Abstract
The effect of vacuum annealing on superconductivity is investigated in Fe(Se,Te) single crystals. It is found that superconductivity is not enhanced by annealing under high vacuum (\({\sim}10^{-3}\) Pa) or by annealing in a sealed evacuated quartz tube. In a moderate vacuum atmosphere (\({\sim}1\) Pa), iron oxide layers are found to show up on sample surfaces, which would draw excess Fe out of the crystal. Thus, it is suggested that remanent oxygen effectively works to remove excess Fe from the matrix of Fe(Se,Te) crystals, resulting improvement of superconducting transition temperature. Our transport measurements suggest that the excess Fe scatters the carriers on electron- and hole-type channels in a different manner. We discuss how the mobility of two types of carriers correlate with superconductivity. Since both the electron and hole bands are important for the occurrence of superconductivity, excess Fe would suppress superconductivity mainly due to strong scattering of electrons.
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