In a fully gapped superconductor the electronic Raman response has a pair-breaking peak at twice the superconducting gap $\mathrm{\Delta }$, if the Bogoliubov excitations are uncorrelated. Motivated by the iron based superconductors, we study how this peak is modified if the superconducting phase hosts a nematic-structural quantum critical point. We show that, upon approaching this point by tuning, e.g., doping, the growth of nematic correlations between the quasiparticles transforms the pair-breaking peak into a nematic resonance. The mode energy is below $2\mathrm{\Delta }$, and stays finite at the quantum critical point, where its spectral weight is sharply enhanced. The latter is consistent with recent experiments on electron-doped iron based superconductors and provides direct evidence of nematic correlations in their superconducting phases.

• Received 17 April 2015

DOI:https://doi.org/10.1103/PhysRevLett.116.017001