Effect of carrier density and valence states on superconductivity of oxygen annealed Fe1.06Te0.6Se0.4 single crystals.

The variations of carrier density and valence states in oxygen annealed Fe1.06Te0.6Se0.4 single crystals were studied systematically. It was found that the carrier density nH increases after oxygen annealing by Hall coefficient measurements. The X-ray photoelectron spectroscopy experiments reveal that the oxygen annealing changes Fe0 and Te0 states to Fe2+/3+ and Te4+, respectively, while the valence variation of Se is negligible. Our results indicate that the improvement of superconductivity, such as the zero resistance transition temperature Tczero, shielding and Meissner fraction value 4πχ and upper critical field Hc2, could be closely related to the proper manipulation of nH and the valence states by oxygen annealing in the system. [ABSTRACT FROM AUTHOR]