Stronger quantum fluctuation with larger spins: Emergent magnetism in the pressurized high-temperature superconductor FeSe

A counterintuitive enhancement of quantum fluctuation with larger spins, together with a few physical phenomena, is discovered in studying the recently observed emergent magnetism in high-temperature superconductor FeSe under pressure. Starting with an experimental crystalline structure from our high-pressure x-ray refinement, we theoretically analyze the stability of the magnetically ordered state with a realistic spin-fermion model. We find, surprisingly, that in comparison with magnetically ordered Fe pnictides, the larger spins in FeSe suffer even stronger long-range quantum fluctuations that diminish their ordering at ambient pressure. This fail-to-order quantum spin-liquid state then develops into an ordered state above 1 GPa 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 1 GPa. 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 paper establishes a generic exotic paradigm of stronger quantum fluctuation with larger spins that complements the standard knowledge of insulating magnetism.