Fe-based superconductivity with T _c =31 K bordering an antiferromagnetic insulator in (Tl,K) Fe _x Se ₂

Abstract

Up to now, there have been two material families, the cuprates and the\niron-based compounds with high-temperature superconductivity (HTSC). An\nessential open question is whether the two classes of materials share the same\nessential physics. In both, superconductivity (SC) emerges when an\nantiferromagnetical (AFM) ordered phase is suppressed. However, in cuprates,\nthe repulsive interaction among the electrons is so strong that the parent\ncompounds are "Mott insulators." By contrast, all iron-based parents are\nmetallic. One perspective is that the iron-based parents are weakly correlated\nand that the AFM arises from a strong "nesting" of the Fermi surfaces. An\nalternative view is that the electronic correlations in the parents are still\nsufficiently strong to place the system close to the boundary between\nitinerancy and electronic localization. A key strategy to differentiate theses\nviews is to explore whether the iron-based system can be tuned into a Mott\ninsulator. Here we identify an insulating AFM in (Tl,K)FexSe2 by introducing\nFe-vacancies and creating superconductivity in the Fe-planar. With the\nincreasing Fe-content, the AFM order is reduced. When the magnetism is\neliminated, a superconducting phase with Tc as high as 31K (and a Tc onset as\nhigh as 40K) is induced. Our findings indicate that the correlation effect\nplays a crucial role in the iron-based superconductors. (Tl,K)FexSe2,\ntherefore, represents the first Fe-based high temperature superconductor near\nan insulating AFM.\n