Transient spin density wave order induced in the normal state of BaFe2As2 by coherent lattice oscillations

We trace the ultrafast dynamics of the spin density wave gap of the pnictide system BaFe2As2 probing resonantly with broadband multi-terahertz pulses. The photoexcitation in the low-temperature ground state leads to a fast suppression of the spin order followed by a slower recovery process. Surprisingly, in the normal state, we observe periodic oscillations of the spin density wave feature at a frequency as high as 5.5 THz. Our results indicate a transient development of a macroscopic order driven by a coherent lattice oscillation and attest to a pronounced spin–phonon coupling in pnictides.