We use multi-scale techniques to determine the extent of local inhomogeneity and superconductivity in Ca0.86Pr0.14Fe2As2 single crystal. The inhomogeneity is manifested as a spatial variation of praseodymium concentration, local density of states, and superconducting order parameter. We show that the high-Tc superconductivity emerges from clover-like defects associated with Pr dopants. The highest Tc is observed in both the tetragonal and collapsed tetragonal phases, and its filamentary nature is a consequence of non-uniform Pr distribution that develops localized, isolated superconducting regions within the crystals.

We present thermodynamic and transport properties of transition-metal (T) arsenides, TAs with T = Sc to Ni (3d), Zr, Nb, Ru (4d), Hf and Ta (5d). Characterization of these binaries is made with powder X-ray diffraction, temperature and field-dependent magnetization and resistivity, temperature-dependent heat capacity, Seebeck coefficient, and thermal conductivity. All binaries show metallic behavior except TaAs and RuAs. TaAs, NbAs, ScAs and ZrAs are diamagnetic, while CoAs, VAs, TiAs, NiAs and RuAs show approximately Pauli paramagnetic behavior. FeAs and CrAs undergo antiferromagnetic order below T N ≈ 71 K and T N ≈ 260 K, respectively. MnAs is a ferromagnet below T C ≈ 317 K and undergoes hexagonal-orthorhombichexagonal transitions at T S ≈ 317 K and 384 K, respectively. For TAs, Seebeck coefficients vary between + 40 μV/K and -40 μV/K in the 2 K to 300 K range, whereas thermal conductivity values stay below 18 W/(m K). The Sommerfeldcoefficient γ are less than 10 mJ/(K 2 mol). At room temperature with application of 8 Tesla magnetic field, large positive magnetoresistance is found for TaAs (~25%), MnAs (~90%) and for NbAs (~75%).

The thermal conductivity of electron-doped Ba(Fe 1−x Co x ) 2 As 2 single crystals is investigated below 200 K, with an emphasis on the behavior near the magnetic and superconducting (T c ) transition temperatures. An enhancement of the in-plane thermal conductivity κ ab is observed below T c for all samples, with the greatest enhancement observed near optimal doping. The observed trends are consistent with the scattering of heat carriers by low-energy magnetic excitations. Upon entering the superconducting state, the formation of a spin gap leads to reduced scattering and an enhancement in κ(T ). Similarly, an enhancement of κ is observed for polycrystalline BaFe 2 As 2 below the magnetic transition, and qualitative differences in κ(T ) between single crystalline and polycrystalline BaFe 2 As 2 are utilized to discuss anisotropic scattering. This study highlights how measuring κ near T c in novel superconductors can be useful as a means to probe the potential role of spin fluctuations.

The intrinsic Fe local magnetic moment and Fe orbital occupations of iron-based superconductors are unveiled through the local, real-space capability of aberration-corrected scanning transmission electron microscopy/electron energy loss spectroscopy (STEM/EELS). Although the ordering of Fe moments needs to be suppressed for superconductivity to arise, the local, fluctuating Fe magnetic moment is enhanced near optimal superconductivity.

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