Orbital mixing at the onset of high-temperature superconductivity in FeSe_{1−x}Te_{x}/CaF_{2}

We perform systematic high-resolution angle-resolved photoemission spectroscopy of iron-chalcogenide superconductor FeSe_{1−x}Te_{x} films on CaF_{2} which exhibit a unique paramagnetic nematicity at x=0 (pristine FeSe) and a gigantic T_{c} enhancement at the critical Te concentration (x_{c}) of x∼0.2. Upon increasing the Te concentration from x=0, the electronlike Fermi-surface shape at the Brillouin-zone corner shows a clear change associated with a remarkable energy shift of the d_{xz/yz} orbital, indicative of the suppression of nematicity near x_{c}. Evolution of band structure at the Brillouin-zone center is characterized by a drastic upward shift of the d_{xy} band with increasing x, leading to an orbital switching from d_{xz/yz} to d_{xz/yz}+d_{xy} accompanied by a mass enhancement. These results demonstrate that the pristine and high-T_{c}FeSe_{1−x}Te_{x} have distinctly different electronic structures. The present study lays the foundation for understanding the origin of high-T_{c} superconductivity and the interplay with electronic nematicity.