Spectroscopic Signatures of Nonlocal Interfacial Coupling in Superconducting FeSe/SrTiO3 Heterostructures

TL;DR

This study uses low-temperature Raman spectroscopy and VEELS to investigate interfacial coupling effects in FeSe/SrTiO3 heterostructures, revealing phonon softening and charge transfer phenomena that may explain enhanced superconductivity.

Contribution

It demonstrates the application of low-temperature gated Raman and VEELS techniques to probe interfacial effects in FeSe/SrTiO3, providing new insights into the mechanisms of superconductivity enhancement.

Findings

Ambipolar softening of FeSe vibrational modes observed

Interfacial charge transfer indicated by electron effective mass enhancement

Interfacial coupling influences lattice dynamics and superconductivity

Abstract

The mechanism of enhanced superconductivity in the one unit-cell (1UC) FeSe film on a SrTiO3 (STO) substrate has stimulated significant research interest but remains elusive. Using low-temperature, voltage-gated Raman spectroscopy and low-temperature valence electron energy loss spectroscopy (VEELS), we characterize the phonon behavior and interfacial charge transfer in single- and few-layer FeSe films on STO. Raman measurements reveal ambipolar softening of the FeSe vibrational modes, mimicking that of the underlying STO substrate. We attribute this behavior to an interfacial coupling effect of STO on FeSe lattice dynamics. This interfacial coupling effect is further supported by local electron effective mass enhancement, which is determined from the red-shift in the FeSe VEELS spectrum near the FeSe/STO interface. Our work sheds light on the possible interfacial mechanisms contributing to the enhanced superconductivity across the FeSe/STO interface and further unveils the potential of low-temperature gated Raman spectroscopy and VEELS in clarifying a broad category of quantum materials.