Unraveling Enhanced Superconductivity in Single-Layer FeSe through Substrate Surface Terminations

Area of Science:

• Condensed Matter Physics
• Materials Science
• Surface Science

Background:

• Single-layer iron selenide (FeSe) on strontium titanate (SrTiO3) exhibits significantly higher superconducting transition temperatures than bulk FeSe.
• Potential factors driving this enhancement include electron doping, interfacial electron-phonon coupling, and electron correlations.

Purpose of the Study:

• To determine the primary factors responsible for the enhanced superconductivity in single-layer FeSe films grown on different SrTiO3 surface terminations.
• To investigate the correlation between interfacial structure, charge transfer, electron correlations, and superconducting properties.

Main Methods:

• Growth of single-layer FeSe films on SrTiO3(001) substrates with both TiO2 and SrO surface terminations.
• Scanning tunneling spectroscopy (STS) to measure superconducting gaps and work functions.
• Angle-resolved photoemission spectroscopy (ARPES) to confirm charge transfer.
• Scanning transmission electron microscopy (STEM) to analyze interfacial atomic structures.
• Dynamical mean-field theory (DMFT) calculations for theoretical comparison.

Main Results:

• A larger superconducting gap (17.0 meV) was observed on the TiO2-terminated surface compared to the SrO-terminated surface (10.5 meV).
• The SrO surface exhibited a larger work function, leading to reduced charge transfer to FeSe.
• Distinct interfacial atomic structures were identified, with variations in the Se-Fe-Se tetrahedral angle between terminations.
• DMFT calculations suggest optimal electron correlations contribute to enhanced superconductivity on the TiO2 termination.

Conclusions:

• Enhanced superconductivity in single-layer FeSe on TiO2-terminated SrTiO3 arises from a combination of optimal electron correlations and sufficient charge transfer from the substrate.
• Interfacial atomic structure and substrate termination play crucial roles in modulating the electronic properties and superconductivity of FeSe films.