Nematic correlations in iron-based superconductors have been widely studied through phonon softening measured by INS and IXS. Softening of the transverse acoustic phonons near the tetragonal-orthorhombic transition reflects growing nematic fluctuations, which also appear near the superconducting transition, highlighting the competition between nematicity and superconductivity. Our earlier IXS work on FeSe and Co-doped Ba122 showed similar behavior with and without magnetism and across the superconducting transition, suggesting that nematicity may hinder superconductivity.

Our most recent FeSe studies examine the effects of pressure, using both hydrostatic pressure and isoelectronic chemical pressure via S substitution. Both methods introduce magnetic phases and modify the structural transition order, opening a rich, otherwise inaccessible phase diagram. We present these results and compare how nematic fluctuations evolve across the different pressure-induced phases of FeSe.