Thermoelectric properties of type-I and type-II nodal line semimetals: a comparative study

Abstract We investigate the thermoelectric (TE) properties of nodal line semimetals (NLSs) using a combination of semi-analytical calculations within Boltzmann's linear transport theory and the relaxation time approximation along with first-principles calculations for the so-called type-I and type-II NLSs. We consider the conduction and valence bands that cross near the Fermi level of these materials through first-principles calculations of typical type-I (TiS) and type-II (Mg 3 Bi 2 ) NLSs and use the two-band model fit to find the Fermi velocity v F and effective mass m that will be employed as the initial energy dispersion parameters. The optimum curvature value for each energy band is searched by tuning both v F and m to improve the TE properties of the NLSs. By systematically analyzing the calculation results, we observe that tuning v F significantly improves TE properties in both types of NLS compared to tuning m . We also find that in all TE metrics, the type-I NLS surprisingly can surpass the type-II NLS, which seems counter-intuitive to the fact that within the two-band model, the type-I NLS contains a parabolic band while the type-II NLS possesses a higher-order, Mexican-hat band. Our study demonstrates that optimizing the curvature of energy bands by tuning v F can significantly improve the TE performance of NLSs. This approach could guide future efforts in exploring other semimetals as potential TE materials by manipulating their band structures.