Oxygen Vacancy Induced Flat Phonon Mode at FeSe /SrTiO3 interface

Area of Science:

• Condensed Matter Physics
• Materials Science
• Surface Science

Background:

• High-temperature superconductivity observed at the interface between monolayer iron selenide (FeSe) and strontium titanate (STO) substrate is linked to a high-frequency optical phonon mode.
• The precise origin of this influential phonon mode within the STO substrate remained unclear, hindering a complete understanding of the observed superconductivity.

Purpose of the Study:

• To investigate the origin of the high-frequency optical phonon mode assisting superconductivity at the FeSe/STO interface.
• To elucidate the role of oxygen vacancies and charge transfer in modifying this phonon mode.

Main Methods:

• First-principles calculations were employed to simulate the electronic and vibrational properties of the FeSe/STO interface.
• Analysis focused on identifying novel phonon modes and their dispersion characteristics under varying conditions, including the presence of oxygen vacancies.

Main Results:

• A novel polar phonon mode, unique to the surface layers of STO and absent in bulk crystals, was identified.
• Oxygen vacancies near the interface flatten the phonon mode's dispersion and lower its energy.
• Charge transfer between the STO substrate and FeSe monolayer further reduces the phonon energy to 81 meV, aligning with experimental observations.

Conclusions:

• The oxygen-vacancy-induced flat and polar phonon mode is a key factor in high-temperature superconductivity at the FeSe/STO interface.
• This finding provides crucial insights into the mechanism behind the enhanced superconducting gap and replica band separations observed experimentally.