13C-NMR study of charge order state of the organic conductor α′‐(BEDT-TTF)2IBr2

It is said that the quasi-two dimensional organic conductor (BEDT-TTF)2X forming two columns structure like α or θ salts, shows charge order (CO) state at low temperature. To confirm the existence of the CO state and the electron magnetism in α′ phase whose symmetry is different from that of α and θ phases, we investigated 13C-NMR on α′‐(BEDT-TTF)2IBr2, in which one-sided central carbon on BEDT-TTF molecules is substituted with 13C. As a result, the change of spectrum was observed at 200 K which corresponds to CO transition and the angular dependence of spectrum at 15 K indicated that the charge ordering pattern was intra-column separating type. The spectrum in paramagnetic state can be examined by not only the difference of the charge density on on-site molecule but also the contribution from off-site molecules. This result suggested that the hyperfine coupling tensor changes after CO transition in which the charge separates extremely.