Here we report bulk superconductivity in BaFe1.8Co0.2As2 single crystals below Tc=22 K, as demonstrated by resistivity, magnetic susceptibility, and specific heat data. Hall data indicate that the dominant carriers are electrons, as expected from simple chemical reasoning. This is the first example of superconductivity induced by electron doping in this family of materials. In contrast with cuprates, the BaFe2As2 system appears to tolerate considerable disorder in the FeAs planes. First principles calculations for BaFe1.8Co0.2As2 indicate the interband scattering due to Co is weak.

[7,8] These discoveries have generated much interest in the mechanisms and manifestations of unconventional superconductivity in the family of doped quaternary layered oxypnictides LOMPn (L = La, Pr, Ce, Sm; M = Mn, Fe, Co, Ni; Pn=P, As) [9,10], because many features of these materials clearly set them apart from other superconductors. First, ab-initio calculations indicate that superconductivity originates from the d-orbitals of what would normally be expected to be pairbreaking magnetic Fe ions, suggesting that new non-phonon pairing mechanisms are responsible for the high T c [11,12]. Second, F-doped LaFeAsO is a semimetal, which exhibits strong ferromagnetic and antiferromagnetic fluctuations and a possible spin density wave instability around 150K in the parent undoped LaFeAsO [5,[13][14][15][16]. And third, superconductivity emerges on several disconnected pieces of the Fermi surface [11,12,17,18], thus exhibiting the multi-gap pairing, which has recently attracted so much attention in MgB 2 [19].Given the importance of magnetic correlations in the doped oxypnictides, transport measurements at very high magnetic fields are vital to probe the mechanisms of superconductivity. Indeed, first measurements of the upper critical field B c2 at low fields B < 7T have yielded a slope B c2 / (T c ) = dB c2 /dT º 2T/K near T c , for both La and Sm based oxypnictides [2][3][4][5][6]. From the conventional one-band Werthamer-Helfand-Hohenberg (WHH) theory [20] such slopes already imply rather high values B c2 (0) = 0.69T c B c2/ º 36T for LaFeAsO 0.89 F 0.1 , B c2 (0) º 59.3T for SmFeAsO 0.89 F 0.1 , and B c2 (0) º 72 T for PrFeAsO 0.89 F 0.1 , all well above B c2 of Nb 3 Sn. However, studies of the high-field superconductivity in MgB 2 alloys have shown that the upward curvature of B c2 (T) resulting from the multiband effects can significantly increase B c2 (0) as compared to the WHH one-band extrapolation (see, e.g., the review [21] and references therein). To address these important issues, we have performed high-field dc transport measurements on LaFeAsO 0.89 F 0.1 samples up to 45T. We show that B c2 (T) indeed exhibits two-gap behavior similar to that in MgB 2 , and B c2 (0) values exceed the WHH extrapolation by the factor ~ 2. Moreover, the observed B c2 (0) also exceeds the BCS paramagnetic limitPolycrystalline LaFeAsO 0.89 F 0.11 samples were made by solid state synthesis [4]. A sample ~ 3 x 1 x 0.5 mm was used for our four probe transport measurements in the 45T Hybrid magnet at the NHMFL, supplemented by low field measurements in a 9T superconducting magnet. Our low-field data agreed well with the earlier data taken at ORNL on the same sample [4], indicating its good temporal and atmospheric stability. The 45T Hybrid magnet was swept only from 11.5T to 45T due to the constant 11.5T background of the outsert magnet while lower fields were swept from 0T to 9T in a PPMS with resistivity measured in AC mode using a 5mA excitation current, whereas the high field resistance R(B) was measured by a Keithley nanovo...

We demonstrate that the changes in the elastic properties of the FeAs systems, as seen in our resonant ultrasound spectroscopy data, can be naturally understood in terms of fluctuations of emerging nematic degrees of freedom. Both the softening of the lattice in the normal, tetragonal phase as well as its hardening in the superconducting phase are consistently described by our model. Our results confirm the view that structural order is induced by magnetic fluctuations.

We report the first NMR investigation of spin dynamics in the overdoped nonsuperconducting regime of Ba(Fe1-xCox)2As2 up to x=0.26. We demonstrate that the absence of interband transitions with large momentum transfer Q{AF} approximately (pi/a,0) between the hole and electron Fermi surfaces results in complete suppression of antiferromagnetic spin fluctuations for x greater than or approximately 0.15. Our experimental results provide direct evidence for a correlation between T{c} and the strength of Q{AF} antiferromagnetic spin fluctuations.

