The Role of SrTiO3 Phonon Penetrating into thin FeSe Films in the Enhancement of Superconductivity

TL;DR

This study reveals that SrTiO3 Fuchs-Kliewer phonons penetrate into FeSe films, strongly coupling with electrons and significantly enhancing superconductivity at the interface.

Contribution

It provides direct experimental evidence of phonon penetration and mode-specific electron-phonon coupling as the mechanism behind superconductivity enhancement in FeSe/SrTiO3.

Findings

F-K phonon field penetrates FeSe and interacts with electrons

Electron-phonon coupling constant for ~92 meV phonon is ~0.25

Penetration depth of phonon field decays exponentially with FeSe thickness

Abstract

The significant role of interfacial coupling on the superconductivity enhancement in FeSe films on SrTiO3 has been widely recognized. But the explicit origination of this coupling is yet to be identified. Here by surface phonon measurements using high resolution electron energy loss spectroscopy, we found electric field generated by Fuchs-Kliewer (F-K) phonon modes of SrTiO3 can penetrate into FeSe films and strongly interact with electrons therein. The mode-specific electron-phonon coupling (EPC) constant for the ~92 meV F-K phonon is ~0.25 in the single-layer FeSe on SrTiO3. With increasing FeSe thickness, the penetrating field intensity decays exponentially, which matches well the observed exponential decay of the superconducting gap. It is unambiguously shown that the SrTiO3 F-K phonon penetrating into FeSe is essential in the interfacial superconductivity enhancement.