  Ultrafast light pulses can modify the electronic properties of quantum
materials by perturbing the underlying, intertwined degrees of freedom. In
particular, iron-based superconductors exhibit a strong coupling among
electronic nematic fluctuations, spins, and the lattice, serving as a
playground for ultrafast manipulation. Here we use time-resolved x-ray
scattering to measure the lattice dynamics of photo-excited BaFe2As2. Upon
optical excitation, no signature of an ultrafast change of the crystal symmetry
is observed, but the lattice oscillates rapidly in time due to the coherent
excitation of an A1g mode that modulates the Fe-As-Fe bond angle. We directly
quantify the coherent lattice dynamics and show that even a small photo-induced
lattice distortion can induce notable changes in the electronic and magnetic
properties. Our analysis implies that transient structural modification can
generally be an effective tool for manipulating the electronic properties of
multi-orbital systems, where electronic instabilities are sensitive to the
orbital character of bands near the Fermi level.
