Direct imaging of electron transfer and its influence on superconducting pairing at FeSe/SrTiO3 interface

TL;DR

This study reveals how electron transfer at the FeSe/SrTiO3 interface enhances superconductivity, showing that interface effects and electron-phonon interactions significantly boost Tc in monolayer FeSe.

Contribution

It provides direct evidence of electron transfer and band-bending at the interface, and demonstrates how backgate tuning can further enhance superconducting transition temperature.

Findings

Electrons transfer from SrTiO3 to FeSe, accumulating within two atomic layers.

Positive backgate enhances Tc by modulating electron-phonon coupling.

Interface electron transfer is key to Tc enhancement in FeSe/SrTiO3.

Abstract

The exact mechanism responsible for the tenfold enhancement of superconducting transition temperature (Tc) in a monolayer iron selenide (FeSe) on SrTiO3(STO) substrate over that of bulk FeSe, is an open issue. We present here a coordinated study of electrical transport and low temperature electron energy-loss spectroscopy (EELS) measurements on FeSe/STO films of various thicknesses. Our EELS mapping across the FeSe/STO interface shows direct evidence of band-bending caused by electrons transferred from STO to FeSe layer. The transferred electrons were found to accumulate only within the first two atomic layers of FeSe films near the STO substrate. Our transport results found a positive backgate applied from STO is particularly effective in enhancing Tc of the films while minimally changing the carrier density. We suggest that the positive backgate tends to 'pull' the transferred electrons in FeSe films closer to the interface and thus further enhances both their coupling to interfacial phonons and the electron-electron interaction within FeSe films, thus leading to a huge enhancement of Tc in FeSe films.