Undergraduates
Why does the space shuttle returning to Earth cause two separate sonic booms?
After receiving a B.S. in physics from the National Taiwan University in 1971, Professor Chiang received his Ph.D. in physics from the University of California-Berkeley in 1978. He joined the Department of Physics at the University of Illinois in 1980 after working as a postdoctoral fellow at the IBM T.J. Watson Research Center in Yorktown Heights, NY.
Professor Chiang has done seminal work on bulk, surface, and interface states of metals and semiconductors using photoemission techniques. Using synchrotron-radiation photoemission spectroscopy, scanning tunneling microscopy, and molecular beam epitaxy techniques, he has examined the growth processes and the resulting physical properties of various surface and interface systems that are of fundamental scientific interest and technological relevance.
He was one of the first to demonstrate that atoms of single-crystal surfaces have binding energies different from the bulk atoms and that the energy shifts are detectable with photoemission, using synchrotron radiation as a light source. He has pioneered the application of angle-resolved and core-level photoemission to interface, quantum-well, and superlattice research and expanded it to include novel configurations and magnetic systems. He is currently carrying out X-ray scattering and diffraction experiments at the Advanced Photon Source at Argonne National Laboratory.
Most recently, Professor Chiang and his students have fabricated miniature electron interferometers containing atomically smooth mirrors spaced by a few atomic layers. Exploiting the fact that electrons bounce back and forth between two interfaces and create standing waves, Professor Chiang's group are able to measure the electron wavelength in their samples with very high accuracy. In addition to his exceptional experimental achievements, Professor Chiang is an outstanding theorist who is able to develop theoretical models for his experimental results.
Atomically Uniform Films
Recent advances in crystal growth have made it possible to prepare atomically uniform films of Ag on an iron whisker. Films prepared by thermal evaporation onto a low-temperature substrate followed by annealing are investigated by angle-resolved photoemission. Electrons in these films, confined by the substrate potential, form discrete quantum well states. These states show atomic layer resolution and can be used as spectroscopic fingerprints for layer thickness identification. Our goal is to understand the kinetics and atomic processes involved in the formation of such uniform films that have been thought to be impossible to prepare until now.
Semiconductor and Ceramic Surfaces and Interfaces
Photoemission and x-ray diffraction and scattering techniques are employed to determine the electronic properties and atomic structure of surfaces, interfaces, and thin films. The behavior of crystal growth by molecular beam epitaxy, chemical vapor deposition, laser ablation, and magnetron sputtering is being investigated. Key issues of interest include chemical reactions and atomic interactions during thermal and energetic beam deposition leading to the formation of surface reconstructions, nonequilibrium compositions, and metastable structures. Fundamental surface behaviors such as charge density wave transitions and metal insulator transitions are also of interest, and these phenomena are probed by photoelectron holography and x-ray diffraction.
Electronic Properties of Impurities, Surfaces, and Quantum Structures
High-resolution angle resolved photoemission is employed to investigate the electron properties of metal surfaces and films. Atomically uniform films are prepared, and the resulting quantum well states can be understood in terms of Fabry-Perot modes in a solid state electron interferometer. An interferometric analysis yields a band structure and quasi-particle lifetime broadening that are the most accurate to date. Effects of impurity and defect scattering, both at the surface and in the interior of a film, will be studied by doping during film growth. Temperature dependent phonon scattering, electron-electron scattering, and structural stability will also be investigated.
Development of an Ultrahigh Resolution Photoemission System for Studies of Quantum Structures
Recent advances in experimental capabilities at national synchrotron radiation facilities including the development of new monochromators and high-intensity undulator beamlines have enabled ultrahigh-resolution measurements which can provide detailed information about quasi-particle interactions and electron correlation effects in solids. To take advantage of these developments, we are setting up an ultrahigh resolution angle-resolved photoemission system that will match the performance of the light source. A two-dimensional detector will be employed that allows multi-channel energy- and angle-dependent data to be taken.