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 BaFe 1.84 Co 0.16 As 2 single crystal was determined via resistivity, for the inter-plane ( H ⊥ a b ) and in-plane ( H ∥ a b ) directions in pulsed and static magnetic fields of up to 60 T. Suppressing superconductivity in a pulsed magnetic field at 3 He 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 ∥ a b [ H c2|| (0)] and H ⊥ a b [ H c2⊥ (0)] are ∼55 and ∼50 T respectively. These values are ∼1.2–1.4 times larger than the weak-coupling Pauli paramagnetic limit ( H p = 1.84 T c ), indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. While H c2 ∥ a b is saturated at low temperature, H c2 with H ⊥ a b ( H c2⊥ ) 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 ∼3.4 near...