Valley-dependent electron-phonon scattering in thermoelectric semimetal Ta$_2$PdSe$_6$

Abstract

Quasi-one-dimensional transition-metal chalcogenide Ta$_2$PdSe$_6$ is a promising thermoelectric semimetal due to the strong electron-hole asymmetry in the carrier lifetime. However, the microscopic origin of such a strong asymmetry remains unclear. In this study, we theoretically investigate electron-phonon scattering in Ta$_2$PdSe$_6$. There is a soft phonon mode mainly consisting of atomic displacements in PdSe$_4$ chains. This soft mode is strongly coupled with the highest valence band at the $Γ$ point, which lies slightly below the Fermi energy, and causes strong electron-phonon scattering. The bottom of the electron pocket energetically overlapped with that band also suffers from strong intervalley scattering, by which the imaginary part of the electron self-energy exhibits a sharp change near the Fermi level. On the other hand, the imaginary part of the self-energy for carriers in the hole pocket shows a moderate energy dependence. Thus, we find that electron-phonon scattering is strongly valley-dependent. Our finding will help us to understand the distinctive transport properties observed in Ta$_2$PdSe$_6$.

Figures (9)

• Figure 1: Crystal structure of Ta$_2$PdSe$_6$ depicted using the VESTA software vesta.
• Figure 2: (a) Electronic band structure and (b) phonon band structure along the ${\bm k}$ path shown in the Brillouin zone (c). In panels (a)--(b), black broken and red solid lines represent first-principles band structure and that obtained by Wannier interpolation, respectively. Definitions of special ${\bm k}$ points are as follows: Y$'=0.5({\bm b}_1 + {\bm b}_2)$, V$=0.5 {\bm b}_2$, L$=0.5({\bm b}_2 + {\bm b}_3)$, A$=0.5 {\bm b}_3$, and Y$=0.5({\bm b}_1 - {\bm b}_2)$. Note that Y and Y$'$ are equivalent while the latter is not in the first Brillouin zone.
• Figure 3: The lowest (soft) phonon mode at the midpoint of the Y$'$-$\Gamma$ line, i.e., ${\bm q}=0.25({\bm b}_1+{\bm b}_2)$.
• Figure 4: Electronic band structure with a weight of (a) Ta-$d$, (b) Pd-$d$, (c) Se(1)(2)-$p$, and (d) Se(3)-$p$ orbitals. These band dispersions were calculated with the tight-binding model consisting of the Wannier orbitals. For panel (b), the orbital weight is enlarged with a factor of two for clarity.
• Figure 5: The imaginary part of the electron self-energy $\Sigma_{n{\bm k}}$ plotted against the Kohn--Sham energy $\epsilon_{n{\bm k}}$ on an $80\times 80\times 8$${\bm k}$ mesh at 300 K. The Fermi level at 300 K was set to zero in the horizontal axis. Blue dots represent the calculated values of $\mathrm{Im}\Sigma_{n{\bm k}}$. For comparison, a red line that is proportional to the electronic density of states $\mathrm{DOS}(\epsilon)$ is also shown.