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

Abstract

Metals and semimetals are often considered poor thermoelectric (TE) materials due to their low
Seebeck coefficients. However, we will show that topological semimetals in the class of nodal-line
semimetals (NLSs) may potentially exhibit better performance as TE materials. The NLSs are semimetals
with an intersection between the conduction band and valence band in the forms of a line (thus called the
nodal 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. We
calculate TE properties of the NLSs using the semiclassical Boltzmann transport theory and the relaxation
time approximation. By varying the band parameters in our model, we find that the type-II NLS generally
has better TE performance than the type-I NLS. The type-II NLS, in particular, possesses a Seebeck
coefficient with a value possibly larger than twice that of normal metals. The origin of this feature might
be the presence of a discontinuity in the density of states due to the intersection of the valence and
conduction bands.