Local Spin Susceptibility of α-D2I3 (D = bis(ethylendithio)tetraselenafulvalene (BETS) and bis(ethylendithio)dithiadiselenafulvalene (BEDT-STF)) Studied by 77Se NMR

We performed static susceptibility and Se-77 NMR measurements on alpha-(BETS)(2)I-3 and alpha-(BEDT-STF)(2)I-3 which are analogues of the zero gap system, alpha-(BEDT-TTF)(2)I-3 to systematically compare the electronic properties of the alpha-type I-3 systems, where BETS, BEDT-STF, and BEDT-TTF are bis(ethylendithio) tetraselenafulvalene, bis(ethylendithio) dithiadiselenafulvalene, and bis(ethylendithio) tetrathiafulvalene, respectively. From the analysis of the NMR lineshape for a polycrystalline sample at high temperatures, which are explained as an admixture of several powder patterns, we found that the local spin susceptibility is not uniform but at least one of the molecular sites has very small susceptibility. It was confirmed from single crystal data that there are three kinds of molecules with different local susceptibilities in a unit cell. The analysis of the angular dependence on a single crystal clarified that the relative magnitude of the local susceptibilities of A, B, and C molecules, chi(loc)(A), chi(loc)(B), and chi(loc)(C), are similar to those of the BEDT-TTF salt; chi(loc)(C) > chi(loc)(A) > chi(loc)(B). On the contrary, the temperature dependence of spin susceptibility and relaxation rate suggested a gap opening well above the metal-insulator transition, whereas no indication of symmetry breaking of the lattice was detected.