  The role of nematic fluctuations for unconventional superconductivity has
been subject of intense discussions for many years. In iron-based
superconductors, the most established probe for electronic-nematic
fluctuations, i.e. the elastoresistivity seems to imply that superconductivity
is reinforced by electronic-nematic fluctuations, since the elastoresistivity
amplitude peaks at or close to optimal $T_C$. However, on the over-doped side
of the superconducting dome, the diminishing elastoresistivity suggests a
negligible importance in the mechanism of superconductivity. Here we introduce
the Nernst coefficient as a genuine probe for electronic nematic fluctuations,
and we show that the amplitude of the Nernst coefficient tracks the
superconducting dome of two prototype families of iron-based superconductors,
namely Rh-doped $BaFe_{2}As_{2}$ and Co-doped $LaFeAsO$. Our data thus provide
fresh evidence that in these systems nematic fluctuations foster the
superconductivity throughout the phase diagram.
