Atomic-scale observation of localized phonons at FeSe/SrTiO3 interface

TL;DR

This study directly observes localized phonon modes at the FeSe/SrTiO3 interface, revealing their strong electron interactions and potential role in enhancing superconductivity.

Contribution

It provides the first atomic-scale characterization of phonon modes at the interface, combining imaging, spectroscopy, and ab initio calculations to reveal their properties.

Findings

Localized phonon modes (~1.3 nm) identified at the interface.

One phonon mode (~83 meV) interacts strongly with electrons.

Electron-phonon coupling comparable to long-range FK phonons.

Abstract

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.