Raman spectroscopy evidence for dimerization and Mott collapse in $\alpha \text{−}{\mathrm{RuCl}}_3$ under pressures

We perform Raman spectroscopy studies on $\alpha \text{−}{\mathrm{RuCl}}_3$ at room temperature to investigate its phase transitions of magnetism and structure under pressure. The Raman measurements resolve two critical pressures at $p_1=1.1$ GPa and $p_2=1.7$ GPa with very different structural and magnetic behaviors. With increasing pressure, a stacking order phase transition of $\alpha \text{−}{\mathrm{RuCl}}_3$ occurs at $p_1$, indicated by the new Raman modes. The appearance of infrared active modes in Raman spectrum usually results from the inversion symmetry breaking, which is confirmed by the second harmonic generation measurement. The second transition at $p_2$ is signaled by the in-plane Ru-Ru bond dimerization accompanied by the Mott collapse and the system becomes a correlated band insulator. Our studies demonstrate the competition among spin-orbit coupling, magnetism, and chemical bondings in Kitaev compounds.

Figure 1

Evolution of the Raman spectra of $\alpha \text{−}{\mathrm{RuCl}}_3$ under different pressures at room temperature.

Figure 2

The layer dependence of IR (a) and Raman (b) spectra of $\alpha \text{−}{\mathrm{RuCl}}_3$.

Figure 3

(a) The Raman spectrum and the corresponding atomic displacements of the five Raman active modes $\left(4E_g+1A_{1g}\right)$ in $\alpha \text{−}{\mathrm{RuCl}}_3$ under ambient pressure, where only one of the doubly degenerated $E_g$ modes is given. (b) Pressure dependence of the frequencies of the five Raman peaks. $p_1=1.1$ GPa and $p_2=1.7$ GPa are two critical pressures.

Figure 4

Second harmonic generation (SHG) of $\alpha \text{−}{\mathrm{RuCl}}_3$ under different pressures.

Figure 5

Pressure dependence of the magnetic Raman conductivity ${\chi }^{″}/\omega$ and the magnetic Raman susceptibility ${\chi }_R$, in (a) and (b), respectively. (c) The phonon Raman spectra (after subtracting the magnetic continuum) for the $116\text{−}{\mathrm{cm}}^{-1}$ mode (peak 1) and the $161\text{−}{\mathrm{cm}}^{-1}$ mode (peak 2) under various pressures. The spectra under 0.0 GPa and 0.8 GPa were fitted by Fano lineshapes. The spectrum under 2.4 GPa was fitted by Lorentz line shape. (d) Pressure dependence of the full width at half maximum (FWHM) and the Fano asymmetry parameter $1/\left|q\right|$ (the inset) for the peak 1 and 2. The vertical dash lines in (b) and (d) indicate the critical pressures.

Figure 6

Schematic phase diagram of $\alpha \text{−}{\mathrm{RuCl}}_3$ under pressures. The crystal structure changes from $C2/m$ to $P{3}_{1}12$ at $p_1$ signaled by inversion symmetry breaking, and further to $C2$ at $p_2$ due to the Ru-Ru bond dimerization. For $p<p_2$, the system is a Mott insulator; after the bond dimerization for $p>p_2$, it becomes a correlated band insulator with Mott collapse.

Figure 7

Evolution of Raman spectrum of $\alpha \text{−}{\mathrm{RuCl}}_3$ with increasing pressure (a) and decreasing pressure(b) excited by 488-nm laser.

Figure 8

(a) Raman spectra measured in (XX) and (XY) polarization for $\alpha \text{−}{\mathrm{RuCl}}_3$ by a 633-nm laser at ambient pressure. (b) The Raman spectrum of $\alpha \text{−}{\mathrm{RuCl}}_3$ by a 488-nm laser at ambient pressure.

Figure 9

The atomic displacement of the Raman active modes along with the IR-active asymmetrical layer breathing modes (the frequency unit is ${\mathrm{cm}}^{-1})$.