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Structure and anisotropic properties of single crystals nickel doped barium iron arsenide

Description: The crystal structure, anisotropic electrical resistivity and magnetic susceptibility, as well as specific heat results of the pure single crystals of BaFe{sub 2}As{sub 2}, BaFeNiAs{sub 2}, and BaNi{sub 2}As{sub 2} are surveyed. BaFe{sub 2}As{sub 2} properties demonstrate the equivalence of C(T), Fisher's d({chi}T)/dT and d{rho}/dT results in determining the antiferromagnetic transition at T{sub N} = 132(1) K. BaNi{sub 2}As{sub 2} shows a structural phase transition from a high-temperature tetragonal phase to a low-temperature triclinic pol symmetry at T{sub 0} 131 K, with superconducting critical temperature well below at {Tc} 0.69 K. BaFeNiAs{sub 2} does not show any sign of superconductivity and gives behavioral similarity to BaCo{sub 2}As{sub 2}, a renomalized paramagnetic metal.
Date: January 1, 2009
Creator: Ronning, Filip; Bauer, Eric D; Sefat, A S; Jin, R; Mcguire, M A; Sales, B C et al.
Partner: UNT Libraries Government Documents Department

Neutron scattering investigation of the magnetic order in single crystalline BaFe2As2

Description: The magnetic structure of BaFe{sub 2}As{sub 2} was determined from polycrystalline neutron diffraction measurements soon after the ThCr{sub 2}Si{sub 2}-type FeAs-based superconductors were discovered. Both the moment direction and the in-plane antiferromagnetic wavevector are along the longer a-axis of the orthorhombic unit cell. There is only one combined magnetostructural transition at {approx}140 K. However, a later single-crystal neutron diffraction work reported contradicting results. Here, we show neutron diffraction results from a single-crystal sample, grown by a self-flux method, that support the original polycrystalline work.
Date: January 1, 2008
Creator: Bao, Wei; Qiu, Y; Kofu, M; Lee, S - H; Chang, S; Wu, T et al.
Partner: UNT Libraries Government Documents Department

Barium iron arsenide, barium cobalt arsenide, barium nickel arsenide single crystals and superconductivity upon cobalt doping

Description: The crystal structure and physical properties of BaFe{sub 2}As{sub 2}, BaCo{sub 2}As{sub 2}, and BaNi{sub 2}As{sub 2} single crystals are surveyed. BaFe{sub 2}As{sub 2} gives a magnetic and structural transition at T{sub N} = 132(1) K, BaCo{sub 2}As{sub 2} is a paramagnetic metal, while BaNi{sub 2}As{sub 2} has a structural phase transition at T{sub 0} = 131 K, followed by superconductivity below {Tc} = 0.69 K. The bulk superconductivity in Co-doped BaFe{sub 2}As{sub 2} 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 BaFe{sub 1.84}Co{sub 0.16}As{sub 2} inter-band scattering due to Co is weak.
Date: January 1, 2009
Creator: Ronning, Filip; Sefat, A S; Mcguire, M M; Sales, B; Jin, R & Mandrus, D
Partner: UNT Libraries Government Documents Department

Proximity detector circuits: an attractive alternative to tunnel diode oscillators for contactless measurements in pulsed magnetic fields

Description: A new radio frequency oscillator circuit based on a proximity detector integrated circuit is described as an alternative for the traditional tunnel diode oscillator used for pulsed magnetic field measurements at low temperatures. The new circuit has been successfully applied to measure the superconducting upper critical field in Ba{sub 0.55}K{sub 0.45}Fe{sub 2}As{sub 2} single crystfl.ls up to 60 T. The new circuit design avoids many of the problems associated with tunnel diode circuits while keeping the advantages of contact less measurements in pulsed magnets.
Date: January 1, 2009
Creator: Altarawneh, Moaz M & Mielke, Charles H
Partner: UNT Libraries Government Documents Department

ARPES studies of the electronic structure of LaOFe(P,As)

Description: We report a comparison study of LaOFeP and LaOFeAs, two parent compounds of recently discovered iron-pnictide superconductors, using angle-resolved photoemission spectroscopy. Both systems exhibit some common features that are very different from well-studied cuprates. In addition, important differences have also been observed between these two ferrooxypnictides. For LaOFeP, quantitative agreement can be found between our photoemission data and the LDA band structure calculations, suggesting that a weak coupling approach based on an itinerant ground state may be more appropriate for understanding this new superconducting compound. In contrast, the agreement between LDA calculations and experiments in LaOFeAs is relatively poor, as highlighted by the unexpected Fermi surface topology around ({pi},{pi}). Further investigations are required for a comprehensive understanding of the electronic structure of LaOFeAs and related compounds.
Date: June 2, 2010
Creator: Analytis, J.G.
Partner: UNT Libraries Government Documents Department

Symmetry-Breaking Orbital Anisotropy Observed for Detwinned Ba(Fe1-xCox)2As2 above the Spin Density Wave Transition

Description: Nematicity, defined as broken rotational symmetry, has recently been observed in competing phases proximate to the superconducting phase in the cuprate high temperature superconductors. Similarly, the new iron-based high temperature superconductors exhibit a tetragonal to orthorhombic structural transition (i.e. a broken C{sub 4} symmetry) that either precedes or is coincident with a collinear spin density wave (SDW) transition in undoped parent compounds, and superconductivity arises when both transitions are suppressed via doping. Evidence for strong in-plane anisotropy in the SDW state in this family of compounds has been reported by neutron scattering, scanning tunneling microscopy, and transport measurements. Here we present an angle resolved photoemission spectroscopy study of detwinned single crystals of a representative family of electron-doped iron-arsenide superconductors, Ba(Fe{sub 1-x}Co{sub x}){sub 2}As{sub 2} in the underdoped region. The crystals were detwinned via application of in-plane uniaxial stress, enabling measurements of single domain electronic structure in the orthorhombic state. At low temperatures, our results clearly demonstrate an in-plane electronic anisotropy characterized by a large energy splitting of two orthogonal bands with dominant d{sub xz} and d{sub yz} character, which is consistent with anisotropy observed by other probes. For compositions x > 0, for which the structural transition (T{sub S}) precedes the magnetic transition (T{sub SDW}), an anisotropic splitting is observed to develop above T{sub SDW}, indicating that it is specifically associated with T{sub S}. For unstressed crystals, the band splitting is observed close to T{sub S}, whereas for stressed crystals the splitting is observed to considerably higher temperatures, revealing the presence of a surprisingly large in-plane nematic susceptibility in the electronic structure.
Date: August 19, 2011
Creator: Yi, Ming
Partner: UNT Libraries Government Documents Department