Nematicity and quantum paramagnetism in FeSe

TL;DR

This paper proposes that quantum spin fluctuations, rather than orbital ordering, drive nematicity in FeSe, introducing a nematic quantum paramagnetic phase rooted in topological spin defects and Berry's phase effects.

Contribution

It introduces a novel quantum spin fluctuation mechanism for nematicity in FeSe, supported by an exactly solvable model and a field theory of the phase transition.

Findings

Quantum fluctuations of spin-1 moments can produce nematic quantum paramagnetism.

The nematic phase is linked to topological defects and Berry's phase effects.

A field theory describes a Landau-forbidden transition between Néel and nematic phases.

Abstract

In common with other iron-based high temperature superconductors, FeSe exhibits a transition to a ``nematic'' phase below 90Kelvin in which the crystal rotation symmetry is spontaneously broken. However, the absence of strong low-frequency magnetic fluctuations near or above the transition has been interpreted as implying the primacy of orbital ordering. In contrast, we establish that quantum fluctuations of spin-1 local moments with strongly frustrated exchange interactions can lead to a nematic quantum paramagnetic phase consistent with the observations in FeSe. We show that this phase is a fundamental expression of the existence of a Berry's phase associated with the topological defects of a N\'eel antiferromagnet, in a manner analogous to that which gives rise to valence bond crystal order for spin 1/2 systems. We present an exactly solvable model realizing the nematic quantum paramagnetic phase, discuss its relation with the spin-1 $J_1-J_2$ model, and construct a field theory of the Landau-forbidden transition between the N\'eel state and this nematic quantum paramagnet.