¹³C NMR Study on charge ordered insulator, θ-(BEDT-TTF)₂TlCo(SCN)₄

The BEDT-TTF salts with θ-type molecular arrangement, θ-(BEDT-TTF)₂X, which have a triangular lattice show a wide verity of electronic states according to X. X=RbZn(SCN)₄ salt and title compound undergo a transition from a conducting state to a charge ordered (CO) insulating state[1] due to large inter-site Coulomb interaction. On the other hand, X=CsZn(SCN)₄ salt does not show CO transition but has inhomogeneous charge distribution. It is believed that this difference comes from competition between various charge patterns in a triangular lattice. We measure ¹³C NMR of X=TlCo(SCN)₄ salt. Below 235 K, the spectra and 1/T₁ give microscopic evidence of appearance of CO state. The 1/T₁ below transition temperature gives the ratio of charge rich site and poor site is at least 10. This is larger than X=RbZn(SCN)₄ salt. Moreover, we observed that slow dynamics above transition temperature like as X=RbZn(SCN)₄ and CsZn(SCN)₄ by comparing line width and 1/T₂. While the increase of 1/T₂, different from two salts, it does not take a peak since CO transition occurs during slowing down of charge fluctuations.
[1]H. Mori et al.,PRB 57, 12023 (1998), [2]H. Seo, JPSJ, 69, 805 (2000), [3]R. Chiba et al., PRB, 77, 115113 (2008).