Quantum effects in a half-polarized pyrochlore antiferromagnet

TL;DR

This paper investigates quantum effects in a half-polarized pyrochlore antiferromagnet, predicting a symmetry-broken state and magnon Bose-Einstein condensation near the magnetization plateau, with implications for experimental observations.

Contribution

It introduces a quantum fluctuation-based theory predicting a novel symmetry-broken state and magnon condensation in a half-polarized pyrochlore antiferromagnet.

Findings

Prediction of a symmetry-broken spin arrangement on the plateau

Identification of a lattice symmetry reduction from $Fd\bar{3}m$ to $P4_332$

Existence of a magnon Bose-Einstein condensate just above the plateau

Abstract

We study quantum effects in a spin-3/2 antiferromagnet on the pyrochlore lattice in an external magnetic field, focusing on the vicinity of a plateau in the magnetization at half the saturation value, observed in CdCr$_2$O$_4$, and HgCr$_2$O$_4$. Our theory, based on quantum fluctuations, predicts the existence of a symmetry-broken state on the plateau, even with only nearest-neighbor microscopic exchange. This symmetry broken state consists of a particular arrangement of spins polarized parallel and antiparallel to the field in a 3:1 ratio on each tetrahedron. It quadruples the lattice unit cell, and reduces the space group from $Fd\bar{3}m$ to $P4_332$. We also predict that for fields just above the plateau, the low temperature phase has transverse spin order, describable as a Bose-Einstein condensate of magnons. Other comparisons to and suggestions for experiments are discussed.