BaT2As2 Single Crystals (T = Fe, Co, Ni) and Superconductivity upon Co-doping

The crystal structure and physical properties of BaFe2As2, BaCo2As2, and BaNi2As2 single crystals are surveyed. BaFe2As2 gives a magnetic and structural transition at TN = 132(1) K, BaCo2As2 is a paramagnetic metal, while BaNi2As2 has a structural phase transition at T0 = 131 K, followed by superconductivity below Tc = 0.69 K. The bulk superconductivity in Co-doped BaFe2As2 below Tc = 22 K is demonstrated by resistivity, magnetic susceptibility, and specific heat data. In contrast to the cuprates, the Fe-based system appears to tolerate considerable disorder in the transition metal layers. First principles calculations for BaFe1.84Co0.16As2 indicate the inter-band scattering due to Co is weak.Comment: Accepted to Physica

59-Co and 75-As NMR Investigation of Electron-Doped High Tc Superconductor BaFe(1.8)Co(0.2)As(2) (Tc = 22K)

We report an NMR investigation of the superconductivity in BaFe(2)As(2) induced by Co doping (Tc=22K). We demonstrate that Co atoms form an alloy with Fe atoms and donate carriers without creating localized moments. Our finding strongly suggests that the underlying physics of iron-pnictide superconductors is quite different from the widely accepted physical picture of high Tc cuprates as doped Mott insulators. We also show a crossover of electronic properties into a low temperature pseudo-gap phase with a pseudo-gap Delta 560K, where chi(spin) constant and resisitivty T. The NMR Knight shift below Tc decreases for both along the c-axis and ab-plane, and is consistent with the singlet pairing scenario.Comment: Accepted for publication in J. Phys. Soc. Jpn. (4 pages

Iron substitution in NdCoAsO: crystal structure and magnetic phase diagram

The effects of replacing small amounts of Co with Fe in NdCoAsO are reported. Polycrystalline materials with compositions NdCo1-xFexAsO (x = 0.05, 0.10, 0.15, and 0.20) are studied and the results compared to previous reports for NdCoAsO. Rietveld analysis of powder x-ray diffraction data shows that as Fe replaces Co on the transition metal (T) site, the T-As distance increases, and the As tetrahedra surrounding the T-site become more regular. Electrical resistivity and magnetization measurements indicate that the three magnetic phase transitions in NdCoAsO are suppressed as Co is replaced by Fe, and these transitions are not observed above 1.8 K for x = 0.20. Based on these results, the magnetic phase diagram for the Co-rich side of the NdCoAsO-NdFeAsO system is constructed.Comment: Accepted for publication in Physical Review B, revised text and figures, 5 pages, 5 figure

NMR Search for the Spin Nematic State in LaFeAsO Single Crystal

We report a 75-As single crystal NMR investigation of LaFeAsO, the parent phase of a pnictide high Tc superconductor. We demonstrate that spin dynamics develop a strong two-fold anisotropy within each orthorhombic domain below the tetragonal-orthorhombic structural phase transition at T[TO]~156 K. This intermediate state with a dynamical breaking of the rotational symmetry freezes progressively into a spin density wave (SDW) below T[SDW]~142 K. Our findings are consistent with the presence of a spin nematic state below T[TO] with an incipient magnetic order.Comment: Revised manuscript accepted for publication in Phys. Rev. Let