Search for Unconventional Superconductors at the Itinerant-to-Local Moment Crossover

In searching for novel optimal superconductors, three strategic routes based on theoretical and experimental knowledge from the known high-Tc superconductors are followed. CaFe4As3 is a newly discovered 3D compound, with Fe2+ in tetrahedral coordination, similar to that in the parent compounds of the known superconductors. The thermodynamic and transport properties reveal a spin density wave (SDW) transition at TN = 88 K, and an incommensurate-to-commensurate SDW transition at T2=26.4 K. A large electronic specific heat coefficient γ=0.02 J/molK^2 and an unusually high Kadowaki-Woods (KW) ratio A/γ^2=55×10E−5 μΩcm mol^2K^2/mJ^2 point to strong electron correlations. While the commensurate SDW state below T2 is suppressed in Co-doped CaFe4As3, neither doping with P, Yb, Co and Cu, nor application of hydrostatic pressures up to 5 GPa, is able to fully suppress the robust incommensurate SDW order in this system. The new layered compound SrMnBi2 has been studied as a promising candidate for high Tc superconductivity as suggested by theoretical calculations. We found that SrMnBi2 is structurally similar to, but more two dimensional than the known Fe superconductors. Two phase transitions at T1=292 K and T2=252 K have been observed. A large electronic specific heat coefficient γ=36.5 mJ/molK^2 and a KW ratio of 9.38×10E−5 μΩcm mol^2K^2/mJ^2 indicate enhanced electron correlations. DFT calculations have revealed metallic Sr-Bi layers in SrMnBi2, as well as Dirac-cone like features in the band structure. Doping experiments on the Mott insulator Sr2F2Fe2OS2 have been carried out to search for superconductivity at the localized-to-itinerant moment crossover. Increasing amounts of T=Mn in Sr2F2(Fe1−xTx)2OS2 suppress the long range magnetic ordering at x≈0.2, and the subsequent increase in x results in a spin glass behavior for 0.2≤x≤0.5, and possibly a new magnetic order for x≥0.5. By contrast, Co-doping increases the AFM transition from TN=106 K for x=0 up to 124 K for x=0.3. The excitation gap determined from the electrical resistivity is minimized but remains finite around x=0.5 for T=Mn. In addition, a study has been done on a rare binary type I superconductor YbSb2. Besides the superconducting transition at Tc=1.30 K, a possible second superconducting phase is observed below Tc(2)=0.41 K. From thermodynamic and transport measurements, there is strong, unambiguous evidence for the type I nature of the superconductivity in YbSb2