Spectrally resolved femtosecond reflectivity relaxation dynamics in undoped spin-density wave 122-structure iron-based pnictides

We systematically investigate temperature- and spectrally-dependent optical reflectivity dynamics in AAs2Fe2, (A=Ba, Sr and Eu), iron-based superconductors parent spin-density-wave (SDW) compounds. Two different relaxation processes are identified. The behavior of the slower process, which is strongly sensitive to the magneto-structural transition, is analyzed in the framework of the relaxation-bottleneck model involving magnons. The results are compared to recent time resolved angular photoemission results (TR-ARPES) and possible alternative assignment of the slower relaxation to the magneto-structural order parameter relaxation is discussed.