Nearly isotropic superconductivity in (Ba,K)Fe2As2

• Published in: Nature (London) Vol. 457; no. 7229; pp. 565 - 568
• Main Authors: Yuan, H. Q., Singleton, J., Balakirev, F. F., Baily, S. A., Chen, G. F., Luo, J. L., Wang, N. L.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 29.01.2009; Nature Publishing Group
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Effects of material synthesis, crystal structure, and chemical composition; Electronic structure; Exact sciences and technology; Humanities and Social Sciences; letter; multidisciplinary; Physics; Properties of type I and type II superconductors; Science; Science (multidisciplinary); Superconductivity; Transition temperature variations; Barium Iron Potassium Arsenides Mixed; Dimensionality; Electronic structure; Anisotropy; Superconducting transitions; High-Tc superconductors; Magnetic field effects; Fermi surface; Critical field; Magnetoresistance; Iron based arsenide
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature07676

Iron-arsenic superconductors: a sense of direction Superconductivity has been observed in iron-arsenic (or iron pnictide) compounds with critical transition temperatures as high as 56 K, inviting comparison with the high- T c copper oxides. The copper oxides have quasi-two-dimensional electronic properties, which led to speculation that extreme anisotropy (direction dependence) is a requirement for superconductivity at temperatures above 40 K. Yuan et al . report that this is not the case: the superconducting properties of (Ba,K)Fe 2 As 2 are in fact quite isotropic. This behaviour is strikingly different from all previously known layered superconductors. It is shown that the superconducting properties of (Ba,K)Fe 2 As 2 are quite isotropic. Such behaviour is strikingly different from all previously known layered superconductors, and indicates that reduced dimensionality in these compounds is not a prerequisite for 'high-temperature' superconductivity. Superconductivity was recently observed 1 , 2 , 3 , 4 , 5 , 6 , 7 in iron-arsenic-based compounds with a superconducting transition temperature ( T c ) as high as 56 K, naturally raising comparisons with the high- T c 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 view 7 , 10 . Here we report measurements of the electrical resistivity in single crystals of (Ba,K)Fe 2 As 2 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 superconductors 9 , 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.