Phase diagram of Fe$_{1+y}$(Te$_{1-x}$Se$_{x}$): evolution from antiferromagnetism to superconductivity

Iron chalcogenide Fe$_{1+y}$(Te$_{1-x}$Se$_{x}$) is the simplified version of Fe-based superconductors [1,2] and has a unique antiferromagnetic (AFM) structure in the parent compound Fe$_{1+y}$Te [3,4]. In iron pnictide superconductor parent compounds, the AFM wavevector $Q_{AF}$ is along the FS nesting direction [5-7], while in Fe$_{1+y}$Te, $Q_{AF}$ is rotated 45$^{\circ}$ from the FS nesting direction. Understanding the magnetic and superconducting properties of this system is considered critical [8]. In this talk I will discuss the phase diagram of Fe$_{1+y}$(Te$_{1-x}$Se$_{x}$) that we recently established. We found that long-range AFM order is gradually suppressed by Se substitution, disappearing near 9\% Se, above which short-range AFM order coexists with non-bulk superconductivity (NBSC). Bulk superconductivity (BSC) does not appear until the Se content is greater than \~30\%. The normal state exhibits distinct properties between the NBSC and BSC regions: metallic behavior is observed above $T_{c}$ for the BSC region, while the NBSC region exhibits weak localization behavior above $T_{c}$. These observations, together with our results of neutron scattering studies, suggest that the short-range magnetic order near $Q_{AF}$ leads to weak charge carrier localization, and is thus unfavorable to superconducting pairing. \\[4pt] [1] F.C. Hsu \textit{et al.}, Proc. Natl. Acad. Sci. USA. \textbf{105}, 14262 (2008).\\[0pt] [2] M.H. Fang \textit{et al.}, Phys. Rev. B \textbf{78}, 224503 (2008). \\[0pt] [3] W. Bao \textit{et al.}, Phys. Rev. Lett. \textbf{102}, 247001 (2009). \\[0pt] [4] S.L. Li \textit{et al.}, Phys. Rev. B \textbf{79}, 054503 (2009).\\[0pt] [5] C. Cruz \textit{et al.}, Nature \textbf{453}, 899 (2008).\\[0pt] [6] Q. Huang \textit{et al.}, Phys. Rev. Lett \textbf{101}, 257003 (2008). \\[0pt] [7] J. Zhao \textit{et al.}, Phys. Rev. Lett. \textbf{101}, 167203 (2008).\\[0pt] [8] A.V. Balatsky and D. Parker, Viewpoint, Physics \textbf{2}, 59 (2009).