Nematic Fluctuations in an Orbital Selective Superconductor FeTeSe

We present a systematic study of the nematic fluctuations in the iron chalcogenide superconductor Fe Te Se () using the elastoresistivity technique. Near , in proximity to the double-stripe magnetic order of Fe Te, a diverging nematic susceptibility is observed. Upon increasing , despite the absence of magnetic order, the nematic susceptibility increases and becomes dominant, closely following the strength of the spin fluctuations. Over a wide range of compositions (), while the nematic susceptibility follows a Curie temperature dependence (with zero Weiss temperature) at low temperatures, it shows deviations from Curie-Weiss behavior for temperatures higher than . This is the opposite of what is observed in typical iron pnictides, where Curie-Weiss deviations are seen at low temperatures. We attribute this unusual temperature dependence to a loss of coherence of the orbital, which is supported by our theoretical calculations. Our results highlight the importance of orbital differentiation on the nematic properties of iron-based materials.