Excited states of quantum many-body interacting systems: A variational coupled-cluster description

TL;DR

This paper extends the variational coupled-cluster method to describe excitation states in quantum many-body systems, successfully reproducing known quasiparticle excitations and discovering new collective modes in quantum antiferromagnets.

Contribution

It introduces a novel application of the variational coupled-cluster method to excitation states, including the prediction of previously missing spin-zero magnon-density-wave excitations.

Findings

Reproduces known spin-wave excitations in antiferromagnets

Predicts new spin-zero magnon-density-wave excitations

Discusses implications of collective modes

Abstract

We extend recently proposed variational coupled-cluster method to describe excitation states of quantum many-body interacting systems. We discuss, in general terms, both quasiparticle excitations and quasiparticle-density-wave excitations (collective modes). In application to quantum antiferromagnets, we reproduce the well-known spin-wave excitations, i.e. quasiparticle magnons of spin $\pm 1$. In addition, we obtain new, spin-zero magnon-density-wave excitations which has been missing in Anserson's spin-wave theory. Implications of these new collective modes are discussed.