Terahertz pulse-driven collective mode in the nematic superconducting state of Ba${}_{1-x}$K${}_x$Fe${}_2$As${}_2$

Abstract

We investigate 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 non-linear 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 new superconducting collective mode activated by nematicity.

25 pages, 3 figures. Supplementary Material available upon request

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