Spectroscopic evidence of nematic fluctuations in LiFeAs

The role of nematic fluctuations in the pairing mechanism of iron-based superconductors is frequently debated. Here, we present a method to reveal such fluctuations by identifying the energy and momentum of the corresponding nematic boson through the detection of a boson-assisted resonant amplification of Friedel oscillations. Using Fourier-transform scanning tunneling spectroscopy, we observe for the unconventional superconductor LiFeAs strong signatures of bosonic states at momentum $q\ensuremath{\sim}0$ and energy $\mathrm{\ensuremath{\Omega}}\ensuremath{\approx}8$ meV. We show that these bosonic states survive in the normal conducting state, and, moreover, that they are in perfect agreement with well-known strong above-gap anomalies in the tunneling spectra. Attributing these small-$q$ boson modes to nematic fluctuations, we provide a spectroscopic approach to the nematic boson in an unconventional superconductor.