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Calendar: ALS Calendar
Title: ALS Special Seminar | TeYu Chien
When: 10/16/2012 10:00 AM - 11:00 AM
Description: ALS Special Seminar

Tue, Oct 16, 2012
10 - 11am

Eli Rotenberg

Presented by:
TeYu Chien
Department of Materials Science and Engineering, Northwestern University

Electron-Phonon Coupling in a system with broken symmetry: Surface of

The electron-boson coupling (EBC) is a many-body physics that governs
many physical properties in novel materials, such as superconductivity. In general,
EBC alters the self-energy of the electron/quasiparticle and renormalize the electronic
dispersion relation near Fermi energy. By using angle-resolved photoemission
spectroscopy (ARPES), the band renormalization near Fermi energy due to EBC
could be studied in details and the underlying mechanism could be further discussed.
For examples, the kink features found in the cuprate electron band dispersion were
studied intensively in order to obtain insight of the origin of superconductivity, which
is believed to be the result of EBC. However, up-to-date, the relationship between
ARPES data and the coupled boson modes are not clearly demonstrated yet. Toward
this goal, here, we utilize ARPES to investigate electron-phonon coupling (EPC) on
an isotropic, free-electron-like 2D electronic band structure system – Be(0001) Γ
surface state, where phonon is the sole type of boson in the system.
The EPC on Be(0001) Γ surface state was found to be anisotropic by
measuring the momentum-dependent mass enhancement factor, λ, which represent
the strength of the EPC. Furthermore, the contribution to EPC from each phonon
mode (including bulk phonon and surface phonon modes) was revealed by comparing
the phonon density of states (DOS) and dispersion relations to the extracted
Eliashberg function (ELF), F( k )

2 , α ω , which represent the EPC detail as function of
phonon energy, ω, and electron momentum, k

. It is found that the presence of the
surface phonon enhances EPC dramatically on Be(0001). However, the most striking
result is the observation of the low energy coupling features which might originate
from the interaction of electron to the mixing of bulk phonon mode (BPM) and
surface phonon mode (SPM). The mixing of the BPM and SPM was not considered in
state-of-the-art theoretical calculation, thus could be the reason for the inconsistency
to the experimental results.
The detail analysis and comparison in this study demonstrates the pathway
toward understanding the EBC. High quality ARPES data and detail information of
the boson mode are essential for gaining the insight of EBC in complex materials.
Location: 6-2202 Conf Room
Author: Adriana Reza