Material Science Theory and Computation for Advanced Light Source Experiments
M. A. Van Hove and C. S. Fadley, Lawrence Berkeley National Laboratory and University of California, Davis
J. Carter, Y. Chen, P. N. Ross, S. Sachs, G. Stone, P. Tang, V. Zhuang, Lawrence Berkeley National Laboratory
Research Objectives
Offer theoretical and computational support to nationwide experimental users of Berkeley Lab's Advanced Light Source (ALS) and other synchrotron facilities in the U.S. and abroad. Develop computational capabilities that permit experimentalists to plan and simulate future experiments, to rapidly evaluate and fine-tune ongoing experiments, and to fully analyze and refine finished experiments. For example, develop methods to deal with the complex electron multiple scattering processes that underlie experimental techniques like photoelectron diffraction and holography, x-ray absorption spectroscopies, and spectromicroscopies.
Computational Approach
Computational methods utilize NERSC capabilities to tackle problems of unprecedented scale and to open up the field to research on new classes of materials. Further extensions of existing methods and new techniques are introduced in response to the needs and requests of synchrotron radiation users.
Accomplishments
A new code, the MSCD Package (Multiple Scattering in Cluster Diffraction), was developed and implemented for the interpretation of photoelectron diffraction measurements. The sequential version of the code has been implemented on supercomputers and Sun workstations in UNIX, as well as on PC and Macintosh desktop computers. Extensive comparisons of code performance on the Cray T3E and on cluster parallel architectures have been conducted under MPI.
The Fast Fourier Transform approach has been enhanced to permit new and efficient methods for holographic reconstruction of 3D atomic maps of surfaces and interfaces, a valuable method to obtain structural and magnetic detail about materials from photoelectron diffraction and similar data.
Global optimization by means of genetic algorithms has been implemented for parallel processing on the Cray T3E and clusters of workstations. This is an essential step in searching for structural solutions of very complex surfaces.
We have developed a molecular-orbital approach to calculate core-level relaxation energies in photoelectron spectroscopy. This capability is valuable for identifying atomic and molecular species attached to surfaces.
Significance
The ALS produces a rapidly increasing amount of experimental data that requires advanced numerical treatment and simulation to yield scientifically useful results. Important information can be obtained about the atomic-scale structural, electronic, magnetic, chemical, corrosive, tribological, and biological properties of surfaces and interfaces of materials with technological relevance. Examples include electronic nanostructures, magnetic storage materials, chemical catalysts and sensors, corrosion protection, hard coatings, and biological membranes.
The MSCD Package was used to create this photoelectron diffraction simulation, which shows calculated Cu 3p photoelectron intensities, as a function of multiple scattering order, from clean Cu(111) in a fixed forward-scattering emission direction, 35.23
off-normal, and for emission from the third layer. A photoelectron can be scattered once or twice consecutively along this forward-scattering path. The 4th order Rehr-Albers approximation is compared with exact calculations.
Publications
Y. Chen, F. J. García de Abajo, A. Chassé, R. X. Ynzunza, A. P. Kaduwela, M. A. Van Hove, and C. S. Fadley, "Convergence and Reliability of the Rehr-Albers Formalism in Multiple Scattering Calculations of Photoelectron Diffraction," Phys. Rev. B, in press.
Y. Chen and M. A. Van Hove, MSCD Package User Guide: Simulation of Photoelectron Diffraction Using Rehr-Albers Separable Representation, http://electron.lbl.gov/mscdpack/mscdpack.html.
Y. Chen, G. Zhuang, P. N. Ross, M. A. Van Hove, and C. S. Fadley, "Equivalent-Core Calculation of Core-Level Relaxation Energies in Photoelectron Spectroscopy: A Molecular-Orbital Approach," J. Chem. Phys., in press.