intermetallic compounds, ceramics, and semiconductors. I did this work in the Nuclear Solid State Physics group at Washington State University, in the Atomistic Simulation group at Imperial College, and with John A. Gardner (emeritus professor) at Oregon State University. My principal activities are listed below.

Snapshot of a simulation
of a monolayer of Ar on
a Ca surface.
Computer Simulation

Predicted defect formation enthalpies and defect-defect interactions in CeO₂ [6], ZrO₂ [9], and cement phases [4, 8, 10, 16].
Investigated the use of cellular automata for modeling grain growth by simulating the evolution of inert gas atoms on a surface of calcium [5, 7]. A snapshot of a simulation involving the adsorption of Ar atoms on a Ca (111) surface is shown to the left. Colors indicate different domains of Ar atoms according to the crystallographic position of the Ar with respect to the terminal Ca layer. (Reference 7 is available online at http://www.tms.org/pubs/journals/JOM/9908/Atkinson/Atkinson-9908.html).
Worked with a combined energy minimization-Monte Carlo technique in order to simulate systems with large concentrations of defects including non-stoichiometric compounds [10, 16].

PAC spectra of ¹¹¹Cd in In₃La.

Experiment

Studied indium-solute site occupation in intermetallic compounds and found striking examples of how site occupation can change as a function of composition (in Ni₂Al₃, Ni₂Ga₃, and similar compounds) [15, 24] and as a function of temperature (in GdAl₂) [22].
Found that the method of perturbed angular correlation of gamma rays (PAC) can be used to measure the atomic jump frequency of a diffusing atom [23] and used this technique to measure Cd-jump frequencies in a series of rare earth tri-indides. The spectra shown to the left are for Cd jumping on the In sublattice in In₃La. At low temperature (429 K), the PAC spectrum shows the classic signal of nuclei in a static electric field gradient (EFG). At elevated temperature, there is a loss of coherence (exponential damping of the PAC signal) due to stochastic reorientation of the EFGs at the nuclei. The reorientation of EFGs comes about because the Cd atoms jump among lattice sites for which EFGs have different orientations. See reference 23 for more information.
Determined defect-defect interactions of indium and transition metal vacancies in NiAl [12] and FeAl [20].

References

S. Dattagupta, Hyperfine Interactions 11, 77-126 (1981).
M. Leslie, DL/SCI/TM31T. Tech. Rep., (SERC Daresbury Laboratory: 1982).
J.D. Gale, JCS Faraday Trans. 93 , 629 (1997); J.D. Gale, Phil. Mag. B 73 , 3 (1996); J.D. Gale and A.L. Rohl, Mol. Simul. 29, 291 (2003).

Home Page of Matthew Zacate
Department of Physics and Geology
Northern Kentucky University

Page last updated in September of 2009.
Material on this page does not necessarily reflect the views of the acknowledged funding agencies and of Northern Kentucky University (disclaimer). Indeed, if something is found to be controversial, it may not necessarily reflect the views of the author. Copyright (C) 2009 Matthew O. Zacate.

Matthew ZacateAssociate Professor of Physics