13C NMR study on the charge-disproportionated conducting state in the quasi-two-dimensional organic conductor α-(BEDT-TTF)₂I ₃

The conducting state of the quasi-two-dimensional organic conductor α-(BEDT-TTF)₂I₃ at ambient pressure is investigated with 13C NMR measurements, which separate the local electronic states at three nonequivalent molecular sites (A, B, and C). The spin susceptibility and electron correlation effect are revealed in a locally resolved manner. While there is no remarkable site dependence around room temperature, the local spin susceptibility gradually disproportionates among the nonequivalent sites with decreasing temperature. The disproportionation ratio yields 5:4:6 for A:B:C molecules at 140 K. Distinct site and temperature dependences are also observed in the Korringa ratio K_i(_1/T1_T)iKi-2 (i = A, B, and C), which is a measure of the strength and the type of electron correlations. The values of K_i point to sizable antiferromagnetic spin correlation. We argue the present results in terms of the theoretical prediction of the peculiar site-specific reciprocal-space (k-space) anisotropy on the tilted Dirac cone, and discuss the k-dependent profiles of the spin susceptibility and electron correlation on the cone.