Nematic fluctuations at nonzero q in FeSe vs. Ba(Fe1-xCox)2As2*

FeSe and Ba(Fe1-xCox)2As2 (Ba122) have several shared phase transitions such as the tetragonal-to-orthorhombic structural transition associated with nematic electronic order as well as a superconducting transition. There are also important differences, including magnetic order found in the Ba122 but not the FeSe compounds. Our recent inelastic X-ray and neutron scattering measurements of TA phonons in these compounds focused on anomalies in their dispersion. These anomalies reflect nematic fluctuations at nonzero q. We obtained nematic correlation length as a function of temperature from fits to these anomalous dispersions and found that it is similar in FeSe and Ba(Fe0.96Co0.04)2As2: compounds whose structural transition temperatures are close. In addition, we measured TA phonon dispersions and linewidths in optimally-doped Ba122 in detail. I will discuss the implications of the observed phenomena, focusing on the relation between nematic correlation length and the structural transitions, magnetic order, and superconductivity.