Molecular Dependence of the Large Seebeck Effect in τ-Type Organic Conductors

Abstract

We study the Seebeck effect in the τ -type organic conductors, τ -(EDO-S,S -DMEDT-TTF)2(AuBr2)1+y and τ -(P-S,S -DMEDT-TTF)2(AuBr2)1+y, where EDO-S,S -DMEDT-TTF and P-S,S -DMEDT-TTF are abbreviated as OOSS and NNSS, respectively, both experimentally and theoretically. Theoretically in particular, we perform first-principles band calculation for the two materials and construct a two-orbital model, on the basis of which we calculate the Seebeck coefficient. We show that the calculated temperature dependence of the Seebeck coefficient S is semi-quantitatively consistent with the experimental observation. In both materials, the absolute value of the Seebeck coefficient is maximum at a certain temperature, and this temperature is lower for NNSS than for OOSS. From a band structure viewpoint, we find that this can be traced back to the narrowness of the band gap between the upper and the lower pudding-mold type bands. On the other hand, the Seebeck coefficient of NNSS in the low temperature regime steeply increases with increasing temperature, which is due to the narrowness of the upper band. These differences in thermoelectric properties demonstrate the effectiveness of controlling the band structure through molecular modification.

Ferrimagnetism Induced by Off-Site Coulomb Interaction in an Itinerant Electron System

“…59,61,62,[65][66][67][68][69][70][71] We have focused on the ordered state in a quasi-twodimensional τ -type MC (τ -MC), 72,73) where the τ -type represents the molecular pattern with space group I4 1 22 and the molecules are related to the glide symmetry operation. In our previous paper, 74) we reported a characteristic temperature dependence of the Seebeck coefficient and clarified its origin by applying the tight-binding model [Fig. 1(a)], which reproduces the first-principles band structure [Fig.…”

“…We adopt a two-band extended Hubbard model, which consists of the tight-binding model obtained from τ -MC 74) and on-site and NN off-site interactions:…”

“…We use the hopping parameters obtained from the first-principles calculation for τ -MC. 74) These parameters are t 1 = 0.161, t 2 = −0.157, t 3 = −0.011, t 4 = 0.021, and t 5 = −0.003 in eV units; refer to the notation for t i in Fig. 1(a), where the tightbinding model was used in our previous work and gives a clear understanding of the origin of the thermoelectric effect in τ -MC.…”

“…1(a), where the tightbinding model was used in our previous work and gives a clear understanding of the origin of the thermoelectric effect in τ -MC. 74) In this study, the on-site interaction U , NN off-site interaction V , and temperature T are variables.…”

“…Figure 1(d) shows that the density of states (DOS) is large on the upper and lower parts from the center of the band structure, and a small energy gap separates the large DOS. τ -MC is formed with the chemical formula D (1+y)/2 2 A −1 1+y , [73][74][75][76][77][78][79][80][81] where D represents the donor molecule, A is a linear anion such as AuBr 2 , I 3 , and so on, and y is the anion content. Therefore, the average number of electrons on the donor molecule n is written as n = (3 − y)/2.…”