Anisotropy of the Upper Critical Field in a Co-Doped BaFe2As2 Single Crystal

The temperature dependence of the upper critical magnetic field (H-c2) in a BaFe1.84Co0.16As2 single crystal was determined via resistivity, for the inter-plane (H perpendicular to ab) and in-plane (H parallel to ab) directions in pulsed and static magnetic fields of up to 60 T. Suppressing superconductivity in a pulsed magnetic field at He-3 temperatures permits us to construct an H-c2-T phase diagram from quantitative H-c2(0) values and determine its behavior in low temperatures. H-c2(0) with H parallel to ab [H-c2 parallel to(0)] and H perpendicular to ab [H-c2 perpendicular to(0)] are similar to 55 and similar to 50 T respectively. These values are similar to 1.2-1.4 times larger than the weak-coupling Pauli paramagnetic limit (H-p = 1.84T(c)), indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. While H-c2 parallel to ab is saturated at low temperature, H-c2 with H perpendicular to ab (H-c2 perpendicular to) exhibits almost linear temperature dependence towards T = 0 K which results in reduced anisotropy of H-c2 in low temperature. The anisotropy of H-c2 was similar to 3.4 near T-c, and decreases rapidly with lower temperatures reaching similar to 1.1 at T = 0.7 K.