In single unit-cell FeSe grown on SrTiO3, the superconductivity transition temperature features a significant enhancement. Local phonon modes at the interface associated with electron-phonon coupling may play an important role in the interface-induced enhancement. However, such phonon modes have eluded direct experimental observations. Indeed, the complicated atomic structure of the interface brings challenges to obtain the accurate structure-phonon relation knowledge from either experiment or theory, thus hindering our understanding of the enhancement mechanism. Here, we achieve direct characterizations of atomic structure and phonon modes at the FeSe/SrTiO3 interface with atomically resolved imaging and electron energy loss spectroscopy in a scanning transmission electron microscope. We find several phonon modes highly localized (~1.3 nm) at the unique double layer Ti-O termination at the interface, one of which (~ 83 meV) engages in strong interactions with the electrons in FeSe based on ab initio calculations. The electron-phonon coupling strength for such a localized interface phonon with short-range interactions is comparable to that of Fuchs-Kliewer (FK) phonon mode with long-rang interactions. Thus, our atomic-scale study provides new insights into understanding the origin of superconductivity enhancement at the FeSe/SrTiO3 interface.
@article{arxiv.2309.02714,
title = {Atomic-scale observation of localized phonons at FeSe/SrTiO3 interface},
author = {Ruochen Sh and Qize Li and Xiaofeng Xu and Bo Han and Ruixue Zhu and Fachen Liu and Ruishi Qi and Xiaowen Zhang and Jinlong Du and Ji Chen and Dapeng Yu and Xuetao Zhu and Jiandong Guo and Peng Gao},
journal= {arXiv preprint arXiv:2309.02714},
year = {2024}
}