We report a ${}^{35}\mathrm{Cl}$ nuclear magnetic resonance study in the honeycomb lattice $\alpha$-${\mathrm{RuCl}}_3$, a material that has been suggested to potentially realize a Kitaev quantum spin liquid (QSL) ground state. Our results provide direct evidence that $\alpha$-${\mathrm{RuCl}}_3$ exhibits a magnetic-field-induced QSL. For fields larger than $\sim 10\mathrm{T}$, a spin gap opens up while resonance lines remain sharp, evidencing that spins are quantum disordered and locally fluctuating. The spin gap increases linearly with an increasing magnetic field, reaching $\sim 50\mathrm{K}$ at 15 T, and is nearly isotropic with respect to the field direction. The unusual rapid increase of the spin gap with increasing field and its isotropic nature are incompatible with conventional magnetic ordering and, in particular, exclude that the ground state is a fully polarized ferromagnet. The presence of such a field-induced gapped QSL phase has indeed been predicted in the Kitaev model.

Raman scattering has been employed to investigate lattice and magnetic excitations of the honeycomb Kitaev material $\alpha -{\mathrm{RuCl}}_3$ and its Heisenberg counterpart ${\mathrm{CrCl}}_3$. Our phonon Raman spectra give evidence for a first-order structural transition from a monoclinic to a rhombohedral structure for both compounds. Significantly, only $\alpha -{\mathrm{RuCl}}_3$ features a large thermal hysteresis, consistent with the formation of a wide phase of coexistence. In the related temperature interval of $70–170$ K, we observe a hysteretic behavior of magnetic excitations as well. The stronger magnetic response in the rhombohedral compared to the monoclinic phase evidences a coupling between the crystallographic structure and low-energy magnetic response. Our results demonstrate that the Kitaev magnetism concomitant with fractionalized excitations is susceptible to small variations of bonding geometry.

The pure Kitaev honeycomb model harbors a quantum spin liquid in zero magnetic fields, while applying finite magnetic fields induces a topological spin liquid with non-Abelian anyonic excitations. This latter phase has been much sought after in Kitaev candidate materials, such as α-RuCl₃. Currently, two competing scenarios exist for the intermediate field phase of this compound (B = 7 − 10 T), based on experimental as well as theoretical results: (i) conventional multiparticle magnetic excitations of integer quantum number vs. (ii) Majorana fermionic excitations of possibly non-Abelian nature with a fractional quantum number. To discriminate between these scenarios a detailed investigation of excitations over a wide field-temperature phase diagram is essential. Here, we present Raman spectroscopic data revealing low-energy quasiparticles emerging out of a continuum of fractionalized excitations at intermediate fields, which are contrasted by conventional spin-wave excitations. The temperature evolution of these quasiparticles suggests the formation of bound states out of fractionalized excitations.

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.

We report on magnetization $M(H)$, dc and ac magnetic susceptibility $\chi (T)$, specific heat $C_m(T)$ and muon spin relaxation ($\mu \mathrm{SR}$) measurements of the Kitaev honeycomb iridate ${\mathrm{Cu}}_2{\mathrm{IrO}}_3$ with quenched disorder. In spite of the chemical disorders, we find no indication of spin glass down to 260 mK from the $C_m(T)$ and $\mu \mathrm{SR}$ data. Furthermore, a persistent spin dynamics observed by the zero-field muon spin relaxation evidences an absence of static magnetism. The remarkable observation is a scaling relation of $\chi [H,T]$ and $M[H,T]$ in $H/T$ with the scaling exponent $\alpha =0.26–0.28$, expected from bond randomness. However, $C_m[H,T]/T$ disobeys the predicted universal scaling law, pointing towards the presence of additional low-lying excitations on the background of bond-disordered spin liquid. Our results signify a many-faceted impact of quenched disorder in a Kitaev spin system due to its peculiar bond character.