Jacob Warshauer: Transient gap generation in BaFe2As2 phonon driven state

Iron-based superconductors provide a rich platform to investigate the interplay between nematicity, magnetism, and unconventional superconductivity. These materials show extreme sensitivity to lattice distortions. Coherent excitation of the A1g phonon by a femtosecond laser directly modulates the anion height; an increase of this parameter favors an enhanced Fe magnetic moment. This means to control the height of the pnictogen or chalcogen above the Fe square-lattice layer led to the discovery of a light-induced gap resembling the spin-density wave gap at temperatures above the spin-density wave transition temperature. This transient order leaves us with several questions: why does this robust transient state not appear in the spin-density wave state when driving the same phonon and is there a displacive response of the spin-density wave gap corresponding to the excitation of the A1g phonon. Here, we cover measurements taken with time-resolved broadband terahertz spectroscopy to investigate the dynamics of BaFe2As2 in the A1g phonon driven state. For the first time, a transient gap generation is observed at early time delays below the magnetic transition temperature. Temperature dependent measurements conclude the same transient feature persists in the normal state, even up to room temperature. Fluence dependent measurements reveal a blue shifting of the transient gap as the material is excited with higher pump fluence.