Dynamics of Photoexcited Carriers in Ba(Fe1-xCox)2As2 Single Crystals with Spin-Density-Wave Ordering

We apply the femtosecond optical pump-probe spectroscopy to study the relaxation dynamics in photoexcited Co-doped Ba(Fe1-xCox)(2)As-2 single crystals in the underdoped spin-density wave (SDW) state region of the x - T phase diagram. Underdoped SDW samples with Co-0 % and Co-2.5 % show a bottleneck in the relaxation as a consequence of the partial charge gap opening in the orthorhombic SDW phase, similar to previous results in other SDW iron-pnictides. Moreover, the charge gap magnitude decreases with increasing doping. The sample with Co-5.1 % displays both a SDW ordering and superconductivity at low T. We were able to observe a 2-fold anisotropy in our samples, existing up to similar to 200 K, without any applied uniaxial stress. We associate the 2-fold symmetry breaking in nominally tetragonal phase with nematic ordering of the Fe d orbitals.