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A new family of superconductors containing layers of iron arsenide has attracted considerable interest because of their high transition temperatures (Tc), some of which are >50 K, and because of similarities with the high-Tc copper oxide superconductors. In both the iron arsenides and the copper oxides, superconductivity arises when an antiferromagnetically ordered phase has been suppressed by chemical doping. A universal feature of the copper oxide superconductors is the existence of a resonant magnetic excitation, localized in both energy and wavevector, within the superconducting phase. This resonance, which has also been observed in several heavy-fermion superconductors, is predicted to occur when the sign of the superconducting energy gap takes opposite values on different parts of the Fermi surface, an unusual gap symmetry which implies that the electron pairing interaction is repulsive at short range. Angle-resolved photoelectron spectroscopy shows no evidence of gap anisotropy in the iron arsenides, but such measurements are insensitive to the phase of the gap on separate parts of the Fermi surface. Here we report inelastic neutron scattering observations of a magnetic resonance below Tc in Ba0.6K0.4Fe2As2, a phase-sensitive measurement demonstrating that the superconducting energy gap has unconventional symmetry in the iron arsenide superconductors. [ABSTRACT FROM AUTHOR]

We have studied the magnetic structure of a Cr-doped iron-arsenide compound CaFe4As3 by means of single crystal neutron diffraction. The neutron data reveal that below 90 K, an antiferromagnetic structure with propagation vector k = 0 is adopted. Refinement of the magnetic structure using one of the modes allowed by symmetry analysis indicates that two of the four Fe sites, including the one where the selective substitution Fe/Cr happens, bear reduced magnetic moments. Density functional theory calculations confirm the stability of such a magnetic arrangement. [ABSTRACT FROM AUTHOR]

We report the results of a systematic investigation of the phase diagram of the iron-based superconductor Ba1-xKxFe2As2 from x = 0 to x = 1.0 using high-resolution neutron and x-ray diffraction and magnetization measurements. The polycrystalline samples were prepared with an estimated compositional variation of &Dgr;x≲ 0.01, allowing a more precise estimate of the phase boundaries than reported so far. At room temperature, Ba1-xKxFe2As2crystallizes in a tetragonal structure with the space group symmetry of 14/mmm, but at low doping, the samples undergo a coincident first-order structural and magnetic phase transition to an orthorhombic (O) structure with the space group Fmmm and a striped antiferromagnet (AF) with the space group Fcmm'm'. The transition temperature falls from a maximum of 139 K in the undoped compound to 0 K at x = 0.252, with a critical exponent as a function of doping of 0.25(2) and 0.12(1) for the structural and magnetic order parameters, respectively. The onset of superconductivity occurs at a critical concentration of ;c = 0.130(3), and the superconducting transition temperature grows linearly with x until it crosses the AF/0 phase boundary. Below this concentration, there is microscopic phase coexistence of the AF/O and superconducting order parameters, although a slight suppression of the AF/O order is evidence that the phases are competing. At higher doping, superconductivity has a maximum Tc of 38 K at x = 0.4 that falls to 3 K at x = 1.0. We discuss reasons for the suppression of the spin density wave order and the electron-hole asymmetry in the phase diagram. [ABSTRACT FROM AUTHOR]

We report a high resolution neutron diffraction investigation of the coupling of structural and magnetic transitions in Ba1-xKxFe2As2. The tetragonal-orthorhombic and antiferromagnetic transitions are suppressed with potassium doping, falling to zero at x≲0.24. However, unlike Ba(Fe1-xCox)2As2, the two transitions are first order and coincident over the entire phase diagram, with a biquadratic coupling of the two order parameters. The phase diagram is refined showing that the onset of superconductivity is at x=0.133 with all three phases coexisting until x⩽0.24. [ABSTRACT FROM AUTHOR]