Nematic fluctuations in an orbital selective superconductor Fe_1+yTe_1−xSe_x

Despite a significant amount of research, there are still many unknowns about the underlying mechanisms of the superconductivity in Fe-based superconductors, in particular the roles of magnetism and nematicity. Here, the authors investigate the compositional dependence of the nematic susceptibility in Fe_1+yTe_1−xSe_x and the interplay between nematic and spin fluctuations, as well as the effect of orbital differentiation on nematic instability. Fe_1+ y Te_1− x Se_ x is characterized by its complex magnetic phase diagram and highly orbital-dependent band renormalization. Despite this, the behavior of nematicity and nematic fluctuations, especially for high tellurium concentrations, remains largely unknown. Here we present a study of both B _1 g and B _2 g nematic fluctuations in Fe_1+ y Te_1− x Se_ x (0 ≤ x ≤ 0.53) using the technique of elastoresistivity measurement. We discovered that the nematic fluctuations in two symmetry channels are closely linked to the corresponding spin fluctuations, confirming the intertwined nature of these two degrees of freedom. We also revealed an unusual temperature dependence of the nematic susceptibility, which we attributed to a loss of coherence of the d _ x y orbital. Our results highlight the importance of orbital differentiation on the nematic properties of iron-based materials.