A crossover in the phase diagram of NaFe1−xCoxAs determined by electronic transport measurements

We report electronic transport measurements on single crystals of the NaFe1−xCoxAs system. We found that the cotangent of the Hall angle, cot θH, follows T4 for the parent compound with filamentary superconductivity and T2 for the heavily overdoped non-superconducting sample. However, it exhibits approximately T3-dependence in all the samples with bulk superconductivity, suggesting that this behavior is associated with bulk superconductivity in ferropnictides. A deviation develops below a characteristic temperature T* well above the structural and superconducting transitions, accompanied by a departure from power-law temperature dependence in resistivity. The doping dependence of T* resembles the crossover line of the pseudogap phase in cuprates. (paper)