Kitaev materials beyond iridates: order by quantum disorder and Weyl magnons in rare-earth double perovskites

TL;DR

This paper explores a generalized Kitaev-Heisenberg model for rare-earth double perovskites, revealing how quantum fluctuations select magnetic orders and produce Weyl magnon excitations, thus advancing understanding of topological spin waves.

Contribution

It introduces a new model for rare-earth magnets, analyzes the phase diagram, and predicts order by quantum disorder and Weyl magnons in these materials.

Findings

Quantum fluctuations lift continuous degeneracy in the phase diagram.

Predicted nearly gapless pseudo-Goldstone modes in ordered phases.

Existence of Weyl magnon excitations in certain magnetic orders.

Abstract

Motivated by the experiments on the rare-earth double perovskites, we propose a generalized Kitaev-Heisenberg model to describe the generic interaction between the spin-orbit-entangled Kramers' doublets of the rare-earth moments. We carry out a systematic analysis of the mean-field phase diagram of this new model. In the phase diagram, there exist large regions with a continuous U(1) or O(3) degeneracy. Since no symmetry of the model protects such a continuous degeneracy, we predict that the quantum fluctuation lifts the continuous degeneracy and favors various magnetic orders in the phase diagram. From this order by quantum disorder mechanism, we further predict that the magnetic excitations of the resulting ordered phases are characterized by nearly gapless pseudo-Goldstone modes. We find that there exist Weyl magnon excitations for certain magnetic orders. We expect our prediction to inspire further study of Kitaev physics, the order by quantum disorder phenomenon and topological spin wave modes in the rare-earth magnets and the systems alike.