Effects of Co substitution on thermodynamic and transport properties and anisotropic H$_{c2}$ in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$.*

Single crystal samples of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2 }$, x$<$ 0.12, have been characterized by microscopic, thermodynamic and transport measurements. With increasing Co concentration, the features of the structural and magnetic transitions are suppressed at a rate of roughly 15K per percent of Co. Superconductivity is stabilized at low temperatures for 0.038 $\le $x and up through our highest doping level of x = 0.114. The superconducting region has a dome like appearance with maximum T$_{c}$ values ($\sim $ 23 K) found near x $\sim $ 0.07. The T -- x phase diagram shows that either the existence of superconductivity in both the tetragonal and the orthorhombic (AFM) phase or there is a structural phase separation. Anisotropic H$_{c2}$ data clearly show that the superconductivity which occurs in samples that show features associated with the transition to the low temperature orthorhombic state is 50{\%} smaller than that found in samples that remain in the tetragonal phase. These data show that the superconductivity is sensitive to the suppression of the higher temperature phase transition.