Ultrafast nonlinear excitation of collective modes in iron pnictides by intense terahertz pulses

Nonlinear terahertz (THz) spectroscopy has been a promising tool for the study of collective excitations in superconductors, as demonstrated by the observation of Higgs mode and Leggett mode in BCS superconductors with phonon-assisted pairing. The nonlinear nature of the excitation is essential as these modes arise from exotic symmetries and do not linearly couple to the electromagnetic field. Unconventional superconductors with strong inter-band pairing channels and multiple spin/charge fluctuations are expected to host new and rich collective excitations which have not yet been explored. Here we report the observation of collective modes in iron pnictides excited by intense THz pulses. The modes diminishes at both high temperatures and field strengths when there is strong quenching of the superconducting coherence. Our results clearly show out of equilibrium driving of multiband superconducting systems can induce nonlinear couplings between collective excitations of order parameters, leading to the formation of novel phase.