368 research outputs found
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
- …