Enhanced thermoelectric performance by van Hove singularities in the density of states of type-II nodal-line semimetals
Nguyen Tuan Hung, Jyesta M. Adhidewata, Ahmad R. T. Nugraha, Riichiro Saito
Abstract
The effects of the unique density of states (DOS) of a topological type-II nodal-line semimetal (NLS) on its thermoelectric (TE) transport properties are investigated through a combination of semianalytical and first-principles methods with ``spinless ${\mathrm{Mg}}_{3}{\mathrm{Bi}}_{2}$'' as the artificial material. The DOS in such a type-II NLS possesses two van Hove singularities near the energy of the nodal line that leads to a large $S$ value compared to the normal metals. Combined with the linear band at the nodal line that gives high electrical conductivity $\ensuremath{\sigma}$, the type-II NLS can exhibit a relatively high TE power factor ($\mathrm{PF}={S}^{2}\ensuremath{\sigma}$) at the nodal line. In particular, we find $\text{PF}\ensuremath{\sim}60$ $\text{\ensuremath{\mu}}\mathrm{W}\text{/}{\mathrm{cm}\phantom{\rule{0.16em}{0ex}}\mathrm{K}}^{2}$ at 300 K for the $n$-type ${\mathrm{Mg}}_{3}{\mathrm{Bi}}_{2}$ by considering the electron-phonon scattering, in which the relaxation time $\ensuremath{\tau}$ of carriers can be expressed as $\ensuremath{\tau}\ensuremath{\propto}{\text{DOS}}^{\ensuremath{-}1}$ for the type-II NLS. Furthermore, we optimize parameters for the TE power factor of type-II NLSs in general by adopting the two-band model with the DOS-dependent relaxation-time approximation. Our results suggest the type-II NLSs as a potential class of high-performance TE materials among metals and semimetals, which are traditionally considered inadequate TE materials compared to semiconductors.