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      Determination of the phase diagram of the electron doped superconductor Ba(Fe\(_{1-x}\)Co\(_x\))\(_2\)As\(_2\)

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          Abstract

          Systematic measurements of the resistivity, heat capacity, susceptibility and Hall coefficient are presented for single crystal samples of the electron-doped superconductor Ba(Fe\(_{1-x}\)Co\(_x\))\(_2\)As\(_2\). These data delineate an \(x-T\) phase diagram in which the single magnetic/structural phase transition that is observed for undoped BaFe\(_2\)As\(_2\) at 134 K apparently splits into two distinct phase transitions, both of which are rapidly suppressed with increasing Co concentration. Superconductivity emerges for Co concentrations above \(x \sim 0.025\), and appears to coexist with the broken symmetry state for an appreciable range of doping, up to \(x \sim 0.06\). The optimal superconducting transition temperature appears to coincide with the Co concentration at which the magnetic/structural phase transitions are totally suppressed, at least within the resolution provided by the finite step size between crystals prepared with different doping levels. Superconductivity is observed for a further range of Co concentrations, before being completely suppressed for \(x \sim 0.018\) and above. The form of this \(x-T\) phase diagram is suggestive of an association between superconductivity and a quantum critical point arising from suppression of the magnetic and/or structural phase transitions.

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          Most cited references 5

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          Unconventional Superconductivity with a Sign Reversal in the Order Parameter ofLaFeAsO1−xFx

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            Superconductivity at 38 K in the Iron Arsenide\(({\mathrm{Ba}}_{1-x}{\mathrm{K}}_{x}){\mathrm{Fe}}_{2}{\mathrm{As}}_{2}\)

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              Magnetic Order versus superconductivity in the Iron-based layered La(O1-xFx)FeAs systems

               G. Chen,  H. Mook,  J. Lynn (2008)
              In high-transition temperature (high-Tc) copper oxides, it is generally believed that antiferromagnetism plays a fundamental role in the superconducting mechanism because superconductivity occurs when mobile electrons or holes are doped into the antiferromagnetic parent compounds. The recent discovery of superconductivity in the rare-earth (R) iron-based oxide systems [RO1-xFxFeAs] has generated enormous interest because these materials are the first noncopper oxide superconductors with Tc exceeding 50 K. The parent (nonsuperconducting) LaOFeAs material is metallic but shows anomalies near 150 K in both resistivity and dc magnetic susceptibility. While optical conductivity and theoretical calculations suggest that LaOFeAs exhibits a spin-density-wave (SDW) instability that is suppressed with doping electrons to form superconductivity, there has been no direct evidence of the SDW order. Here we use neutron scattering to demonstrate that LaOFeAs undergoes an abrupt structural distortion below ~150 K, changing the symmetry from tetragonal (space group P4/nmm) to monoclinic (space group P112/n) at low temperatures, and then followed with the development of long range SDW-type antiferromagnetic order at ~134 K with a small moment but simple magnetic structure. Doping the system with flourine suppresses both the magnetic order and structural distortion in favor of superconductivity. Therefore, much like high-Tc copper oxides, the superconducting regime in these Fe-based materials occurs in close proximity to a long-range ordered antiferromagnetic ground state. Since the discovery of long
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                Author and article information

                Journal
                15 November 2008
                Article
                10.1103/PhysRevB.79.014506
                0811.2463

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

                Custom metadata
                Phys. Rev. B 79, 014506 (2009)
                cond-mat.supr-con cond-mat.str-el

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