Understanding the cooperative optimization of FeSe/SrTiO₃ thin films

One unit-cell-thick FeSe on SrTiO₃ substrates displays a superconducting gap opening temperature near 70 K, which exceeds the counterpart in bulk FeSe by one order of magnitude. Revealing the superconducting mechanism of FeSe/SrTiO₃ can provide a road map toward high temperature superconductivity. In this talk I will discuss a series of in situ experiments aimed at highlighting the critical ingredients for superconductivity in FeSe/SrTiO₃. Using time- and angle-resolved photoemission spectroscopy, we launch the coherent Se A_1g mode. The mode frequency is abruptly softened from 5.25 to 5.00 terahertz in the one-unit-cell limit [1]. We quantify the electron-phonon coupling strength by combining two time-domain experiments into a "coherent lock-in" measurement in the terahertz regime. X-ray diffraction tracks the atomic displacement; photoemission monitors the corresponding electron energy shift [2]. Comparison with theory suggests a substantial enhancement of electron-phonon coupling in FeSe owing to strong correlations. In addition, we compare these spectroscopic results to new in situ 4-point resistivity measurements on FeSe thin films, where a complete characterization of the superconducting transition temperature as a function of thickness and surface doping is performed [3].
[1] S.-L. Yang et al. Nano Lett. 15, 4150 (2015)
[2] S. Gerber*, S.-L. Yang* et al. Science 357, 71 (2017)
[3] B. D. Faeth*, S.-L. Yang* et al. in preparation (2017)