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

• Published in: Nature (London) 2024-11, Vol.635 (8038), p.332-336
• Main Authors: Yang, Hongbin, Zhou, Yinong, Miao, Guangyao, Rusz, Ján, Yan, Xingxu, Guzman, Francisco, Xu, Xiaofeng, Xu, Xianghan, Aoki, Toshihiro, Zeiger, Paul, Zhu, Xuetao, Wang, Weihua, Guo, Jiandong, Wu, Ruqian, Pan, Xiaoqing
• Format: Article
• Language: English
• Subjects: 639/301/119/544; 639/766/119/1003; Anisotropy; Coupling; Electron energy; Electron energy loss spectroscopy; Electrons; Energy; Energy loss; Humanities and Social Sciences; Interfaces; Interlayers; Microscopy; Molecular beam epitaxy; multidisciplinary; Oxygen; Oxygen atoms; Phonons; Science; Science (multidisciplinary); Simulation; Spectroscopy; Spectrum analysis; Strontium titanates; Substrates; Superconductivity; Titanium oxides; Transition temperature; Transition temperatures; Unit cell; Vibrations
• ISSN: 0028-0836; 1476-4687; 1476-4687
• DOI: 10.1038/s41586-024-08118-0

The remarkable increase in superconducting transition temperature ( T c ) observed at the interface of one-unit-cell FeSe films on SrTiO 3 substrates (1 uc FeSe/STO) 1 has attracted considerable research into the interface effects 2 – 6 . Although this high T c is thought to be associated with electron–phonon coupling (EPC) 2 , 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 x 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 x terminated STO. These findings shed light on the microscopic origin of the interfacial EPC and provide insights into achieving large and consistent T c enhancement in FeSe/STO and potentially other superconducting systems. Phonons at the FeSe/STO interface are imaged at atomic scale, uncovering new optical phonon modes that couple strongly with electrons, shedding light on the microscopic origin of the interfacial EPC and providing insights into achieving superconducting transition temperature enhancement.