Quasi-one-dimensional transition-metal chalcogenide ${\mathrm{Ta}}_2{\mathrm{PdSe}}_6$ 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 ${\mathrm{Ta}}_2{\mathrm{PdSe}}_6$. There is a soft phonon mode mainly consisting of atomic displacements in ${\mathrm{PdSe}}_4$ chains. This soft mode is strongly coupled with the highest valence band at the $\mathrm{\Gamma }$ 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 ${\mathrm{Ta}}_2{\mathrm{PdSe}}_6$.

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• Electron-phonon coupling
• Electronic structure
• Thermoelectric effects
• Valley degrees of freedom
• Semimetals
• Density functional calculations