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

Abstract

We examine the impact of temperature (T) on the Seebeck coefficient S, i.e., the T dependenceof 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 thermalconductivity to the electrical conductivity. The present paper demonstrates theoretically the novelresult of large anisotropy in the Seebeck coefficient components of three-dimensional Dirac electronsin a molecular conductor. The conductor exhibits a nodal line with the energy variation aroundthe chemical potential and provides the density of states (DOS) with a minimum. Using a threedimensionaltight-binding (TB) model in the presence of both impurity and electron–phonon (e–p)scatterings, we study the Seebeck coefficient Sy for the molecular stacking and the most conductingdirection. The impact of T on Sy exhibits a sign change, where Sy > 0 with a maximum at hightemperatures and Sy < 0 with a minimum at low temperatures. The T dependence of Sy suggeststhat 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 Sx andSz are much smaller than Sy and present no sign change, in contrast to Sy. These results are analyzedin terms of the spectral conductivity as a function of the energy ϵ close to the chemical potential μ.