Possible Superconductivity above 25K in Single-Crystalline Co-Doped BaFe_2As_2(Condensed matter: electronic structure and electrical, magnetic, and optical properties)

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We present the superconducting properties of single-crystalline Ba(Fe_<0.9>Co_<0.1>)_2As_2 by measuring its magnetization, resistivity, upper critical field, Hall coefficient, and magneto-optical images. The magnetization measurements reveal a fish-tail hysteresis loop at high temperatures and a relatively high critical current density above J_c=10^5A/cm^2 at low temperatures. The upper critical field determined by resistive transition is anisotropic with an anisotropic parameter of 〜3.5. Hall effect measurements indicate that Ba(Fe_<0.9>Co_<0.1>)_2As_2 is a multiband system and that the mobility of electrons is dominant. Magneto-optical imaging reveals a prominent Bean-like penetration of vortices, although there is a slight inhomogeneity in a sample. Moreover, we observe distinct superconductivity above 25K, which leads us to speculate that a higher transition temperature can be realized by fine-tuning the Co-doping level.
2009-02-15