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

## TL;DR

This study reveals that in FeSe under pressure, larger spins lead to stronger quantum fluctuations, causing a transition from a quantum spin liquid to an ordered magnetic state, challenging conventional understanding of magnetism.

## Contribution

It introduces the novel finding that larger spins can enhance quantum fluctuations, leading to unexpected magnetic behavior in high-temperature superconductor FeSe under pressure.

## Key findings

- Quantum fluctuations in FeSe increase with larger spins.
- FeSe transitions from a quantum spin liquid to an ordered state above 1GPa.
- Enhanced magnetism correlates with pressure-induced changes in structure and carrier density.

## Abstract

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.