Research: Spurious Symmetry in Linear Spin Wave Theory

Linear spin wave theory (LSWT) is the standard technique to compute the spectra of magnetic excitations in quantum materials. Beside the great success of the technique, it is known to fail in particular instances called order by disorder, where spurious symmetries appear in the ground state. However, less is known on what are the shortcomings of LSWT higher up in the excitation spectrum. A group of researchers from the Okinawa Institute of Science and Technology in Japan,  Johannes Gutenberg University in Mainz and the Max Planck Institute for the Physics of Complex Systems (MPI-PKS) in Dresden, discovered and systematized cases where the technique fails to implement the symmetries of the underlying ordered magnetic Hamiltonian leading to spurious degeneracies in the full excitation spectrum. With growing interest in magnon band topology, it is indeed fundamental to understand how to implement those symmetries correctly as these provide important constraints on the possible topological band structures that can arise. The researchers showed how the correct symmetries may be restored at the level of LSWT and with them the correct topological features.

M. Gohlke, A. Corticelli, R. Moessner, P. A. McClarty, A. Mook,
Spurious Symmetry Enhancement in Linear Spin Wave Theory and Interaction-Induced Topology in Magnons,
Phys. Rev. Lett. 131, 186702 (2023) (arXiv)