Angle-resolved photoemission study of the Kitaev candidate $\alpha -{\mathrm{RuCl}}_3$

$\alpha -{\mathrm{RuCl}}_3$ has been hinted to be a spin-orbital-assisted Mott insulator in proximity to a Kitaev spin liquid state. Here we present angle-resolved photoemission measurements on single-crystal $\alpha -{\mathrm{RuCl}}_3$ in both the pristine and electron-doped states, and combine them with Local Density Approximation (LDA)+Spin Orbital Coupling (SOC)+U calculations performed for several low-energy competing magnetically ordered states as well as the paramagnetic state. A large Mott gap is found in the measured band structure of the pristine compound that persists to more than 30 times beyond the magnetic ordering temperature, though the paramagnetic calculation shows almost no gap. Upon electron doping, spectral weight is transferred into the gap but the new states still maintain a sizable gap from the Fermi edge. These findings are most consistent with a Mott insulator with a somewhat exotic evolution out of the Mott state with both temperature and doping, likely related to unusually strong spin fluctuations.

Figure 1

Isoenergy plot for ${\mathrm{RuCl}}_3$ showing a 6-fold symmetry at (a) 5.5 eV, (b) 3.8 eV, and (c) 1.2 eV below $E_F$ near the first Brillouin zone. Data were taken in Advanced Light Source (ALS) beam line 4.0.3 with $h\nu =75\mathrm{eV}$ and linear horizontally polarized light.

Figure 2

ARPES spectra showing band structures for ${\mathrm{RuCl}}_3$ at (a) ALS beam line 10.0.1 with $h\nu =55\mathrm{eV}$ and $s$-polarized light; (b) ALS beam line 10.0.1 with $h\nu =55\mathrm{eV}$ and $p$-polarized light, and (c) ALS beam line 4.0.3 with $h\nu =75\mathrm{eV}$ and linear horizontally polarized light (mixed $s$ and $p$ polarization). (d) Comparison of the normalized spectral weight (integrated in $k$ space) for measurements with (a) $s$-polarized light and (b) $p$-polarized light respectively. (e) Broadened DFT calculation with spin-orbital coupling and in the paramagnetic phase; (f) with a zigzag magnetic order; (g) with FM magnetic order; and (h) with AF magnetic order. Band dispersions along the $\mathrm{\Gamma }$-M direction are represented by dashed lines.

Figure 3

ARPES spectra along the $\mathrm{\Gamma }$-K direction for ${\mathrm{RuCl}}_3$ with (a) no Rb dosing; (b) 1 Rb dosing; (c) 5 Rb dosing; and (d) 14 Rb dosing (saturation) at 300 K. Significant weights are shifted into the gap from the Ru valence bands while the Cl bands remain largely unaffected. (e) ARPES spectra for as-grown ${\mathrm{K}}_{0.5}{\mathrm{RuCl}}_3$ at 300 K. Data were taken in ALS beam line 4.0.3 with $h\nu =75\mathrm{eV}$ and linear horizontally polarized light.

Figure 4

(a) Core-level scan of ${\mathrm{RuCl}}_3$ with in situ Rb dosing. Inset: spectral weight transfer into the gap (zoomed in near EF).

Figure 5

(a) Sketch of an ordinary Mott transition, in which the spectral weights (solid line) is transferred to the new states (dashed line) at the middle of the Mott gap at the chemical potential $\mu$. (b) Sketch of another ordinary Mott transition, in which the chemical potential is lowered towards the lower Hubbard band. (c) Sketch of the Mott transition observed in this study, in which the spectral weights (solid line) is transferred to new states (dashed line) that remains gapped from the chemical potential.

Figure 6

${\mathrm{RuCl}}_3$ (cross) in the potential phase diagram as described by the $J-K-\mathrm{\Gamma }$ model [26]. The competition between the Kitaev term $K$ and other spin exchange terms $J$ results in a continuous quantum critical area above Neel temperature in which our ARPES observations were made.