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Fractional Magnetic Excitations and Kitaev physics in honeycomb α-RuCl3  

Stephen Nagler, Oak Ridge National Laboratory
Thursday, February 7, 2019 - 12:30pm to 1:30pm
PAT C-520

In 2006 Alexei Kitaev put forth an exactly solvable  model of S=1/2 spins on a honeycomb lattice [1].  The ground state of this model is a quantum spin liquid (QSL) exhibiting Majorana fermion excitations.   Subsequently Jackeli and Khaliullin showed how the Kitaev Hamiltonian might actually  be realized in the laboratory [2].  This stimulated a great deal of experimental activity, initially on various iridates, and more recently on the honeycomb magnet a-RuCl3, including a rather spectacular claim that a quantized thermal Hall effect is observed [3].  This seminar will briefly introduce the Kitaev model and its relationship to physical materials along with a brief discussion of fractional magnetic excitations. The focus will then turn to the physics seen in a-RuCl3, with an emphasis on our group’s neutron scattering measurements [4-6].   At low temperaturesa-RuCl3orders in an antiferromagnetic zigzag structure. Despite this ordering, inelastic neutron scattering and spectroscopic measurements  yield evidence for a fractionalized magnetic excitation spectrum. The magnetic order can be suppressed by a magnetic field of 7.5 Tesla applied appropriately in the honeycomb plane, and this results in a disordered quantum phase that is likely some sort of quantum spin liquid. Measurements of the excitation spectrum as a function of field show features that may be related to a high field topological phase transition.

  1. Kitaev, A. Anyons in an exactly solved model and beyond. Phys. 321, 2-111 (2006).
  2. Jackeli, G. & Khaliullin, G. Mott insulators in the strong spin-orbit coupling limit: From Heisenberg to a quantum compass and Kitaev models. Rev. Lett. 102, 017205 (2009).
  3. Kasahara, Y. et al. Majorana quantization and half-integer thermal quantum Hall effect in a Kitaev spin liquid. Nature 559, 227-231 (2018).
  4. Banerjee, A. et al. Proximate Kitaev quantum spin liquid behaviour in a honeycomb magnet. Mater. 15, 733-740 (2016).
  5. Banerjee, A. et al. Neutron scattering in the proximate quantum spin liquid α-RuCl3. Science 356, 1055-1059 (2017).
  6. Banerjee, A. et al., Excitations in the field-induced quantum spin liquid state of α-RuCl3. npj Quantum Materials 3, 8 (2018).