Unconventional superconductivity usually originates from several strongly coupled degrees of freedom, such as magnetic, charge and elastic. A highly anisotropic electronic phase, not driven by lattice degrees of freedom, has been proposed in some of these superconductors, from cuprates to iron-based compounds. In the iron pnictide BaFe 2 As 2 , this nematic phase arises in the paramagnetic phase and is present for wide doping and temperature ranges. Here we probe the in-plane electronic anisotropy of electron-and hole-doped BaFe 2 As 2 compounds. Unlike other materials, the resistivity anisotropy behaves very differently for electron-and hole-type dopants and even changes sign on the hole-doped side. This behaviour is explained by Fermi surface reconstruction in the magnetic phase and spin-fluctuation scattering in the paramagnetic phase. This unique transport anisotropy unveils the primary role played by magnetic scattering, demonstrating the close connection between magnetism, nematicity and unconventional superconductivity.

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The low-temperature specific heat (SH) of overdoped La 2−x Sr x CuO 4 single crystals (0.178 x 0.290) has been measured. For the superconducting samples (0.178x 0.238), the derived gap values (without any adjusting parameters) approach closely onto the theoretical prediction ∆ 0 = 2.14k B T c for the weak-coupling d-wave BCS superconductivity. In addition, the residual term γ(0) of SH at H = 0 increases with x dramatically when beyond x ∼ 0.22, and finally evolves into the value of a complete normal metallic state at higher doping levels, indicating growing amount of unpaired electrons. We argue that this large γ(0) cannot be simply attributed to the pair breaking induced by the impurity scattering, instead the phase separation is possible. PACS numbers: 74.25.Bt,74.20.Rp, 74.25.Dw, 74.72.Dn c = 1 − 82.6(x − 0.16) 2 with T max c = 38 K. The SH measurements were performed on an Oxford Maglab cryogenic system using the thermal relaxation technique, as described in detail previously. 19 The temperature was down to 1.9 K and the magnetic field was applied parallel to c-axis in the measurements.