Phonon modes and electron-phonon coupling at the FeSe/SrTiO<sub>3</sub> interface.
basic_science · Level V
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- Record sourced from PubMed, PMID 39478222.
- Also identified by DOI 10.1038/s41586-024-08118-0.
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Abstract
The remarkable increase in superconducting transition temperature (T<sub>c</sub>) observed at the interface of one-unit-cell FeSe films on SrTiO<sub>3</sub> substrates (1 uc FeSe/STO)<sup>1</sup> has attracted considerable research into the interface effects<sup>2-6</sup>. Although this high T<sub>c</sub> is thought to be associated with electron-phonon coupling (EPC)<sup>2</sup>, the microscopic coupling mechanism and its role in the superconductivity remain elusive. Here we use momentum-selective high-resolution electron energy loss spectroscopy to atomically resolve the phonons at the FeSe/STO interface. We uncover new optical phonon modes, coupling strongly with electrons, in the energy range of 75-99 meV. These modes are characterized by out-of-plane vibrations of oxygen atoms in the interfacial double-TiO<sub>x</sub> layer and the apical oxygens in STO. Our results also demonstrate that the EPC strength and superconducting gap of 1 uc FeSe/STO are closely related to the interlayer spacing between FeSe and the TiO<sub>x</sub> terminated STO. These findings shed light on the microscopic origin of the interfacial EPC and provide insights into achieving large and consistent T<sub>c</sub> enhancement in FeSe/STO and potentially other superconducting systems.