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      A Model of the Normal State Susceptibility and Transport Properties of Ba(Fe1-xCox)2As2: Why does the magnetic susceptibility increase with temperature?

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          Abstract

          A simple two-band model is used to describe the magnitude and temperature dependence of the magnetic susceptibility, Hall coefficient and Seebeck data from undoped and Co doped BaFe2As2. Overlapping, rigid parabolic electron and hole bands are considered as a model of the electronic structure of the FeAs-based semimetals. The model has only three parameters: the electron and hole effective masses and the position of the valence band maximum with respect to the conduction band minimum. The model is able to reproduce in a semiquantitative fashion the magnitude and temperature dependence of many of the normal state magnetic and transport data from the FeAs-type materials, including the ubiquitous increase in the magnetic susceptibility with increasing temperature.

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          Superconductivity at 41 K and its competition with spin-density-wave instability in layered CeO1-xFxFeAs.

          A series of layered CeO1-xFxFeAs compounds with x=0 to 0.20 are synthesized by the solid state reaction method. Similar to the LaOFeAs, the pure CeOFeAs shows a strong resistivity anomaly near 145 K, which was ascribed to the spin-density-wave instability. F doping suppresses this instability and leads to the superconducting ground state. Most surprisingly, the superconducting transition temperature could reach as high as 41 K. Such a high T_{c} strongly challenges the classic BCS theory based on the electron-phonon interaction. The closeness of the superconducting phase to the spin-density-wave instability suggests that the magnetic fluctuation plays a key role in the superconducting pairing mechanism. The study also reveals that the Ce 4f electrons form local moments and are ordered antiferromagnetically below 4 K, which could coexist with superconductivity.
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            Superconductivity at 22 K in Co-doped BaFe2As2 Crystals

            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 to the cuprates, the BaFe2As2 system appears to tolerate considerable disorder in the FeAs planes. First principles calculations for BaFe1.8Co0.2As2 indicate the inter-band scattering due to Co is weak.
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              Effects of Co substitution on thermodynamic and transport properties and anisotropic \(H_{c2}\) in Ba(Fe\(_{1-x}\)Co\(_x\))\(_2\)As\(_2\) single crystals

              Single crystalline samples of Ba(Fe\(_{1-x}\)Co\(_x\))\(_2\)As\(_2\) with \(x < 0.12\) have been grown and characterized via microscopic, thermodynamic and transport measurements. With increasing Co substitution, the thermodynamic and transport signatures of the structural (high temperature tetragonal to low temperature orthorhombic) and magnetic (high temperature non magnetic to low temperature antiferromagnetic) transitions are suppressed at a rate of roughly 15 K per percent Co. In addition, for \(x \ge 0.038\) superconductivity is stabilized, rising to a maximum \(T_c\) of approximately 23 K for \(x \approx 0.07\) and decreasing for higher \(x\) values. The \(T - x\) phase diagram for Ba(Fe\(_{1-x}\)Co\(_x\))\(_2\)As\(_2\) indicates that either superconductivity can exist in both low temperature crystallographic phases or that there is a structural phase separation. Anisotropic, superconducting, upper critical field data (\(H_{c2}(T)\)) show a significant and clear change in anisotropy between samples that have higher temperature structural phase transitions and those that do not. These data show that the superconductivity is sensitive to the suppression of the higher temperature phase transition.
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                Author and article information

                Journal
                11 June 2009
                2009-07-07
                Article
                0906.2134

                http://arxiv.org/licenses/nonexclusive-distrib/1.0/

                Custom metadata
                Physica C Vol 470, 304 (2010)
                5 pages, 4 figures, submitted to PRB, version 2, model parameters adjusted to reflect electron-hole symmetry for pure BaFe2As2
                cond-mat.supr-con

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