H. Q. Yuan

Fe-based superconductivity with Tc=31 K bordering an antiferromagnetic insulator in (Tl,K) FexSe2

(Tl,K)FexSe2 single crystals were first successfully synthesized with the Bridgeman method. The physical properties are characterized by electrical resistivity, magnetic susceptibility and Hall coefficient. We found that the (Tl,K)FexSe2 (1.30u2009≤u2009xu2009≤u20091.65) compounds show an antiferromagetic (AFM) insulator behavior, which may be associated with the Fe-vacancy ordering in the crystals. While in the 1.70u2009≤u2009xu2009<u20091.78 crystals, superconductivity (SC) coexists with an insulating phase. As Fe content further increases, the bulk SC with Tc=31u2009K (and a Tconset as high as 40u2009K) appears in the 1.78u2009≤u2009xu2009≤u20091.88 crystals. Our discovery represents the first Fe-based high-temperature superconductivity (HTSC) at the verge of an AFM insulator.Up to now, there have been two material families, the cuprates and the iron-based compounds with high-temperature superconductivity (HTSC). An essential open question is whether the two classes of materials share the same essential physics. In both, superconductivity (SC) emerges when an antiferromagnetical (AFM) ordered phase is suppressed. However, in cuprates, the repulsive interaction among the electrons is so strong that the parent compounds areMott insulators.By contrast, all iron-based parents are metallic. One perspective is that the iron-based parents are weakly correlated and that the AFM arises from a strongnestingof the Fermi surfaces. An alternative view is that the electronic correlations in the parents are still sufficiently strong to place the system close to the boundary between itinerancy and electronic localization. A key strategy to differentiate theses views is to explore whether the iron-based system can be tuned into a Mott insulator. Here we identify an insulating AFM in (Tl,K)FexSe2 by introducing Fe-vacancies and creating superconductivity in the Fe-planar. With the increasing Fe-content, the AFM order is reduced. When the magnetism is eliminated, a superconducting phase with Tc as high as 31K (and a Tc onset as high as 40K) is induced. Our findings indicate that the correlation effect plays a crucial role in the iron-based superconductors. (Tl,K)FexSe2, therefore, represents the first Fe-based high temperature superconductor near an insulating AFM.

Superconductivity at 32 K and anisotropy in Tl0.58Rb0.42Fe1.72Se2 crystals

Tl0.58Rb0.42Fe1.72Se2 single crystals were successfully synthesized with the Bridgeman method. The physical properties are characterized by electrical resistivity, magnetization and Hall resistivity. Tl0.58Rb0.42Fe1.72Se2 undergoes a superconducting transition at Tconset=32u2009K and Tczero=31.4u2009K. Extrapolation of the upper critical field Hc2 at Tc to zero temperature gives a large value of Hc2(0) (Hc2(0)=221u2009T for H∥ab and Hc2(0)=44.2u2009T for H∥c). The Hall coefficient changes sign upon cooling down, indicating a possible multi-band behavior. The normal-state magnetic susceptibility decreases with decreasing temperature, suggesting strong antiferromagnetic (AFM) spin fluctuations, which might be related to the superconductivity (SC).

Weak anisotropy of the superconducting upper critical field in Fe 1.11 Te 0.6 Se 0.4 single crystals

We have determined the resistive upper critical field Hc2 for single crystals of the superconductor Fe1.11Te0.6Se0.4 using pulsed magnetic fields of up to 60T. A rather high zero-temperature upper critical field of mu0Hc2(0) approx 47T is obtained, in spite of the relatively low superconducting transition temperature (Tc approx 14K). Moreover, Hc2 follows an unusual temperature dependence, becoming almost independent of the magnetic field orientation as the temperature T=0. We suggest that the isotropic superconductivity in Fe1.11Te0.6Se0.4 is a consequence of its three-dimensional Fermi-surface topology. An analogous result was obtained for (Ba,K)Fe2As2, indicating that all layered iron-based superconductors exhibit generic behavior that is significantly different from that of the high-Tc cuprates.

Revised phase diagram for the FeTe1−xSexsystem with fewer excess Fe atoms

We observed bulk superconductivity in the FeTe1-xSex crystals with a low Se concentration after the excess Fe atoms existing unavoidably in crystals as-grown are partially removed by means of annealing in the air. A revised magnetism and superconductivity phase diagram is obtained via resistivity and magnetic susceptibility measurements for FeTe1-xSex system. It is found that bulk superconductivity coexists with antiferromagnetic order in the crystals with 0.05<x<0.20. The phase diagram is very similar to the case of the K-doping or Co-doping BaFe2As2, as well as of SmFeAsO1-xFx iron-pnictide system, perhaps indicating that all iron-based systems have a generic phase diagram, although antiferromagnetic wave vector in the parent compounds of these systems is remarkable different.

Upper critical field and thermally activated flux flow in single-crystalline Tl 0 . 58 Rb 0 . 42 Fe 1 . 72 Se 2

The upper critical field

Magnetic and Superconducting Properties in Single Crystalline Fe1+δTe1-xSex (

mu_0H_{c2}(T_c)

x<0.50

of Tl

) System

_{0.58}

Evidence for two distinct superconducting phases in EuBiS

Rb

_2

_{0.42}