Role of multiband effects and electron-hole asymmetry in the superconductivity and normal state properties of Ba(Fe_(1-x)Co_(x))₂As₂

We report a systematic investigation, together with a theoretical analysis, of the resistivity and Hall effect in single crystals of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$, over a wide doping range. We find a surprisingly great disparity between the relaxation rates of the holes and the electrons, in excess of an order of magnitude in the low-doping, low-temperature regime. The ratio of the electron to hole mobilities diminishes with temperature and doping (away from the magnetically ordered state) and becomes more conventional. We also find a straightforward explanation of the large asymmetry (compared to cuprates) of the superconducting dome: in the underdoped regime the decisive factor is the competition between AF and superconductivity (SC), while in the overdoped regime the main role is played by degradation of the nesting that weakens the pairing interaction. Our results indicate that spin-fluctuations due to interband electron-hole scattering play a crucial role not only in the superconducting pairing, but also in the normal transport.