Two-triplon excitations of the Kitaev-Heisenberg-Bilayer

TL;DR

This paper analyzes the two-triplon excitations in a bilayer Kitaev-Heisenberg model, revealing complex scattering continua and bound states, with implications for magnetic Raman scattering experiments.

Contribution

It provides a detailed series expansion study of two-triplon excitations in a bilayer Kitaev magnet, highlighting the effects of broken SU(2) symmetry and stacking on the excitation spectrum.

Findings

Presence of rich two-triplon scattering continua.

Identification of several (anti)bound states.

Strong sensitivity of Raman intensity to interactions and geometry.

Abstract

We study the spectrum of a bilayer of Kitaev magnets on the honeycomb lattice, coupled by Heisenberg exchange in the quantum-dimer phase at strong interlayer coupling. Using the perturbative Continuous Unitary transformation (pCUT) we perform series expansion starting from the fully dimerized limit, to evaluate the elementary excitations, reaching up to and focusing on the two-triplon sector. In stark contrast to conventional bilayer quantum magnets, and because of the broken $SU(2)$-invariance, encoded in the intralayer directional compass-exchange, the bilayer Kitaev magnet is shown to exhibit a rich structure of two-triplon scattering-state continua, as well as several collective two-triplon (anti)bound states. Direct physical pictures for the occurrence of the latter are provided and the (anti)bound states are studied versus the stacking type, the spin components, and the exchange parameters. In addition to the two-triplon spectra, we investigate a corresponding experimental probe by evaluating the magnetic Raman-scattering intensity. We find a very strong sensitivity of this intensity on the two-triplon interactions and the scattering geometry, however a signal from the (anti)bound states appears only in very close proximity to the continuum.