Neutron spectroscopy evidence for a possible magnetic-field-induced gapless quantum-spin-liquid phase in a Kitaev material $\alpha$-RuCl${}_3$

Abstract

As one of the most promising Kitaev quantum-spin-liquid (QSL) candidates, $\alpha$-RuCl${}_3$ has received a great amount of attention. However, its ground state exhibits a long-range zigzag magnetic order, which defies the QSL phase. Nevertheless, the magnetic order is fragile and can be completely suppressed by applying an external magnetic field. Here, we explore the evolution of magnetic excitations of $\alpha$-RuCl${}_3$ under an in-plane magnetic field, by carrying out inelastic neutron scattering measurements on high-quality single crystals. Under zero field, there exist spin-wave excitations near the $M$ point and a continuum near the $\mitΓ$ point, which are believed to be associated with the zigzag magnetic order and fractional excitations of the Kitaev QSL state, respectively. By increasing the magnetic field, the spin-wave excitations gradually give way to the continuous excitations. On the verge of the critical field ${\mu }_0{H}_{\mathrm{c}}=7.5$ T, the former vanish and only the latter is left, indicating the emergence of a pure QSL state. By further increasing the field strength, the excitations near the $\mitΓ$ point become more intense. By following the gap evolution of the excitations near the $\mitΓ$ point, we are able to establish a phase diagram composed of three interesting phases, including a gapped zigzag order phase at low fields, possibly-gapless QSL phase near ${\mu }_0{H}_{\mathrm{c}}$, and gapped partially polarized phase at high fields. These results demonstrate that an in-plane magnetic field can drive $\alpha$-RuCl${}_3$ into a long-sought QSL state near the critical field.

9 pages, 4 figures

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• Kitaev magnetism in honeycomb RuCl3 with intermediate spin-orbit coupling
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• Theory of the field-revealed Kitaev spin liquid
• Unconventional spin dynamics in the honeycomb-lattice material $\alpha$-RuCl${}_3$: high-field ESR studies
• Identification of Magnetic Interactions and High-field Quantum Spin Liquid in $\alpha$-RuCl${}_3$
• Thermodynamic perspective on the field-induced behavior of $\alpha$-RuCl${}_3$
• Nature of Magnetic Excitations in the High-Field Phase of $\alpha$-RuCl${}_3$
• Angle-dependent thermodynamics of $\alpha$-RuCl${}_3$
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• Disorder-induced broadening of the spin waves in a triangular-lattice quantum-spin-liquid candidate YbZnGaO${}_4$
• Evidence for magnetic fractional excitations in a Kitaev quantum-spin-liquid candidate $\alpha$-RuCl${}_3$

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• Dilution of the magnetic lattice in the Kitaev candidate $\alpha$-RuCl${}_3$ by Rh${}^{3+}$ doping
• Ground state and magnetic transitions of the orthorhombic antiferromagnet CaCo${}_2$TeO${}_6$
• Field tuning Kitaev systems for spin fractionalization and topological order
• Dominant Kitaev Interaction and Field-induced Quantum Disordered Phase in the Cobaltate Na${}_2$Co${}_2$TeO${}_6$
• Spin disorder state induced by Mg${}^{2+}$ doping in a Kitaev material Na${}_3$Co${}_2$SbO${}_6$