"Electron-phonon coupling in a system with broken symmetry: Surface of Be(0001)"

TeYu Chien (簡德宇): Xiaobo He, Sung-Kwan Mo, Makoto Hashimoto, Zahid Hussain, Zhi-Xun Shen, and E. W. Plummer; Physical Review B, 08/21/2015.

Additional Authors: Xiaobo He, Sung-Kwan Mo, Makoto Hashimoto, Zahid Hussain, Zhi-Xun Shen, and E. W. Plummer

Abstract:

The momentum-resolved Eliashberg function (ELF) α2F(ω, k) for the Be(0001) zone-center surface state was extracted from the high-quality angle-resolved photoemission spectroscopy (ARPES) data at the Fermi energy in the Γ→M direction, displaying ten peaks. A comparison of the peaks in the ELF to the bulk phonon density of states (DOS) and the bulk and surface phonon dispersion allows for an identification of the origin of all but two of the peaks. The five high energy peaks (>52meV) are associated with the coupling of the surface state to bulk phonon modes. The peaks at 44.5 and at 49.0 meV have contributions from both the bulk and surface phonons. The most intense peak at 37.5 meV is evidently having a contribution from electron-phonon coupling (EPC) of the surface state to the surface Rayleigh phonon mode. Surprisingly, the two lowest energy modes, which must be associated with surface Rayleigh phonon, cannot be attributed to a high phonon DOS at the surface nor to any Fermi surface nesting. After detail analysis, the three lowest energy peaks are associated with momentum dependence in the EPC matrix, reflected in the phonon linewidth changes. As a result of the broken symmetry at the surface, coupling of the initial surface state due to the presence of the surface phonons contributes ∼48.5±12.5% of the spectral weight in the ELF and ∼66.5±10.5% to the mass enhancement (λ).