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arXiv:1706.06157 · Physical Review Letters

Magnetic Excitations and Continuum of a Possibly Field-Induced Quantum Spin Liquid in α−RuCl3

Zhe Wang, S. Reschke, D. Hüvonen, Seung-Hwan Do, Kwang‐Yong Choi, Michael Gensch, U. Nagel, T. Rõõm, Alois Loidl

Published Nov 28, 2017• 184 citations
arXivHTMLPDF

Abstract

We report on terahertz spectroscopy of quantum spin dynamics in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, a system proximate to the Kitaev honeycomb model, as a function of temperature and magnetic field. We follow the evolution of an extended magnetic continuum below the structural phase transition at ${T}_{s2}=62\text{ }\text{ }\mathrm{K}$. With the onset of a long-range magnetic order at ${T}_{N}=6.5\text{ }\text{ }\mathrm{K}$, spectral weight is transferred to a well-defined magnetic excitation at $\ensuremath{\hbar}{\ensuremath{\omega}}_{1}=2.48\text{ }\text{ }\mathrm{meV}$, which is accompanied by a higher-energy band at $\ensuremath{\hbar}{\ensuremath{\omega}}_{2}=6.48\text{ }\text{ }\mathrm{meV}$. Both excitations soften in a magnetic field, signaling a quantum phase transition close to ${B}_{c}=7\text{ }\text{ }\mathrm{T}$, where a broad continuum dominates the dynamical response. Above ${B}_{c}$, the long-range order is suppressed, and on top of the continuum, emergent magnetic excitations evolve. These excitations follow clear selection rules and exhibit distinct field dependencies, characterizing the dynamical properties of a possibly field-induced quantum spin liquid.

DOI
10.1103/physrevlett.119.227202

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A Kitaev quantum spin liquid involves fractionalized quasiparticles called itinerant Majorana fermions and localized gauge fluxes. The topological character with chiral excitations under a magnetic field is currently in an active debate. For a candidate spin liquid $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$, we report low-energy spin excitations through nuclear magnetic and quadrupole resonance measurements down to temperatures three orders of magnitude lower than the exchange energy. In a field-driven disordered state, we found the evolution of two kinds of gapped spin excitations with distinct field dependences. The result is indicative of a field-induced hybridization between fractionalized quasiparticles, as inferred by the perturbation theory for the Kitaev model. The detailed field dependence of spin dynamics across the quantum critical region shows the emergence of a gapless intermediate phase between the gapped zigzag order phase and spin-disordered phases.
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