Band topology of magnetic excitations in anisotropic quantum magnets
Invited
Abstract
Excitations of ordered magnets offer a natural path to realize magnetic analogs of topologically nontrivial Z2 and Chern bands. Here we explore the topology of multiplet excitations arising due to the local entanglement of spin and/or orbital degrees of freedom. In particular, we study the fate of the fragile Z2 topological bands realized by non-Kramers magnetic excitations in bilayer as well as spin-orbital systems.
Z2 bands formed by magnetic excitations have been studied in bilayer honeycomb and kagome models involving magnons[1] and triplons[2]. In these models, the magnetic excitations form time-reversal (TR) partners, which play the role of the Kramers pairs of electrons in the model of Kane and Mele. However, bosonic excitations do not enjoy the same symmetry-protection as Kramers pairs of electrons and can be mixed by interactions. The consequences of such mixing for the Z2 topological phases of a quantum paramagnet remains an open question.
We show that a symmetric-exchange anisotropy, allowed by the symmetries of most bilayer models, can destroy the Z2 topology. We investigate possible symmetries that can provide an analog to the TR symmetry protecting the Kramers doublets in the case of electron bands.
Furthermore, we examine the TR-breaking case and the formation of Chern bands. We focus on the multipolar characters of the topological bands formed by spin-multiplet excitations[3].
[1] H. Kondo, Y. Akagi, and H. Katsura, Phys. Rev. B 99, 041110 (2019)
[2] D. G. Joshi and A. P. Schnyder, Physical Review B 100, 020407 (2019)
[3] J. Romhányi Phys. Rev. B 99, 014408 (2019)
Z2 bands formed by magnetic excitations have been studied in bilayer honeycomb and kagome models involving magnons[1] and triplons[2]. In these models, the magnetic excitations form time-reversal (TR) partners, which play the role of the Kramers pairs of electrons in the model of Kane and Mele. However, bosonic excitations do not enjoy the same symmetry-protection as Kramers pairs of electrons and can be mixed by interactions. The consequences of such mixing for the Z2 topological phases of a quantum paramagnet remains an open question.
We show that a symmetric-exchange anisotropy, allowed by the symmetries of most bilayer models, can destroy the Z2 topology. We investigate possible symmetries that can provide an analog to the TR symmetry protecting the Kramers doublets in the case of electron bands.
Furthermore, we examine the TR-breaking case and the formation of Chern bands. We focus on the multipolar characters of the topological bands formed by spin-multiplet excitations[3].
[1] H. Kondo, Y. Akagi, and H. Katsura, Phys. Rev. B 99, 041110 (2019)
[2] D. G. Joshi and A. P. Schnyder, Physical Review B 100, 020407 (2019)
[3] J. Romhányi Phys. Rev. B 99, 014408 (2019)
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Presenters
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Judit Romhanyi
University of California, Irvine, University of California Irvine
Authors
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Judit Romhanyi
University of California, Irvine, University of California Irvine
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Andreas Thomasen
Okinawa Institute of Science and Technology Graduate University (OIST)
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Nicholas Shannon
Okinawa Inst of Sci & Tech, Theory of Quantum Matter Unit, OIST, Okinawa Institute of Science and Technology, Okinawa Institute of Science and Technology Graduate University (OIST), OIST, Theory of Quantum Matter Unit, Okinawa Institute of Science and Technology
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Karlo Penc
Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, Hungarian Academy of Sciences, Wigner Research Centre for Physics