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Nearly isotropic superconductivity in (Ba,K)Fe2As2


Superconductivity was recently observed1,2,3,4,5,6,7 in iron-arsenic-based compounds with a superconducting transition temperature (Tc) as high as 56 K, naturally raising comparisons with the high-Tc copper oxides. The copper oxides have layered crystal structures with quasi-two-dimensional electronic properties, which led to speculation that reduced dimensionality (that is, extreme anisotropy) is a necessary prerequisite for superconductivity at temperatures above 40 K (refs 8, 9). Early work on the iron-arsenic compounds seemed to support this view7,10. Here we report measurements of the electrical resistivity in single crystals of (Ba,K)Fe2As2 in a magnetic field up to 60 T. We find that the superconducting properties are in fact quite isotropic, being rather independent of the direction of the applied magnetic fields at low temperature. Such behaviour is strikingly different from all previously known layered superconductors9,11, and indicates that reduced dimensionality in these compounds is not a prerequisite for ‘high-temperature’ superconductivity. We suggest that this situation arises because of the underlying electronic structure of the iron-arsenic compounds, which appears to be much more three dimensional than that of the copper oxides. Extrapolations of low-field single-crystal data incorrectly suggest a high anisotropy and a greatly exaggerated zero-temperature upper critical field.

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Figure 1: In-plane electrical resistivity of single crystal (Ba,K)Fe 2 As 2 measured in pulsed high magnetic fields.
Figure 2: Electrical resistivity versus temperature at selected magnetic fields.
Figure 3: The upper critical field Hc2(T).


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We acknowledge S. Riggs and J. Betts for experimental assistance and F. C. Zhang, P. Goddard and S. Blundell for discussions. Work at NHMFL-LANL is performed under the auspices of the National Science Foundation, Department of Energy and State of Florida. The experiments reported here are supported by the DOE BES program ‘Science in 100T’, the NHMFL-UCGP, the National Science Foundation of China, the National Basic Research Program of China (973 Program) and the Chinese Academy of Sciences. H.Q.Y. is also supported by PCSIRT of the Ministry of Education of China.

Author Contributions H.Q.Y. designed this study, did the main experiments and analysed the data. F.F.B. and J.S. provided experimental support. S.A.B. measured the angle dependence of Hc2. Samples were grown by G.F.C., J.L.L. and N.L.W. The paper was written by H.Q.Y. and J.S.

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Yuan, H., Singleton, J., Balakirev, F. et al. Nearly isotropic superconductivity in (Ba,K)Fe2As2. Nature 457, 565–568 (2009).

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