Broadband optical conductivity of the chiral multifold semimetal PdGa

We present an optical conductivity study of the multifold semimetal PdGa, performed in a broad spectral range ($100–\mathrm{20 000} {\mathrm{cm}}^{-1}$; 12 meV–2.5 eV) down to $T=10$ K. The conductivity at frequencies below $4000 {\mathrm{cm}}^{-1}$ is dominated by free carriers while at higher frequencies interband transitions provide the major contribution. The spectra do not demonstrate a significant temperature evolution: Only the intraband part changes as a function of temperature with the plasma frequency remaining constant. The interband contribution to the conductivity exhibits a broad peak at around $5500 {\mathrm{cm}}^{-1}$ and increases basically monotonously at frequencies above $9000 {\mathrm{cm}}^{-1}$. The band-structure-based computations reproduce these features of the interband conductivity and predict its linear-in-frequency behavior as frequency diminishes.

Figure 1

PdGa optical reflectivity (a) and the real part of optical conductivity (b) at selected temperatures as indicated. The insets show (c) the dc resistivity vs $T$ (line) together with the inverse optical conductivity in the $\nu \to 0$ limit used in the fits of Fig. 2 (bold dots), and (d) zoomed permittivity spectra near the zero-line crossing. Note that in (a), (b), and (d), the experimental curves are presented for all indicated temperatures.

Figure 2

Drude-Lorentz fits (lines) of the measured optical conductivity spectra (symbols) at 10 and 295 K.

Figure 3

Temperature-dependent parameters of the two Drude terms (narrow and broad) used in the Drude-Lorentz fits. Note different vertical scales for the plasma frequencies (a) and the scattering rates (b).

Figure 4

Low-energy electronic band structure of PdGa with the spin-orbit coupling included. The multifold fermions are supposed to exist near the $\mathrm{\Gamma }$ and $R$ points.

Figure 5

Calculated bulk Fermi surface of PdGa (left) and its cut at the middle of the Brillouin zone (right). The $k_z$ direction is perpendicular to the picture plane.

Figure 6

Comparison of the measured (solid line) and calculated (dashed and dotted lines) optical conductivity in PdGa. The interband portion of ${\sigma }_1\left(\omega \right)$ (blue dotted line) is calculated from the band structure. Adding two Drude terms (cyan dashed line) to this curve provides a good qualitative match (red dashed line) to the experimental spectrum. The green dashed line is the fit from Fig. 2. The quasilinear interband conductivity at low frequencies is due to the transitions between the multiple bands in the vicinity of the Fermi level, as shown in the inset.