Charge ordering in a quasi-two-dimensional organic conductor

The first nuclear magnetic resonance (NMR) evidence of charge ordering in a quasi-two-dimensional organic conductor is presented. We have performed ¹³C NMR shift and relaxation rate measurements for the organic conductor, θ-(BEDT-TTF)₂RbZn(SCN)₄, with a quasi-two-dimensional quarter-filled band. Below the metal-insulator transition at 195 K, the NMR spectrum splits into two kinds of lines. The spin shifts and relaxation rates are quite different between the two lines, showing charge disproportionation of 1:6. This result demonstrates a route to the correlation-induced insulator transition in organic conductors.