Nematic Fluctuations in an orbital-selective superconductor Fe_1+yTe_1-xSe_x

Optimal superconductivity is found in the vicinity of a fully suppressed nematic phase for most families of iron-based high-temperature superconductors, leading to speculation that nematic quantum critical fluctuations enhance or even mediate this superconductivity. While there has been extensive investigation of nematic fluctuations in several families of iron pnictides, much less is known about iron chalcogenides, in particular the family of Fe_1+yTe_1-xSe_x. Fe_1+yTe_1-xSe_x is special in several regards. It is one of the earliest discovered iron-based superconductors with the simplest crystal structure. Its phase diagram hosts the most complicated evolution of spin dynamics. It is also recognized as the most strongly correlated iron-based superconductors, as seen from the highly orbital dependent band renormalization. In this talk, I will present a systematic study of both B_1g and B_2g nematic fluctuations in Fe_1+yTe_1-xSe_x using the technique of elastoresistivity measurement, which showed that the two nematic fluctuations closely track the corresponding spin fluctuations, confirming the intertwined nature of the two degrees of freedom. I will also discuss the unusual temperature dependence of the nematic susceptibility, which revealed the highly orbital selective nature of the nematic instability, a property that has been long speculated by the theory.