Evolution of soft phonons in FeSe under pressure

FeSe, as the simplest iron-based superconductor (Fe-SC), is of great interest for furthering our understanding of the Fe-SC materials. It shares a common tetragonal-to-orthorhombic phase transition at 90 K with many of the Fe-SCs, and becomes superconducting at 9 K, but in contrast displays no magnetic order. Previous studies have shown an enhancement of the superconducting transition temperature with increasing pressure, with a maximum of 37 K at approximately 6.3 GPa, and magnetic order concurrent with the structural transition above 1.6 GPa. Here we present IXS measurements of TA phonon dispersion from which we can extract the nematic correlation length. The phonon renormalization effects observed in our experiments have important implications for nematicity and superconductivity. This includes a significant reduction in the phonon softening at the high-pressure structural-magnetic transition and a new phonon hardening effect at the superconducting transition. We will discuss the implications of these results in the context of pressure- and doping-dependence in the Fe-SCs.