Thermoelectrics of type-I and type-II nodal line semimetals within the two-band model

Metals and semimetals are often considered poor thermoelectric (TE) materials due to their lowSeebeck coefficients. However, we will show that topological semimetals in the class of nodal-linesemimetals (NLSs) may potentially exhibit better performance as TE materials. The NLSs are semimetalswith an intersection between the conduction band and valence band in the forms of a line (thus called thenodal line). We construct a two-band model using an almost-linear conduction and parabolic (or Mexicanhat) valence bands that overlap each other near the band edge to represent a type-I (or type-II) NLS. Wecalculate TE properties of the NLSs using the semiclassical Boltzmann transport theory and the relaxationtime approximation. By varying the band parameters in our model, we find that the type-II NLS generallyhas better TE performance than the type-I NLS. The type-II NLS, in particular, possesses a Seebeckcoefficient with a value possibly larger than twice that of normal metals. The origin of this feature mightbe the presence of a discontinuity in the density of states due to the intersection of the valence andconduction bands.