  The interplay among charge, spin and lattice degrees of freedom in solids
gives rise to intriguing macroscopic quantum phenomena such as colossal
magnetoresistance, multiferroicity and high-temperature superconductivity.
Strong coupling or competition between various orders in these systems presents
the key to manipulate their functional properties by means of external
perturbations such as electric and magnetic fields or pressure. Ultrashort and
intense optical pulses have emerged as an interesting tool to investigate
elementary dynamics and control material properties by melting an existing
order. Here, we employ few-cycle multi-terahertz pulses to resonantly probe the
evolution of the spin-density-wave (SDW) gap of the pnictide compound BaFe2As2
following excitation with a femtosecond optical pulse. When starting in the
low-temperature ground state, optical excitation results in a melting of the
SDW order, followed by ultrafast recovery. In contrast, the SDW gap is induced
when we excite the normal state above the transition temperature. Very
surprisingly, the transient ordering quasi-adiabatically follows a coherent
lattice oscillation at a frequency as high as 5.5 THz. Our results attest to a
pronounced spin-phonon coupling in pnictides that supports rapid development of
a macroscopic order on small vibrational displacement even without breaking the
symmetry of the crystal.
