Quantum kagome antiferromagnet in a magnetic field: Low-lying non-magnetic excitations versus valence-bond crystal order

TL;DR

This paper investigates the quantum kagome antiferromagnet under a magnetic field, revealing a complex landscape of non-magnetic excitations and evidence for valence-bond crystal order in the ground state.

Contribution

It provides new insights into the stability of the 1/3 magnetization plateau and the nature of the ground state, highlighting differences between quantum and classical calculations.

Findings

Presence of a magnetic gap with many non-magnetic excitations

Existence of a smaller gap above the ground state

Evidence for valence-bond crystal order with nine-spin unit cell

Abstract

We study the ground state properties of a quantum antiferromagnet on the kagome lattice in the presence of a magnetic field, paying particular attention to the stability of the plateau at magnetization 1/3 of saturation and the nature of its ground state. We discuss fluctuations around classical ground states and argue that quantum and classical calculations at the harmonic level do not lead to the same result in contrast to the zero-field case. For spin S=1/2 we find a magnetic gap below which an exponential number of non-magnetic excitations are present. Moreover, such non-magnetic excitations also have a (much smaller) gap above the three-fold degenerate ground state. We provide evidence that the ground state has long-range order of valence-bond crystal type with nine spins in the unit cell.