Pressure-Induced Collapse of the Charge Density Wave and Higgs Mode Visibility in $2H\text{−}{\mathrm{TaS}}_2$

The pressure evolution of the Raman active electronic excitations of the transition metal dichalcogenides $2H\text{−}{\mathrm{TaS}}_2$ is followed through the pressure phase diagram embedding incommensurate charge-density-wave and superconducting states. At high pressure, the charge-density wave is found to collapse at 8.5 GPa. In the coexisting charge-density-wave and superconducting orders, we unravel a strong in-gap superconducting mode, attributed to a Higgs mode, coexisting with the expected incoherent Cooper-pair breaking signature. The latter remains in the pure superconducting state reached above 8.5 GPa. Our report constitutes a new observation of such Raman active Higgs mode since the long-standing unique case $2H\text{−}{\mathrm{NbSe}}_2$.

Figure 1

(a) Raman spectra of $2H\text{−}{\mathrm{TaS}}_2$ at ambient pressure at 13 K in the ICDW state and for the $A_{1g}+E_{2g}$ symmetry. We observe three phonons ($E_{2g}^2$, $E_{2g}^1$, and $A_{1g}$) and two CDW modes labeled according to there symmetry ($E_{\mathrm{CDW}}$ and $A_{\mathrm{CDW}}$). Additional less intense CDW modes (denoted *) are also visible. (b),(c) Temperature dependence of the amplitudons in the pure symmetries. Insets: Energy of the amplitudons as a function of temperature. The black line corresponds to a fit using a mean-field calculation. (d) Raman spectra at various temperatures in the $E_{2g}$ symmetry where a CDW gap opens. Inset: Raman spectra in the $A_{1g}$ symmetry.

Figure 2

(a),(b) Raman spectra of $2H\text{−}{\mathrm{TaS}}_2$ in the $A_{1g}+E_{2g}$ symmetry at 10 (a) and 40 K (b) for various pressures ranging from 1.1 to 9.5 GPa. (c),(d) Pressure evolution of the energy of the CDW amplitudons at 10 K (c) and 40 K (d). The $A_{1g}$ and $E_{2g}$ modes soften with pressure toward zero energy (solid lines are guides for the eye). (e) Phase diagram ($T$,$P$) of $2H\text{−}{\mathrm{TaS}}_2$.

Figure 3

(a) Raman spectra of $2H\text{−}{\mathrm{TaS}}_2$ in the ${\mathrm{A}}_{1g}+{\mathrm{E}}_{2g}$ symmetry at 3.5 and 10 K for various pressure ranging from 5 to 9.5 GPa. (b) Low energy Raman spectra in the $E_{2g}$ symmetry at 6 GPa below $T_c$. The in-gap mode is at $\sim 10{\mathrm{cm}}^{-1}$, below $2\mathrm{\Delta }$ (dotted line), similarly to what is observed in $2H\text{−}{\mathrm{NbSe}}_2$. (c) Fit of the various features observed in the SC state at 8 GPa in the $A_{1g}+E_{2g}$ symmetry: a sharp in-gap Higgs mode, an incoherent Cooper-pair breaking peak (CPBP), and the $E_{2g}^2$ phonon. (d) Subtraction of the normal state response (10 K) from the superconducting state response (3.5 K) at 9.5 GPa in the $A_{1g}+E_{2g}$ symmetry. The SC signature fits a Cooper-pair breaking peak for a BCS response in an $s$-wave superconductor with a gap $2\mathrm{\Delta }=22.5{\mathrm{cm}}^{-1}$ corresponding to a $T_c$ of 8.55 K (purple line). Inset: Raman spectra for the pure $A_{1g}$ (black curve) and $E_{2g}$ (green curve) symmetries. The Cooper-pair breaking peak appears only in the $E_{2g}$ symmetry.