A variational coupled-cluster study of magnon-density-wave excitations in quantum antiferromagnets

TL;DR

This paper extends a variational coupled-cluster method to quantum antiferromagnets, successfully describing known magnon excitations and discovering a new spin-zero magnon-density-wave excitation with distinct energy gap properties.

Contribution

It introduces a novel variational coupled-cluster approach that captures both magnon and magnon-density-wave excitations, including a previously missing spin-zero mode.

Findings

Reproduces known magnon (spin $ eq$ 0) excitations.

Identifies a new spin-zero magnon-density-wave excitation.

Shows the excitation gap depends on lattice geometry.

Abstract

We extend recently proposed variational coupled-cluster method to describe excitation states of quantum antiferromagnetic bipartite lattices. We reproduce the spin-wave excitations (i.e., magnons with spin $\pm 1$). In addition, we obtain a new, spin-zero excitation (magnon-density waves) which has been missing in all existing spin-wave theories. Within our approximation, this magnon-density-wave excitation has a nonzero energy gap in a cubic lattice and is gapless in a square lattice, similar to those charge-density-wave excitations (plasmons) in quantum electron gases.