Scattering Continuum and Possible Fractionalized Excitations in $\alpha \text{−}{\mathrm{RuCl}}_3$

The combination of electronic correlation and spin-orbit coupling is thought to precipitate a variety of highly unusual electronic phases in solids, including topological and quantum spin liquid states. We report a Raman scattering study that provides evidence for unconventional excitations in $\alpha \text{−}{\mathrm{RuCl}}_3$, a spin-orbit coupled Mott insulator on the honeycomb lattice. In particular, our measurements reveal unusual magnetic scattering, typified by a broad continuum. The temperature dependence of this continuum is evident over a large scale compared to the magnetic ordering temperature, suggestive of frustrated magnetic interactions. This is confirmed through an analysis of the phonon linewidths, which show a related anomaly due to spin-phonon coupling. These observations are in line with theoretical expectations for the Heisenberg-Kitaev model and suggest that $\alpha \text{−}{\mathrm{RuCl}}_3$ may be close to a quantum spin liquid ground state.

Figure 1

Structure and polarized Raman response of $\alpha \text{−}{\mathrm{RuCl}}_3$. (a) Lattice structure of a single plane of $\alpha \text{−}{\mathrm{RuCl}}_3$. The Ru atoms are shown in blue while the Cl are indicated in red. (b) 5 K Raman intensity. (c) Low-energy detail at 5 K. The shaded blue region is a guide to the eye and indicates the continuum contribution. (d) 295 K Raman intensity. The Raman spectra evince a series of narrow phonon modes in both channels, a broad continuum extending up to 20–25 meV in ${\mathrm{E}}_g$, and a quasielastic scattering component in ${\mathrm{A}}_{1g}$. The continuum emerges at low temperatures, while the quasielastic scattering is only visible at high temperatures.

Figure 2

Magnetic scattering in ${\mathrm{RuCl}}_3$. (a) Raman intensity in ${\mathrm{E}}_g$ and ${\mathrm{A}}_{2g}$ (XY). (b) ${\mathrm{E}}_g$ and ${\mathrm{A}}_{2g}$ (XY) low-energy spectral weight (SW) vs temperature. (c) Raman intensity in ${\mathrm{A}}_{1g}$ (XX-XY). (d) ${\mathrm{A}}_{1g}$ (XX-XY) low-energy SW vs temperature. As shown in (c), the $A_{1g}$ quasielastic scattering (QES) decreases in intensity as temperature is reduced down to 100 K. In contrast, the intensity of the $E_g$ continuum decreases as temperature is reduced to 100 K before increasing. This is more clearly resolved in the spectral weights shown in (b) and (d). The behavior of the $E_g$ continuum is at odds with the temperature dependence of the thermal factors expected for one [$\mathrm{n}(\omega )+1$] and two [$(\mathrm{n}(\omega )+1{)}^2$] particle scattering and signals a change in the magnetic excitations near 100 K.

Figure 3

Spin-phonon coupling in $\alpha \text{−}{\mathrm{RuCl}}_3$. (a) Data and Fano fit to the low-energy scattering at 5 K. (b) Energy ${\omega }_o$ of the 20 meV phonon. (c) Linewidth $\mathrm{\Gamma }$ of the 20 meV phonon. (d) Magnetic contribution $\mathrm{\Delta }\mathrm{\Gamma }$ to the phonon linewidth. The low-energy data are well-described by the Fano form and indicate a coupling between the magnetic continuum and the lattice. This coupling is also apparent in the temperature dependence of $\mathrm{\Gamma }$, which shows an anomaly near 140 K.