Enhanced thermoelectric performance by van Hove singularities in the density of states of type-II nodal-line semimetals

TL;DR

This study demonstrates that type-II nodal-line semimetals, due to their unique density of states with van Hove singularities, can achieve high thermoelectric performance, challenging the traditional view that metals are poor thermoelectric materials.

Contribution

The paper reveals how the van Hove singularities in the density of states of type-II NLSs enhance thermoelectric properties, providing a new pathway for high-performance thermoelectric materials among metals.

Findings

Type-II NLSs exhibit high power factors (~60 μW/cmK^2 at 300K) due to van Hove singularities.

The relaxation time of carriers is inversely proportional to the DOS in type-II NLSs.

Optimization using a two-band model suggests broad potential for high thermoelectric efficiency in type-II NLSs.

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 semi-analytical and first-principles methods with "spinless Mg$_3$Bi$_2$" as 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 $\sigma$, the type-II NLS can exhibit a relatively high TE power factor ($\text{PF}=S^2\sigma$) at the nodal line. In particular, we find $\text{PF} \sim 60$ $\mu$W/cmK$^2$ at 300 K for the n-type Mg$_3$Bi$_2$ by considering the electron-phonon scattering, in which the relaxation time $\tau$ of carriers can be expressed as $\tau\propto\text{DOS}^{-1}$ 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 not good TE materials compared to semiconductors.