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arXiv · 1710.00670

Signatures of low-energy fractionalized excitations in $\alpha$-RuCl$_3$ from field-dependent microwave absorption

C. Wellm·J. Zeisner·A. Alfonsov·A. U. B. Wolter·M. Roslova·A. Isaeva·T. Doert·M. Vojta·B. Büchner·V. Kataev
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Abstract

Topologically ordered states of matter are generically characterized by excitations with quantum number fractionalization. A prime example is the spin liquid realized in Kitaev's honeycomb-lattice compass model where spin-flip excitations fractionalize into Majorana fermions and Ising gauge fluxes. While numerous compounds have been proposed to be proximate to such a spin-liquid phase, clear-cut evidence for fractionalized excitations is lacking. Here we employ microwave absorption measurements to study the low-energy excitations in $\alpha$-RuCl$_3$ over a wide range of frequencies, magnetic fields, and temperatures, covering in particular the vicinity of the field-driven quantum phase transition where long-range magnetic order disappears. In addition to conventional gapped magnon modes we find a highly unusual broad continuum characteristic of fractionalization which -- most remarkably -- extends to energies below the lowest sharp mode and to temperatures significantly higher than the ordering temperature, and develops a gap of a nontrivial origin in strong magnetic fields. Our results unravel the signatures of fractionalized excitations in $\alpha$-RuCl$_3$ and pave the way to a more complete understanding of the Kitaev spin liquid and its instabilities.

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C. Wellm, J. Zeisner, A. Alfonsov, A. U. B. Wolter, M. Roslova, A. Isaeva, T. Doert, M. Vojta, B. Büchner, V. Kataev. 2017-10-02. Signatures of low-energy fractionalized excitations in $\alpha$-RuCl$_3$ from field-dependent microwave absorption. https://doi.org/10.1103/physrevb.98.184408

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