We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe 1-x Co x ) 2 As 2 . Unlike the high-temperature superconductor cuprates and 1111-phase oxypnictides, Ba(Fe 1-x Co x ) 2 As 2 showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. We report the temperature dependencies of the upper critical field H c2 both parallel and perpendicular to the c-axis, the irreversibility field H irr c (T) and a rather unusual symmetric volume pinning force curve F p (H) suggestive of a strong pinning nanostructure. The anisotropy parameter γ = H c2 ab /H c2 c deduced from the slopes of dH c2 ab /dT = 4.9T/K and dH c2 c /dT = 2.5T/K decreases from ~2 near T c , to ~1.5 at lower temperatures, much smaller than g for 1111 pnictides and high-T c cuprates.

We present results from a detailed experimental investigation of LaFeAsO, the parent material in the series of "FeAs" based oxypnictide superconductors. Upon cooling, this material undergoes a tetragonalorthorhombic crystallographic phase transition at ϳ160 K followed closely by an antiferromagnetic ordering near 145 K. Analysis of these phase transitions using temperature dependent powder x-ray and neutrondiffraction measurements is presented. A magnetic moment of ϳ0.35 B per iron is derived from Mössbauer spectra in the low-temperature phase. Evidence of the structural transition is observed at temperatures well above the transition temperature ͑up to near 200 K͒ in the diffraction data as well as the polycrystalline elastic moduli probed by resonant ultrasound spectroscopy measurements. The effects of the two phase transitions on the transport properties ͑resistivity, thermal conductivity, Seebeck coefficient, and Hall coefficient͒, heat capacity, and magnetization of LaFeAsO are also reported, including a dramatic increase in the magnitude of the Hall coefficient below 160 K. The results suggest that the structural distortion leads to a localization of carriers on Fe, producing small local magnetic moments which subsequently order antiferromagnetically upon further cooling. Evidence of strong electron-phonon interactions in the high-temperature tetragonal phase is also observed.

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Inelastic neutron scattering measurements on single crystals of superconducting BaFe1.84Co0.16As2 reveal a magnetic excitation located at wavevectors (1/2 1/2 L) in tetragonal notation. On cooling below TC, a clear resonance peak is observed at this wavevector with an energy of 8.6(0.5) meV, corresponding to 4.5(0.3) kBTC . This is in good agreement with the canonical value of 5 kBTC observed in the cuprates. The spectrum shows strong dispersion in the tetragonal plane but very weak dispersion along the c-axis, indicating that the magnetic fluctuations are two-dimensional in nature. This is in sharp contrast to the anisotropic three dimensional spin excitations seen in the undoped parent compounds.PACS numbers: 78.70.Nx, 74.20.Mn Understanding the physics of superconductivity in high-T c cuprates and other unconventional superconductors remains a central unresolved problem at the forefront of condensed matter physics. One widespread school of thought maintains that magnetic fluctuations are intimately involved in the pairing mechanism. This view is supported by a growing number of neutron scattering investigations showing the appearance of a magnetic excitation coincident with the onset of superconductivity [1,2,3,4,5,6,7,8]. The spectrum shows a resonance at a wavevector related to the antiferromagnetic order in the non-superconducting parent compounds. The apparent resonance energy scales with T C for different cuprate materials exhibiting a wide range of superconducting transition temperatures [9], providing tantalizing evidence for a common mechanism related to magnetic fluctuations.The discovery of a new family of Fe-based high temperature superconductors with T C as high as 55 K [10,11,12,13,14,15,16] presents an exciting opportunity to examine the relationship of spin excitations to the superconducting condensate in unconventional superconductors. The new materials are composed of Fe containing planes (FeAs or FeSe). Both theory and experiment indicate that simple electron-phonon coupling cannot describe superconductivity in these materials [17,18]. Furthermore, the superconducting state exists in close proximity to magnetism as the parent compounds exhibit spin-density wave order [19,20]. These observations have been put forth as evidence that the superconductivity in the Fe-based materials is unconventional. The presence of the Fe planes suggests quasi-two-dimensionality, as observed in the cuprates. However, neutron scattering investigations of the spin waves in the undoped parent compounds SrFe 2 As 2 [21], BaFe 2 As 2 [22], and CaFe 2 As 2 [23], indicate anisotropic exchange that cannot be classified as two dimensional. Band structure calculations [24,25] indicate that doping should enhance the twodimensionality of the Fermi surface, favoring superconductivity [25]. Directly probing the magnetic fluctuations in superconducting Fe-based systems is crucial for further progress.Recent measurements on a polycrystalline sample of Ba 0.6 K 0.4 Fe 2 As 2 found a spin excitation that appears at the onset...