Phonon modes and electron-phonon coupling at the FeSe/SrTiO₃ interface

Area of Science:

• Condensed Matter Physics
• Materials Science
• Surface Science

Background:

• Superconductivity in one-unit-cell FeSe films on SrTiO3 substrates exhibits a remarkably high transition temperature (Tc).
• The underlying mechanism, particularly the role of electron-phonon coupling (EPC) at the interface, remains poorly understood.

Purpose of the Study:

• To microscopically investigate the phonons and their coupling to electrons at the FeSe/SrTiO3 interface.
• To elucidate the origin of the enhanced superconductivity in this system.

Main Methods:

• Momentum-selective high-resolution electron energy loss spectroscopy (MSR-HREELS) was employed.
• Atomic-level resolution of interfacial phonons was achieved.

Main Results:

• New optical phonon modes were identified at the interface, with energies between 75-99 meV.
• These modes involve out-of-plane vibrations of oxygen atoms in the interfacial TiO2 layer and apical oxygens in SrTiO3.
• Strong coupling between these phonons and electrons was observed.
• The electron-phonon coupling strength and superconducting gap correlate with the interlayer spacing between FeSe and the TiO2-terminated SrTiO3.

Conclusions:

• The study reveals the microscopic origin of interfacial electron-phonon coupling in the FeSe/SrTiO3 system.
• The findings provide crucial insights for enhancing superconducting transition temperatures in FeSe/SrTiO3 and other related superconducting materials.