Breakdown of Magnons in a Strongly Spin-Orbital Coupled Magnet

Abstract

The description of quantized collective excitations stands as a landmark in the quantum theory of condensed matter. A prominent example occurs in conventional magnets, which support bosonic magnons - quantized harmonic fluctuations of the ordered spins. In striking contrast is the recent discovery that strongly spin-orbital coupled magnets, such as $\alpha$-RuCl${}_3$, may display a broad excitation continuum inconsistent with conventional magnons. Due to incomplete knowledge of the underlying interactions unraveling the nature of this continuum remains challenging. The most discussed explanation refers to a coherent continuum of fractional excitations analogous to the celebrated Kitaev spin liquid. Here we present a more general scenario. We propose that the observed continuum represents incoherent excitations originating from strong magnetic anharmoniticity that naturally occurs in such materials. This scenario fully explains the observed inelastic magnetic response of $\alpha$-RuCl${}_3$ and reveals the presence of nontrivial excitations in such materials extending well beyond the Kitaev state.

Final version; supplemental information included

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• Kitaev magnetism in honeycomb RuCl3 with intermediate spin-orbit coupling
• Spontaneous Magnon Decays
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• Dynamics of the Kitaev-Heisenberg Model
• Magnetic anisotropy in the Kitaev model systems Na2IrO3 and RuCl3
• Theoretical investigation of the magnetic dynamics in $\alpha$-RuCl${}_3$
• Importance of anisotropic exchange interactions in honeycomb iridates. Minimal model for zigzag antiferromagnetic order in Na${}_2$IrO${}_3$
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• Field-induced decays in $XXZ$ triangular-lattice antiferromagnets
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