Observation and Exploitation of Coherent Phonon Frequency Combs in Eu(Fe_.8Co_.2)₂As₂

Iron pnictides have gained interest in recent years based on their display of unconventional superconductivity. However, ultrafast studies on many of the alloys of the parent compound EuFe₂As₂ are missing in the literature. In this talk, I will present some preliminary results on a Cobalt doped europium iron pnictide sample that presents a strong pulsed behavior on top of the typical electronic decay. The pulses are found to be caused by the propagation of a coherent phonon wavepacket, which forms a frequency comb upon Fourier transformation of the time domain signal. We find that the pulsed behavior is extremely robust and is primarily dependent on sample thickness, as in picosecond ultrasonics experiments. Additionally, we see evidence that this frequency comb may provide a sensitive measure of the order parameter during phase transitions in the material, as a shift is seen upon crossing the Neel temperature in our experiments.