Orbital mixing at the onset of high-temperature superconductivity in FeSe1-xTex/CaF2

Abstract

We perform systematic high-resolution angle-resolved photoemission spectroscopy of iron-chalcogenide superconductor FeSe1-xTex films on CaF2 which exhibit a unique paramagnetic nematicity at x=0 (pristine FeSe) and a gigantic Tc enhancement at the critical Te concentration (xc) 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 dxz/yz orbital, indicative of the suppression of nematicity near xc. Evolution of band structure at the Brillouin-zone center is characterized by a drastic upward shift of the dxy band with increasing x, leading to an orbital switching from dxz/yz to dxz/yz+dxy accompanied by a mass enhancement. These results demonstrate that the pristine and high-TcFeSe1-xTex have distinctly different electronic structures. The present study lays the foundation for understanding the origin of high-Tc superconductivity and the interplay with electronic nematicity.