Interfacial mode coupling as the origin of the enhancement of Tc in FeSe films on SrTiO3

• Published in: Nature (London) Vol. 515; no. 7526; pp. 245 - 248
• Main Authors: Lee, J. J., Schmitt, F. T., Moore, R. G., Johnston, S., Cui, Y.-T., Li, W., Yi, M., Liu, Z. K., Hashimoto, M., Zhang, Y., Lu, D. H., Devereaux, T. P., Lee, D.-H., Shen, Z.-X.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 13.11.2014; Macmillan, London
• Subjects: 140/146; 639/766/119/1003; 639/766/119/544; Humanities and Social Sciences; letter; MATERIALS SCIENCE; MATSCI, PHYS; multidisciplinary; Science
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature13894

High-resolution angle-resolved photoemission spectroscopy reveals bosonic modes in a SrTiO 3 substrate coupling to electrons in an FeSe overlayer to facilitate high-temperature superconductivity. Strontium titanate boosts supereconductivity Bulk iron selenide (FeSe) is a superconductor with a critical temperature T c = 8 K, but superconductivity is substantially enhanced in single-unit cell films of FeSe grown on strontium titanate (SrTiO 3 or STO) substrates, where superconducting energy gaps open at temperatures close to the boiling point of liquid nitrogen (77 K). This raises the question of whether the substrate has a contributory role in this enhancement. Zhi-Xun Shen and colleagues report high-resolution angle-resolved photoemission spectroscopy (ARPES) results that reveal bosonic modes (thought to be oxygen optical phonons) in the SrTiO 3 substrate coupling to electrons in the FeSe overlayer to facilitate high-temperature superconductivity. Such coupling helps superconductivity in most channels, so the pairing enhancement described here may well work for other superconducting materials, as well as for FeSe. Films of iron selenide (FeSe) one unit cell thick grown on strontium titanate (SrTiO 3 or STO) substrates have recently shown 1 , 2 , 3 , 4 superconducting energy gaps opening at temperatures close to the boiling point of liquid nitrogen (77 kelvin), which is a record for the iron-based superconductors. The gap opening temperature usually sets the superconducting transition temperature T c , as the gap signals the formation of Cooper pairs, the bound electron states responsible for superconductivity. To understand why Cooper pairs form at such high temperatures, we examine the role of the SrTiO 3 substrate. Here we report high-resolution angle-resolved photoemission spectroscopy results that reveal an unexpected characteristic of the single-unit-cell FeSe/SrTiO 3 system: shake-off bands suggesting the presence of bosonic modes, most probably oxygen optical phonons in SrTiO 3 (refs 5 , 6 , 7 ), which couple to the FeSe electrons with only a small momentum transfer. Such interfacial coupling assists superconductivity in most channels, including those mediated by spin fluctuations 8 , 9 , 10 , 11 , 12 , 13 , 14 . Our calculations suggest that this coupling is responsible for raising the superconducting gap opening temperature in single-unit-cell FeSe/SrTiO 3 .