Abstract
The fractionalization of elementary excitations in quantum spin systems is a central theme in current condensed matter physics. The Kitaev honeycomb spin model provides a prominent example of exotic fractionalized quasiparticles, composed of itinerant Majorana fermions and gapped gauge fluxes. However, identification of the Majorana fermions in a three-dimensional honeycomb lattice remains elusive. Here we report spectroscopic signatures of fractional excitations in the harmonic-honeycomb iridates β- and γ-Li2IrO3. Using polarization-resolved Raman spectroscopy, we find that the dynamical Raman response of β- and γ-Li2IrO3 features a broad scattering continuum with distinct polarization and composition dependence. The temperature dependence of the Raman spectral weight is dominated by the thermal damping of fermionic excitations. These results suggest the emergence of Majorana fermions from spin fractionalization in a three-dimensional Kitaev–Heisenberg system.
Highlights
The fractionalization of elementary excitations in quantum spin systems is a central theme in current condensed matter physics
The fractionalization of elementary excitations is a characteristic feature of quantum spin liquids
The exactly solvable Kitaev honeycomb model offers a genuine opportunity of exploring spin liquid physics on a more quantitative level as the spin response functions for spin liquids can be analytically computed[4,5,6,7,8,9]
Summary
The fractionalization of elementary excitations in quantum spin systems is a central theme in current condensed matter physics. For two-dimensional (2D) triangular and Kagome lattices, a quantitative understanding of spinons remains unsatisfactory due to a lack of reliable theoretical methods of handling macroscopic degenerate ground states[2,3] In this context, the exactly solvable Kitaev honeycomb model offers a genuine opportunity of exploring spin liquid physics on a more quantitative level as the spin response functions for spin liquids can be analytically computed[4,5,6,7,8,9]. In search for a new platform for Kitaev magnetism, the harmonic series of hyperhoneycomb lattices, b À and g À Li2IrO3, were discovered[23,24] These structural polytypes have the same tricoordinated network of Ir ions as the layered a-A2IrO3, and are a three-dimensional (3D) analogue of the honeycomb iridate materials. An ensuing question is whether quantum spin liquids are preserved in such a 3D generalization of the Kitaev model[25,26,27,28,29]
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