Date of Award
Doctor of Philosophy
Here, we studied an isotropic, free-electron-like 2D electronic band structure system--the Be(0001) [Gamma-bar] surface state. We revealed that the EPC on Be(0001) is anisotropic. Mass enhancement factors are closely related to the strength of the electron-phonon coupling (EPC). Large values of mass enhancement factors represent a strong EPC. For beryllium, the mass enhancement factors, [lambda], of the surfaces are large when compared with the bulk values. However, the reported values of [lambda] of the surfaces are inconsistent among the values obtained from different experiments or theories. One of the possible reasons is that [lambda] is strongly k -dependent. We did systematic measurements to understand that the inconsistency originated from the anisotropic nature of the EPC on the Be(0001) surface. The details of EPC are described by Eliashberg function (ELF) --so called coupling function. This function describes the coupling between the electron and phonon as a function of energy and momentum. To understand the EPC, ELF is required to be extracted from angle-resolved photoemission spectroscopy experimental data. With a set of extra-high quality data, we accurately extracted the ELF for the Be(0001) [Gamma-bar] surface state for the first time. With comparison to the measured bulk and surface phonon density of states, we found that the bulk phonon contributes to the high energy part of the ELF; while the surface phonon contributes to the low energy part of the ELF. The contribution from the surface phonon to [lambda] is found to be about 77%, equals to 0.72 out of the total value of 0.94. To quantitatively extract [lambda] we did simulations to understand the effects from the linear approximation used for analyzing data--from the energy and momentum resolutions in instruments and from the noise in the data.We concluded that (a) the linear approximation can work in a very wide range; (b) the momentum resolution plays a minor role in determining [lambda] ; (c) the energy resolution would severely distort the extracted dispersion near the Fermi energy and kink, hence, affecting the resulting [lambda] ; and (d) [lambda] is robust against the noise.
Chien, Te-Yu, "Anisotropic electron-phonon coupling on a two-dimensional isotropic fermi contour [Gamma-bar] surface state of Be(0001). " PhD diss., University of Tennessee, 2009.