Speaker: Prof. Wei KuAffiliation: Shanghai Jiao Tong UniversityHost: Prof Hsin LinAbstract Details: Like the high-temperature superconductivity found in other systems, superconductivity in Fe-based systems is often found near an long-range ordered phase. This talk will introduce our understanding of the strong spin- and orbital-correlations. In particular, a counter-intuitive enhancement of quantum ﬂuctuation with larger spins, together with a few novel physical phenomena, is discovered in studying the recently observed emergent magnetism in high-temperature superconductor FeSe under pressure. Starting with experimental crystalline structure from our high-pressure X-ray reﬁnement, we analyze theoretically the stability of the magnetically ordered state with a realistic spin-fermion model. We ﬁnd surprisingly that in comparison with the magnetically ordered Fe-pnictides, the larger spins in FeSe suﬀer even stronger long-range quantum ﬂuctuation that diminishes their ordering at ambient pressure. This ”fail-to-order” quantum spin liquid state then develops into an ordered state above 1GPa due to weakened ﬂuctuation accompanying the reduction of anion height and carrier density. The ordering further beneﬁts from the ferro-orbital order and shows the observed enhancement around 1GPa. We further clarify the controversial nature of magnetism and its interplay with nematicity in FeSe in the same uniﬁed picture for all Fe-based superconductors. In addition, the versatile itinerant carriers produce interesting correlated metal behavior in a large region of phase space. Our study establishes a generic exceptional paradigm of stronger quantum ﬂuctuation with larger spins that complements the standard knowledge of insulating magnetism.