Nontrivial metallic state of ${\mathrm{MoS}}_2$

The electrical conductivity and Raman spectroscopy measurements have been performed on ${\mathrm{MoS}}_2$ at high pressures up to 80 GPa and variable temperatures down to 5 K. We find that the temperature dependence of the resistance in a mixed phase has an anomaly (a hump) which shifts with pressure to higher temperature. Concomitantly, a different Raman phonon mode appears in the mixed state suggesting that the electrical resistance anomaly may be related to a structural transformation. We suggest that this anomalous behavior is due to a charge-density wave state, the presence of which is indicative for an emergence of superconductivity at higher pressures.

Figure 1

Temperature dependence of the resistance of ${\mathrm{MoS}}_2$ at various pressures measured at 300 K. (a) Semiconducting behavior at pressures below 20 GPa. (b) Metal behavior at pressures between 20 and 30 GPa. The arrows indicate the lock-in temperatures at various pressures.

Figure 2

Raman data of ${\mathrm{MoS}}_2$ at various temperatures at 20.9 GPa (sample B) (a), 34.8 GPa (sample C) (b), and 47.6 GPa (sample B) (c). The upper panels are the spectra at various temperatures and the bottom panel is the 2D presentation in the Raman frequency-temperature coordinates.

Figure 3

The temperature dependence of the normalized intensity of $E^{\prime }$ mode (upper panels) and the frequency of $A_{1g}$ mode (lower panels) at 34.8 (red) and 47.6 GPa (blue). The $E^{\prime }$ mode is normalized by the intensity of $E^{\prime }$ mode at 4.6 K and 47.6 GPa.

Figure 4

Raman data of ${\mathrm{MoS}}_2$ at temperature of 5.7 K and various pressures up to 80.0 GPa. (a) The 2D map of the Raman intensity at 5.7 K in the frequency-temperature coordinates. (b) Raman spectra at various pressures at 5.7 K. (c) The pressure dependence of the intensity of the $E^{\prime }$ mode at 5.7 K. The pressure dependence of the Raman frequencies of the $E_{2g}^2$ (d), $A_{1g}$ (e), and $E_{2g}^1$ (f) modes at 5.7 K.

Figure 5

Phase diagram illustrating the results. The dark cyan dot-dashed lines are the phase lines (separating pure $2H_c$, mixed, and $2H_a$ phases determined from Raman spectroscopy in this work at 5.7 K and Ref. [22] at 297 K). The lock-in temperatures of this work are the filled symbols (dark yellow triangle from resistance and green square from Raman spectra). The results from the works of Chi et al. [22, 32] by electrical transport measurement are open symbols (red diamond indicated the anomalous temperature and purple star for $T_c$).