Date of Award
Doctor of Philosophy
Tom A. Callcott, David G. Madrus, Charles S. Feigerle
Symmetry is the beauty of nature. It is the mirror of the way nature minimizes the energy of the system, and achieves the stable state. In the bulk crystal, 3D symmetry has ensured the minimum of free energy contributed by electrostatic energy, vibrational energy and many body self-energy. When the crystal is broken to form two surfaces, the 3D symmetry is destroyed, leading to high free energy on the surface. In order to minimize the free energy, the electronic charge on or near the surface rearranges to form an electronic and lattice structure quite distinct from the bulk. My research is to investigate the interplay between surface electronic structure and lattice structure through electron-phonon coupling of surface states on the open surface of simple metals, such as Be and Mg
Through the technique of angle-resolved photoemission, I investigated the band structures, line shapes, widths of surface states and their temperature dependence. At T = 0, electron structures are in the ground state, but as the temperature was increased, electron-phonon coupling became more important as more electron-hole pairs were excited within energy which corresponded to the phonon energies. On the surface Brillouin zone at the zone boundary there are two surface states (S1 and S2) coexisting in a gap in the bulk projection. These appear on both Beand Mg. Through fitting the temperature-dependent surface state width contributed to by the imaginary part of self-energy from electron-phonon coupling, the electron-phonon coupling strength parameter, l, was determined for both surface states on both surfaces. The l value of S1 and S2 (lS1 = 0.647 and lS2 = 0.491, respectively) on Be is more than two times larger than the bulk value (lbulk = 0.24). However, for Mg, the determined electron-phonon coupling parameters of S1 and S2 surface states are not larger (lS1 = 0.20, lS2 = 0.31) than the bulk value (lbulk = 0.31). According to many previous studies, Be surfaces have very special electronic and lattice behaviors. Therefore, a larger electron -phonon interaction on the Be surface would be expected. Furthermore, according to the comparison of fitting goodness between Einstein and Debye models for the temperature dependence of surface state widths, I found the most localized S1 surface state had dominant coupling with localized high-energy optical phonon at about 64 meV. This large coupling even causes the large distortion of the S1 surface state band on crossing the optical phonon energy as observed in high-resolution photoemission spectrum. Based on these results, I have attempted to construct a picture of the interplay between the electronic structure and dynamic lattice structure for the large negative thermal expansion of the Besurface. The behavior of surface states on Mg shows a big contrast with that on Be, which I attribute to the close relationship between the surface state and bulk state on Mg.
Tang, Shu-Jung, "The Role of Surface States in Electron-Phonon Coupling on the Open Surfaces of Simple Metals. " PhD diss., University of Tennessee, 2003.