Development of novel high-resolution electron energy loss spectroscopy and related studies on surface excitations

Abstract

High-resolution electron energy loss spectroscopy (HREELS) is a powerful technique to probe vibrational and electronic excitations at solid surfaces. A monochromatic electron beam incident on the crystal surface may interact with the vibrations of adsorbed molecules, surface phonons or electronic excitations before being back-scattered. By analyzing the energy and momentum of the scattered electrons, we can obtain the information about the chemical bonds, lattice dynamics, occupation of electronic states, and surface plasmons. However the application of traditional HREELS to dispersion analyses is restricted by its point-by-point measurement of the energy loss spectrum for each momentum. Recently, a new strategy for HREELS was realized by utilizing a specially designed lens system with a double-cylindrical monochromator combined with a commercial Scienta hemispherical electron energy analyzer, which can be used to simultaneously measure the energy and momentum of the scattered electrons. The new system possesses improved momentum resolution, high detecting efficiency and high sampling density with no loss in energy resolution. The new HREELS system was employed to study the mechanism of the superconductivity enhancement at FeSe/SrTiO3 interface. By surface phonon measurements on samples with different film thickness, it is revealed that the electric field associated with phonon modes of SrTiO3 substrate can penetrate into FeSe film and interact with the electrons therein, playing the key role in the superconductivity enhancement. The surface collective modes of three-dimensional topological insulator was also studied by using this new HREELS system. A highly unusual acoustic plasmon mode is revealed on the surface of a typical three-dimensional topological insulator Bi2Se3. This mode exhibits an almost linear dispersion to the second Brouillion zone center without reflecting lattice periodicity, and it remains prominent over a large momentum range, with unusually weak damping unseen in any other system. This observation indicates that the topological protection exists not only in single-particle topological states but also in their collective excitations. The application of the new HREELS system with the ability to measure large momentum range with high-efficiency, will definitely promote the development of related researches on condensed matter physics.