Unconventional magnetic field-temperature superconducting phase-diagram in Co doped BaFe$_2$As$_2$

Here, we report electrical transport and preliminary torque magnetometry measurements in underdoped Ba(Fe$_{1-x}$Co$_2$As$_2$ single crystals under high magnetic fields. For fields perpendicular to the conducting planes one observes an upward curvature for the temperature dependence of the upper critical field $H_{C2} (T)$ which contrasts markedly with one expects for conventional superconductors. While for fields along the conducting planes $H_{C2} (T)$ behaves nearly linearly in temperature down to low temperatures. For both configurations $H_{C2} (T)$ is found to surpass the weak coupling Pauli limiting field e.g., for a sample displaying a superconducting transition temperature $T_c \simeq 9.7$ K one observes $H_{C2} (T \rightarrow 0$ K $\geq 35$ T for fields along an in-plane direction. This value surpasses the expected Pauli limiting field by a factor of two. Thus, the resulting phase-diagram presents strong similarities with the one reported for poly-crystalline LaFeASO$_{1-x}$F$_x$ and which was recently claimed to be described by a model based on a two-gap strong coupled superconducting state. Our on-going torque magnetometry measurements are expected to check the validity of this interpretation.