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

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The temperature dependence of the upper critical magnetic field ($H_{\text{c2}}$) in a BaFe1.84Co0.16As2 single crystal was determined via resistivity, for the inter-plane ($H\perp ab$) and in-plane ($H\parallel ab$) directions in pulsed and static magnetic fields of up to 60 T. Suppressing superconductivity in a pulsed magnetic field at 3He temperatures permits us to construct an $H_{\text{c2}}$–$T$ phase diagram from quantitative $H_{\text{c2}}(0)$ values and determine its behavior in low temperatures. $H_{\text{c2}}(0)$ with $H\parallel ab$ [$H_{\text{c2}\|}(0)$] and $H\perp ab$ [$H_{\text{c2}\bot }(0)$] are ${\sim}55$ and ${\sim}50$ T respectively. These values are ${\sim}1.2--1.4$ times larger than the weak-coupling Pauli paramagnetic limit ($H_{\text{p}} = 1.84 T_{\text{c}}$), indicating that enhanced paramagnetic limiting is essential and this superconductor is unconventional. While $H_{\text{c2}}\parallel ab$ is saturated at low temperature, $H_{\text{c2}}$ with $H\perp ab$ ($H_{\text{c2}\bot}$) exhibits almost linear temperature dependence towards $T=0$ K which results in reduced anisotropy of $H_{\text{c2}}$ in low temperature. The anisotropy of $H_{\text{c2}}$ was ${\sim}3.4$ near $T_{\text{c}}$, and decreases rapidly with lower temperatures reaching ${\sim}1.1$ at $T=0.7$ K.
2009-08-15