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

Abstract

We investigate the collective mode response of the iron-based superconductor Ba1−xKxFe2As2 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 C4 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 C4 symmetric component, signaling the emergence of a superconducting collective mode activated by nematicity.

Contribution of Collective Excitations to Third Harmonic Generation in Multiband Superconductors: the case of MgB$_2$

“…Finally, in Ref. [39] the authors measured the time-resolved signal for a pumping frequency much smaller than both optical gaps, so that one probes in time the square of the applied electric field weighted with K(ω ≈ 0), as theoretically expected by computing the convolution of the kernel with the pump profile [34,35,54]. The results confirm the increase of the non-linear response in the SC state, with the additional observation of a larger B 1g signal when superconductivity coexists with nematic order.…”

“…Also in this case, the claim of a relevance of the Higgs is not supported by realistic theoretical calculations: indeed, the authors of Ref. [39] do not consider the role of disorder, that is crucial not only to enhance all the paramagnetic processes, including the BCS ones, but also to change the symmetry of the non-linear kernel, that is the main focus of Ref. [39].…”

“…Finally, envisioning possible applications of this analysis, we would like to comment on recent experiments in electron-doped [37] or hole-doped [39] 122 multiband ironbased superconductors. So far, the non-linear response in these systems has been probed via time-domain protocols either with multidimensional THz pulses [37] or via THz pump-optical probe response [39].…”

“…Finally, envisioning possible applications of this analysis, we would like to comment on recent experiments in electron-doped [37] or hole-doped [39] 122 multiband ironbased superconductors. So far, the non-linear response in these systems has been probed via time-domain protocols either with multidimensional THz pulses [37] or via THz pump-optical probe response [39]. In the former case, the authors observed marked oscillations of the transmitted signal as a function of the pump-probe delay t pp at a frequency corresponding to twice the smaller gap.…”

“…In the present manuscript we provide a detailed analysis of the non-linear response of multiband superconductors, with the twofold aim from one side to clarify the nature of the collective fluctuations, in particular the Higgs mode, and from the other side to establish the experimental conditions for their detection via non-linear optics, that has been recently applied both to MgB 2 [24,27] and iron-based superconductors [37][38][39]. For what concerns the amplitude fluctuations, we will show that the redistribution of spectral weight between the two peaks at 2∆ α in the spectral functions depends crucially on the structure of the coupling matrix.…”