Optical conductivity of the type-II Weyl semimetal ${\mathrm{WTe}}_2$ under pressure

Tungsten ditelluride ${\mathrm{WTe}}_2$ is a type-II Weyl semimetal with electronic properties highly sensitive to external pressure, as demonstrated by the superconductivity emerging under pressure. Here we study the optical conductivity of the type-II Weyl semimetal ${\mathrm{WTe}}_2$ under external pressure at room temperature. With increasing pressure, a pronounced spectral weight transfer occurs from the high-energy to the low-energy interband transitions, with drastic changes in the profile of the optical conductivity spectrum indicating a high sensitivity of the electronic band structure to external pressure. The detailed analysis of the pressure-dependent optical conductivity furthermore reveals anomalies at the pressures $\sim 2$ and $\sim 4.5\mathrm{GPa}$, where an electronic and a structural phase transition, respectively, were reported in the literature.

Figure 1

(a) Reflectivity spectrum $R_{\text{s-d}}\left(\omega \right)$ of ${\mathrm{WTe}}_2$ for all measured pressures, together with the Drude-Lorentz fit for the spectrum at 1.2 GPa (gray dashed line). Inset: Loss function $\mathrm{\Lambda }$ at the lowest pressure, showing no clear plasmon peak. (b) Real part of the optical conductivity ${\sigma }_1\left(\omega \right)$ of ${\mathrm{WTe}}_2$ for all measured pressures. The gray arrow illustrates the pressure-induced spectral weight transfer from ${\omega }_{\mathrm{high}}$ to ${\omega }_{\mathrm{low}}$, as explained in the text. The gray dashed line marks the fit of the optical conductivity spectrum at 1.2 GPa.

Figure 2

Optical conductivity spectra together with the fits and contributions for (a) 1.2 GPa and (b) 3.7 GPa, illustrating the different models needed to describe the data below and above 2 GPa, respectively. The two gray arrows indicate the splitting of the Lorentz contribution L4 into two contributions L4 and L5 above 2 GPa.

Figure 3

(a) Contour plot of the difference spectra $\mathrm{\Delta }{\sigma }_{1,\mathrm{inter}}$ as defined in the text. The vertical dashed lines indicate the frequency intervals ${\mathrm{\Delta }}_{\mathrm{low}}$ (800–7500 ${\mathrm{cm}}^{-1}$) and ${\mathrm{\Delta }}_{\mathrm{high}}$ (7500–12 500 ${\mathrm{cm}}^{-1}$). (b) Effective number of electrons in the frequency intervals ${\mathrm{\Delta }}_{\mathrm{low}}$ and ${\mathrm{\Delta }}_{\mathrm{high}}$ as a function of pressure, showing the spectral weight transfer from high to low energies with increasing pressure.

Figure 4

(a) Plasma frequency of the Drude component ${\omega }_{\mathrm{pl},\mathrm{D}}$ as a function of pressure. The value of ${\omega }_{\mathrm{pl},\mathrm{D}}$ at ambient pressure was extracted [37] as the average of the two measured polarization directions. (b) Resonance frequency ${\omega }_{0,j}$ of the Lorentz contributions L2 (black square), L3 (red dot), and L4 (blue triangle) as a function of pressure. (c) Oscillator strength, as given by ${\mathrm{\Omega }}_j$ in Eq. (1), for the Lorentz contributions L2 (black square) and L3 (red dot) as a function of pressure. The vertical gray shaded areas mark the critical pressure ranges of the phase transitions as explained in the text.

Figure 5

Contributions to the pressure-dependent optical conductivity ${\sigma }_1$ of ${\mathrm{WTe}}_2$, as obtained from Drude-Lorentz fitting. The contribution L8 was kept constant for all fittings, as it is part of the high-energy extrapolation.

Figure 6

Pressure-dependence of ${\omega }_0$, ${\omega }_{\mathrm{pl},\mathrm{D}}$, $\mathrm{\Omega }$, and $\mathrm{\Gamma }$ of the various contributions of the Drude-Lorentz fittings shown in Fig. 5 and described in the main text.