Elastotransport and Nernst effect in 122- and 1111-Fe-based superconductors: Evidence for superconductivity driven by nematic fluctuations

The role of nematic fluctuations in the appearance of high temperature superconductivity is still controversial. In this work we investigate the phase diagrams of BaFe${}_2$As${}_2$ and LaFeAsO as a function of electron-doping through elastotransport and Nernst effect measurements. We obtain an anomalously large Nernst coefficient in the tetragonal phase, upon doping its magnitude strikingly mimicking the superconducting dome. Similar but slightly different non-monotonic behavior is found for the elastoresistivity. Using a minimal orbital model for iron-based superconductors we show that the Nernst coefficient couples directly to the nematic fluctuations. We explain the difference between elastoresistivity and Nernst effect by incorporating the coupling to a soft phonon mode in our theory. Thus, our experimental results provide supportive evidence that nematic fluctuations are crucial for the formation of the superconducting state in iron-based superconductors.