Unusual Temperature and Field Dependence of Electrical Resistivity of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$

The in-plane electrical resistivity ($\rho _{ab})$ of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ single crystals with x=0.02 and 0.20 was measured as a function of temperature (2 - 400 K) and magnetic field (up to 14 Tesla). At the under-doped end (x = 0.02) of the superconductivity dome, we observe positive transverse (H$\bot $\textit{ab}) magnetoresistance (MR) that varies linearly with H, and T$^{2 }$dependence of $\rho _{ab}$ below and above the structure transition temperature. Surprisingly, the negative MR and T$^{1.5}$ dependence of $\rho _{ab}$ were found for the over-doped end member (x=0.2). The underlying physics of such unusual temperature and field dependence of $\rho _{ab}$ will be discussed.