  Our valence band photoelectron spectroscopic studies show a temperature
dependent spectral weight transfer near the Fermi level in the Fe-based
superconductor FeSe1-xTex. Using theoretical band structure calculations we
have shown that the weight transfer is due to the temperature induced changes
in the Fe(Se,Te)4 tetrahedra. These structural changes lead to shifts in the
electron occupancy from the xz/yz and x2-y2 orbitals to the 3z2-r2 orbitals
indicating a temperature induced crossover from a metallic state to an Orbital
Selective Mott (OSM) Phase. Our study presents the observation of a temperature
induced crossover to a low temperature OSM phase in the family of Fe
chalcogenides.
