Ultrafast melting of spin density wave order in BaFe2As2 observed by time- and angle-resolved photoemission spectroscopy with extreme-ultraviolet higher harmonic generation.

The transient single-particle spectral function of BaFe2As2, a parent compound of iron-based superconductors, has been studied by time- and angle-resolved photoemission spectroscopy with an extreme-ultraviolet laser generated by higher harmonics from Ar gas, which enables us to investigate the dynamics in the entire Brillouin zone. We observed electronic modifications from the spin density wave (SDW) ordered state within ~1 ps after the arrival of a 1.5 eV pump pulse. We observed optically excited electrons at the zone center above EF at 0.12 ps, and their rapid decay. After the fast decay of the optically excited electrons, a thermalized state appears and survives for a relatively long time. From a comparison with the density-functional theory band structure for the paramagnetic and SDW states, we interpret the experimental observations as the melting of the SDW. Exponential decay constants for the thermalized state to recover back to the SDW ground state are ~0.60 ps both around the zone center and the zone corner. [ABSTRACT FROM AUTHOR]