Observation of layer-dependent <i>d_xz</i>/<i>d_yz</i>band splittings in the nematic phase of FeSe/SrTiO₃ films

The effect of interlayer coupling on physical properties of layered materials such as iron-based superconductors is an interesting question. Here, we use angle-resolved photoemission spectroscopy to directly observe the electronic structure of FeSe films with different layers grown on SrTiO₃(001) substrates by in situ molecular beam epitaxy. We find two distinct layer-dependent band splittings in the Brillouin zone center and corner, respectively. In the Brillouin zone corner, the band splitting caused by the nematic phase decreases with the increase in the number of layers. On the contrary, the degeneracy of d_xz/d_yz bands in double-layer FeSe is lifted in multi-layer films and the band splitting in the Brillouin zone center becomes larger with the increase of film thickness. The results of double-layer FeSe grown on SrTiO₃ favor the proposal of d-wave nematic order. Our layer-dependent experiments provide limitations for the theoretical study of the nematic phase in FeSe and shed insights for the study of the interplay between nematicity and superconductivity.