Professor Fadley studies the properties of the surfaces of solids and of microstructures formed on those surfaces. The systems under study include semiconductors and their interactions with metals and other species, magnetic metals as well-ordered epitaxial overlayers and clusters that may contain a few to several hundred atoms, and the interactions of oxygen and other molecules with both semiconductor and metal surfaces. This work is thus relevant to the fabrication of integrated circuits and magnetic storage devices, as well as catalytic reactions at solid surfaces. In all of these applied areas, various surface properties (e.g., atomic positions, magnetic order, diffusion, epitaxial growth, chemical reactivity) are becoming more and more crucial to next-generation technologies.
The experiments are carried out in ultrahigh vacuum environments so that surfaces can be prepared with atomic-scale cleanliness and studied for periods of hours to days without contamination from the surrounding atmosphere. A principal technique in which our group has made a number of fundamental advances is angle-resolved photo-electron spectroscopy. In this technique, the distributions in energy and angle of electrons excited from a surface by ultraviolet or soft-x-ray radiation via the well-known photoelectric effect are measured. From these measurements, the electronic band structure of the material can be mapped out in detail. Also, the angular distributions of electrons emitted from core levels can be viewed as photoelectron diffraction patterns which carry detailed information on the positions of the atoms. Experimental and theoretical studies of such photoelectron diffraction effects in our laboratory have provided a number of new kinds of structural information on surface atomic species. We have recently begun to study the spin-dependent scattering of these photoelectrons and to determine the local orientations of magnetic moments (i.e., the local magnetic order). A further new development our group is studying is the analysis of photoelectron diffraction patterns as holograms; this shows considerable promise for directly generating three-dimensional images of atomic structure or magnetic structure that are not possible with any other current technique.
Beyond these photoelectron-based methods, surfaces and surface structures are characterized by low energy electron diffraction (the modern version of the Davisson-Germer experiment), and by scanning tunneling microscopy (a more newly developed technique that permits directly imaging atoms on a surface). Considerable activity is also directed to the theoretical interpretation of photoelectron diffraction and holography, including programming and calculations on a variety of computers from work stations to supercomputers.
Our work is split between experiments carried out in Davis and at a new national facility - the Advanced Light Source (ALS) at the Lawrence Berkeley Laboratory (LBL), located an hour's drive from Davis. The ALS is a storage ring of 67 m diameter in which electrons circulate at very close to the speed of light; this beam of highly relativistic radially- accelerated electrons generates the most intense beams of ultraviolet and soft-x-ray radiation in the world. This radiation (known as synchrotron radiation) is in turn used to excite photoelectrons from surfaces, as well as to carry out other types of absorption or diffraction experiments. Students working with Professor Fadley thus learn a range of experimental techniques for the characterization of surfaces, design and use highly advanced instrumentation for carrying out such studies, carry out model theoretical calculations to better understand and use the experimental data, and work on interdisclipinary projects related to condensed matter physics, materials science, and chemical physics.
E-mail fadley@physics.ucdavis.edu
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