Recently, London penetration depth measurement down to 50 mK indicates a possible nodal gap 21. In optimally-doped Fe 1+ yTe 1− xSe x, ARPES 13 shows an isotropic superconducting gap, while anisotropic or two-gap features were suggested by angle-resolved specific heat measurement 17, muon spin rotation measurements 18, 19 and optical conductivity measurements 19, 20. In particular, Fe 1+ yTe 1− xSe x exhibits a bi-collinear antiferromagnetic ordering and it can be either commensurate or incommensurate depending on the Fe content 15, 16, different from the common collinear commensurate antiferromagnetic ordering observed in all iron pnictides.Īlthough much attention has been paid to this compound, some crucially fundamental physical properties are still controversial. Iron chalcogenides also manifest some differences compared with iron pnictides. With Se doping, superconductivity emerges and T c goes up to 14 K accompanied by the suppression of SDW. The parent compound Fe 1+ yTe is not superconducting, but exhibits a spin-density wave (SDW) ground state. Moreover, the competition between magnetism and superconductivity, similar to iron pnictides, is also observed in iron chalcogenides 14. Band structure calculation 9, 10 and ARPES 11, 12, 13 results show that Fermi surface of FeTe 1− xSe x is characterized by hole bands around Γ point and electron bands around M point, which is similar to iron pnictides. Among iron chalcogenides, Fe 1+ yTe 1− xSe x are unique in their structural simplicity, composing of only iron-chalcogenide layers, which is favorable for probing the mechanism of superconductivity. Recent angle-resolved photoemission spectroscopy (ARPES) revealed unexpected large superconducting gap ~19 meV in a single-layer FeSe, which suggests a T c as high as 65 K 8. By applying pressure to A xFe 2- ySe 2, T c can even reach ~48 K 7.

Furthermore, by intercalating spacer layers between adjacent FeSe layers, T c has reached ~ 32 K 5 in A xFe 2− ySe 2 ( A = K, Cs, Rb and Tl) and 43 K 6 in Li x(NH 2) y(NH 3) 1− yFe 2Se 2 ( x ~ 0.6 y ~ 0.2). Although the initial T c in FeSe is just 8 K, it increased up to 14 K 1, 2 with appropriate Te substitution and 37 K 3, 4 under high pressure. Superconductivity discovered in iron chalcogenides has stimulated great interests since it is a possible candidate to break the superconducting transition temperature record ( T c ~ 55 K) in the iron-based superconductors. Some fundamental properties were recharacterized and compared with those of as-grown crystals to discuss the influence of excess Fe. After the optimal annealing, the complete removal of excess Fe was demonstrated via STM measurements. Superconductivity was witnessed to evolve mainly from the edge of the crystal to the central part. The optimally annealed crystals can be easily obtained by annealing with ~1.5% molar ratio of oxygen at 400☌ for more than 1 hour. Bulk superconductivity can be gradually induced by increasing the amount of O 2 and annealing time at suitable temperatures.

Here we report a Systematical study of O 2-annealing dynamics in Fe 1+ yTe 1− xSe x by controlling the amount of O 2, annealing temperature and time. On the other hand, the presence of excess Fe is almost unavoidable in Fe(Te,Se) single crystals, which hinders the appearance of bulk superconductivity and causes strong controversies over its fundamental properties. By intercalating spacer layers, superconducting transition temperature has been raised over 40 K. Its less toxic nature compared with iron arsenides is also advantageous for applications of iron-based superconductors. Iron chalcogenide Fe(Te,Se) attracted much attention due to its simple structure, which is favorable for probing the superconducting mechanism.

Polycrystal annealing singlecrystal