Detuning the honeycomb of $\alpha \text{−}{\mathrm{RuCl}}_3$: Pressure-dependent optical studies reveal broken symmetry

The honeycomb Mott insulator $\alpha \text{−}{\mathrm{RuCl}}_3$ loses its low-temperature magnetic order by pressure. We report clear evidence for a dimerized structure at $P>1$ GPa and observe the breakdown of the relativistic $j_{\mathrm{eff}}$ picture in this regime strongly affecting the electronic properties. A pressure-induced Kitaev quantum spin liquid cannot occur in this broken symmetry state. We shed light on the new phase by broadband infrared spectroscopy of the low-temperature properties of $\alpha \text{−}{\mathrm{RuCl}}_3$ and ab initio density functional theory calculations, both under hydrostatic pressure.

Figure 1

(a) Low-temperature conductivity spectra in the range of the 290 and $320{\mathrm{cm}}^{-1}$ phonon modes recorded for different hydrostatic pressure. The values refer to the pressure set at room temperature. (b) Position ${\omega }_0$ and (c) linewidth $\mathrm{\Gamma }$ of peak 1 and peak 2 obtained from fits of the low-temperature conductivity for different pressure applied. The colored lines are guides to the eye.

Figure 2

(a),(b) Top views of the $ab$ plane in undimerized $C2/m$ and dimerized $P\overline{1}$ structures, respectively; the dimerization is indicated by magenta lines. The corresponding calculated in-plane phonon modes (blue arrows) and frequencies are presented in (c)–(e) for $P<P_c$ and in (f) and (g) for $P>P_c$. Ru and Cl atoms are denoted by red and gray spheres, respectively.

Figure 3

(a) Temperature dependence of the conductivity spectrum of $\alpha \text{−}{\mathrm{RuCl}}_3$ around the Mott gap. Three distinct features are indicated by arrows: the small excitonic peak $\mathrm{\Lambda }$, the main absorption maximum $\alpha$, and the shoulder ${\alpha }^{\prime }$. (b)–(d) Temperature evolution of the optical conductivity for selected hydrostatic pressure as indicated. (e) Pressure dependence of the low-temperature optical conductivity. For clarity reasons the curves are shifted with respect to each other. The pressure effects can be followed in the false-color contour plot (f). In panel (g) the spectral weight (SW) is displayed as a function of temperature. The parameters are obtained from fits of the $\alpha$ feature measured at different pressure values as indicated. Pressure dependence of (h),(i) the positions of the $\alpha ,\mathrm{\Lambda }$, and ${\alpha }^{\prime }$ peaks, respectively; (j) the linewidth $\mathrm{\Gamma }$ of the $\alpha$ peak and (k) the spectral weight, obtained from fits of the low-temperature conductivity for different pressure applied. The colored lines are guides to the eye.

Figure 4

(a)–(d) Calculated density of states (DOS) using the GGA+SOC+$U$ method with $U=1.5$ eV at ambient pressure (left panels) and $P>P_c$ (right panels). The orbital-decomposed (top) and $J$-decomposed (bottom) DOS are compared in the two pressure regimes. Note that the $J=5/2$ and $J=3/2$ have most contributions coming from $j_{\mathrm{eff}}=1/2$ and $j_{\mathrm{eff}}=3/2$, respectively. (e) Schematics of Ru-Ru hopping pathways.