Engineering interface-induced high-temperature superconductivity in monolayer FeSe

Recent advances in low-dimensional unconventional superconductors and their unique interface properties require new techniques to probe pairing symmetry, a crucial and often controversial aspect of superconductivity. Fe-chalcogenides (FeCh: Se, Te and S) are particularly interesting systems as their crystal structure is relatively simple compared to those of other Fe-based superconductors. A notable example is monolayer FeSe grown on SrTiO₃ (STO), which shows a superconducting transition temperature T_c > 60 K, nearly ten times higher than that of bulk FeSe [1, 2]. Despite intensive research on monolayer FeSe, its pairing symmetry and mechanism remain unclarified. We fabricated high-quality monolayer FeSe thin films on Nb-doped STO substrates via molecular beam epitaxy. The superconductivity of the FeSe film was finely tuned by controlling the growth condition and post-annealing process. The films were transferred in situ without exposure to air for characterization with angle-resolved photoemission spectroscopy, X-ray photoemission spectroscopy and scanning tunneling microscopy/spectroscopy. The results provide a comprehensive examination of the mechanisms for unconventional superconductivity in monolayer FeSe.

Reference

[1] F.-C. Hsu et al., Superconductivity in the PbO-type structure α-FeSe, PNAS 105, 14262–14264 (2008).

[2] R. Peng et al., Tuning the band structure and superconductivity in single-layer FeSe by interface engineering, Nature Comm. 5, 5044 (2014).