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We use NMR techniques to investigate the magnetic properties of BaCo2As2 single crystals, the non-superconducting end member of the Co-substituted iron-pnictide high Tc superconductor Ba(Fe1-xCox)2As2 with x = 1. We present 75As NMR evidence for enhancement of low frequency spin fluctuations below ~ 100 K. This enhancement is accompanied by that of static uniform spin susceptibility at the wave vector q = 0, suggesting that the primary channel of the spin correlations is ferromagnetic rather than antiferromagnetic. Comparison between the NMR Knight shift 75K and bulk susceptibility chi(bulk) data uncovers the presence of two separate components of spin susceptibility with distinct temperature dependences, presumably because multiple electronic bands crossing the Fermi energy play different roles in the electronic properties of BaCo2As2., Submitted to PRB

We report the first NMR investigation of spin dynamics in the overdoped non-superconducting regime of Ba(Fe[1-x]Co[x])2As2 up to x =0.26. We demonstrate that the absence of inter-band transitions with large momentum transfer Q(AF) ~ (pi, 0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x > 0.15. Our experimental results provide direct evidence for a correlation between Tc and the strength of Q(AF) antiferromagnetic spin fluctuations., Accepted for publication in Phys. Rev. Lett

We will examine the possible link between spin fluctuations and the superconducting mechanism in the iron-based high temperature superconductor Ba(Fe,Co)2As2 based on NMR and high pressure transport measurements., Comment: Invited paper to m2s-IX (2009)

We investigate the electronic properties of Ba[Fe(1-x)Co(x)]2As2 (x =0.02, 0.04) in the lightly electron-doped regime by 59Co and 75As NMR. We demonstrate that Co doping significantly suppresses the magnetic ordering temperature to the SDW state, T_{SDW}. Furthermore, ordered moments below T_{SDW} exhibit large distribution. Strong spin fluctuations remain even below T_{SDW}, persisting all the way down to 4.2 K. We find no signature of additional freezing of spin degrees of freedom unlike the case of the lightly hole-doped stripe phase of the cuprates., Comment: 4 figures are changed to color online. 4 pages

We report comprehensive resistivity measurements of single crystalline samples of the Ba(Fe(1-x)Co(x))2As2 high Tc superconductor under hydrostatic pressure up to 2.75 GPa and over a broad concentration range, 0 < x < 0.099. We show that application of pressure progressively suppresses the SDW transition temperature, T_SDW, in the underdoped regime. There is no sign of pressure-induced superconductivity in the undoped BaFe2As2 down to 1.8 K, but applied pressure dramatically enhances Tc in the underdoped regime. The effect of pressure on Tc is very small in the optimally and overdoped regimes. As a consequence, the dome of the superconducting phase extends to x 0.02 under pressure. We discuss the implications of our findings in the context of a possible quantum phase transition between the SDW and superconducting phases., 6 figures

We report 19-F NMR investigation of the new high temperature superconductor LaFeAsO(0.89)F(0.11) (Tc ~ 28K). We demonstrate that low frequency spin fluctuations exhibit pseudo gap behavior above Tc. We also deduce the London penetration depth lambda from NMR line broadening below Tc., Accepted for publication in Phys. Rev. B (Rapid) with the original submission date