Study of the carrier content, scattering processes and the magnetic transition in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2} $ single crystals by transport measurements

We present resistivity $\rho(T)$ and Hall effect measurements in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals for a large range of Co dopings $x$ [1]. The negative Hall constant R$_{H}$ measured whatever $x$ indicates that electrons always dominate the transport properties at all $T$. More precisely, the analysis of our data at low $T$ indicates that the Hall number $n_{H}$($T$)=1/$e R_{H}$ can be assimilated to the actual values of electron content for $x > 0.04$, in good agreement with ARPES measurements on the same samples [2]. We also show that the electron carriers have archetypal Fermi liquid behavior, with large $T^{2}$ contributions to the electron scattering rate $1/\tau_{e}$. We propose that the unusually large variations of carrier content for $T < 150$K and of $1/\tau_{e}$ might be assigned to the small values of $E_{F}$ found in these compounds. In all the magnetic samples, the spin density wave transition is signalled by a drop in $n_{H}$ resulting from the gap opening associated with the nesting between the electron and hole bands. Moreover the evolution of $R_{H}(T)$ found for low Co dopings indicates that important modifications of the band structure of BaFe$_{2}$As$_{2}$ happen before the emergence of superconductivity at $x > 3$\%. [1] F. Rullier-Albenque et al., Phys. Rev. Lett. \textbf{103}, 057001 (2009). [2] V. Brouet et al., Phys. Rev. B \textbf{80},165115 (2009).