The origin of nematic order in FeSe

TL;DR

This paper investigates the origin of nematic order in FeSe, revealing that small Fermi pockets cause a near-degeneracy of magnetic and charge fluctuations, explaining the nematic transition without magnetic order.

Contribution

The study proposes a mechanism where small Fermi energy causes competing fluctuations to induce nematic order without magnetism in FeSe.

Findings

Small Fermi pockets lead to near-degenerate magnetic and charge fluctuations.

Magnetic and charge fluctuations cooperate for nematic order but compete as primary drivers.

Pressure lifts the near-degeneracy, affecting the nematic transition as observed experimentally.

Abstract

The origin of the 90 K nematic transition in the chalcogenide FeSe, which displays no magnetic order down to T=0, remains a major puzzle for a unifying theory for the iron-based superconductors. We analyze this problem in light of recent experimental data which reveal very small Fermi pockets in this material. We show that the smallness of the Fermi energy leads to a near-degeneracy between magnetic fluctuations and fluctuations in the charge-current density-wave channel. While the two fluctuation modes cooperate to promote the same preemptive Ising-nematic order, they compete for primary order. We argue that this explains why in FeSe the nematic order emerges when the magnetic correlation length is smaller than in other Fe-based materials, and why no magnetism is observed. We discuss how pressure lifts this near-degeneracy, resulting in a non-monotonic dependence of the nematic transition with pressure, in agreement with experiments.