2017年4月26日(水) 13時30分

　The spontaneous appearance of nematicity, a state of matter that breaks rotation but not translation symmetry, is ubiquitous in many iron based superconductors (Fe SC), and has relevance for the cuprates as well. Here I will review recent electronic Raman scattering experiments which report the presence of critical electron nematic fluctuations in the tetragonal phase of several Fe SC systems. I will focus in particular on the compound FeSe where the effect of both chemical (isovalent substitution with of Se with S) and physical pressure on nematic degrees of freedom have been studied. In isovalently doped S-FeSe the nematic fluctuations extend over most of the superconducting dome. Their associated nematic susceptibility shows Curie-Weiss behavior, and its doping dependence suggests the presence of a non-magnetic nematic quantum critical point near optimal TC, a rather unique feature among Fe SC. Similar nematic fluctuations are also observed in FeSe at low hydrostatic pressures. However the fluctuations disappear quickly above 1.5 GPa where pronounced phonon anomalies indicate a possible change in the Fermi surface topology, which appears to be correlated with the onset of a magnetic phase and the opening of a pseudogap below 100K. In light of the difference between chemical and physical pressures on FeSe, I will discuss the relevance of nematic quantum criticality on the superconducting properties of FeSe and other Fe SC.