Terahertz pulse-driven collective mode in the nematic superconducting state of Ba1−xKxFe2As2

We investigate the collective mode response of the iron-based superconductor Ba 1− x K x Fe 2 As 2 using intense terahertz (THz) light. In the superconducting state a THz Kerr signal is observed and assigned to nonlinear THz coupling to superconducting degrees of freedom. The polarization dependence of the THz Kerr signal is remarkably sensitive to the coexistence of a nematic order. In the absence of nematic order the C 4 symmetric polarization dependence of the THz Kerr signal is consistent with a coupling to the Higgs amplitude mode of the superconducting condensate. In the coexisting nematic and superconducting state the signal becomes purely nematic with a vanishing C 4 symmetric component, signaling the emergence of a superconducting collective mode activated by nematicity.