Fig. 1: Observation of clear band splitting and strong superconductivity-induced Bogoliubov back-bending bands in single-layer FeSe/STO films.

a Fermi surface mapping near M3 (–π, –π) measured at 20 K. It is obtained by integrating the spectral weight within [−5, 5] meV energy window with respect to the Fermi level. Two electron-like Fermi surface sheets are resolved marked by black line (FS 1) and blue line (FS 2). b Second derivative constant energy contour near M3 (–π, –π) at a binding energy of 18 meV. Band structure measured along the momentum Cut 1 at 20 K (d) and 83 K (c), respectively. Each image is divided by the corresponding Fermi distribution function. The location of the momentum Cut 1 is shown in (a). e Photoemission image obtained from dividing the data at 20 K (d) by the one at 83 K (c). f Two electron-like bands obtained from (c). The inner and outer bands come from the FS2 and FS1 Fermi surface sheets in (a), respectively. g Two electron-like bands and the superconductivity-induced Bogoliubov back-bending bands obtained from (d). h Representative MDCs of the photoemission image in (e) at different binding energies. The splitting of the Bogoliubov back-bending bands is marked by two colored arrows. Band structure along the momentum Cut 2 measured at 20 K (j) and 103 K (i), respectively. Each image is divided by the corresponding Fermi distribution function. The location of the momentum Cut 2 is shown in (a). k Photoemission image obtained from dividing the data at 20 K (j) by the one at 103 K (i). l Two electron-like bands corresponding to (i). The band marked by solid black line is obtained from (i). The band marked by dashed blue line is expected from the overall measured Fermi surface and band structure. m Two bands corresponding to (j). The band marked by solid blue line is obtained from (j). The band marked by dashed black line is extracted from the band at 103 K (solid black line in (l)) using the BCS formula. n Representative MDCs of the photoemission image in (k) at different binding energies. The blue arrow marks the peak position of Bogoliubov back-bending band.