Impact of Temperature on Seebeck Coefficient of Nodal Line Semimetal in Molecular Conductor

We examine the impact of temperature (T) on the Seebeck coefficient S, i.e., the T dependence of S for a single-component molecular conductor [Pd(dddt)(2)] (dddt = 5,6-dihydro-1,4-dithiin-2,3-dithiolate) with a half-filled band, where the coefficient is obtained from a ratio of the thermal conductivity to the electrical conductivity. The present paper demonstrates theoretically the novel result of large anisotropy in the Seebeck coefficient components of three-dimensional Dirac electrons in a molecular conductor. The conductor exhibits a nodal line with the energy variation around the chemical potential and provides the density of states (DOS) with a minimum. Using a threedimensional tight-binding (TB) model in the presence of both impurity and electron-phonon (e-p) scatterings, we study the Seebeck coefficient S-y for the molecular stacking and the most conducting direction. The impact of T on S-y exhibits a sign change, where S-y > 0 with a maximum at high temperatures and S-y < 0 with a minimum at low temperatures. The T dependence of S-y suggests that the contribution from the conduction (valence) band is dominant at low (high) temperatures. Further, it is shown that the the Seebeck coefficient components for perpendicular directions S-x and S-z are much smaller than S-y and present no sign change, in contrast to S-y. These results are analyzed in terms of the spectral conductivity as a function of the energy & varepsilon; close to the chemical potential mu.