Department of Physics

Abstract: Around 15 years ago Alexei Kitaev devised an exactly solvable model consisting of spin ½ particles on a honeycomb lattice with competing bond-directional dependent interactions [1]. The ground state of this model is a quantum spin liquid, and the excited states include Majorana fermions that could be utilized for topologically protected quantum computing. Subsequently it was shown that the terms in Kitaev’s Hamiltonian might be realized in practice [2], giving rise to a new area of research dubbed “Kitaev Materials” [3]. For several years now α-RuCl3 has been one of the most widely studied examples. This talk will include a brief discussion of collective excitations in quantum magnets and their relationship to elementary particles. After introducing the Kitaev model mentioned above it will specialize to what is known about α-RuCl3 and its excitations, with some emphasis on neutron inelastic scattering measurements [4]. If time permits I will also give an update on a raging controversy surrounding reported observations of the quantized thermal Hall effect in α-RuCl3 [5]. Such an effect was predicted to arise from magnetic Majorana fermions in the Kitaev quantum spin liquid. 

End Notes

1. A. Kitaev, Anyons in an exactly solved model and beyond, Ann. Phys. 321, 2-111 (2006).

2. G. Jackeli & G. Khaliullin, Mott insulators in the strong spin-orbit coupling limit: from Heisenberg to a quantum compass and Kitaev models. Phys. Rev. Lett. 102, 017205 (2009).

3. H. Takagi, T. Takayama, G. Jackeli, G. Khaliullin, & S. E. Nagler, Concept and realization of Kitaev quantum spin liquids, Nat. Rev. Phys. 1, 264 (2019).

4. See, e.g., C. Balz et al., Finite field regime for a quantum spin liquid in α-RuCl3, Phys. Rev. B 100, 060405(R) (2019), A. Banerjee et al., Excitations in the field-induced quantum spin liquid state of α-RuCl3, npj Quantum Materials 3, 8 (2018), and earlier related references in those papers.

5. Although now somewhat dated, for those interested there is a commentary on this subject by Patrick Lee in the Journal Club for Condensed Matter Physics that can be found online via DOI:10.36471/JCCM_November_2021_02.