Spin excitations in the quasi-two-dimensional charge-ordered insulator α-(BEDT-TTF)2I3 probed via 13C NMR

The spin excitations from the nonmagnetic charge-ordered insulating state of alpha-(BEDT-TTF)(2)I-3 at ambient pressure have been investigated by probing the static and low-frequency dynamic spin susceptibilities via siteselective nuclear magnetic resonance at C-13 sites. The site-dependent values of the shift and the spin-lattice relaxation rate 1/T-1 below the charge-ordering transition temperature (T-CO approximate to 135 K) demonstrate a spin density imbalance in the unit cell, in accord with the charge-density ratio reported earlier. The shift and 1/T-1 showactivated temperature dependence with a static (shift) gap Delta(S) approximate to 47-52 meV and a dynamic (1/T-1) gap Delta(R) approximate to 40 meV. The sizes of the gaps are well described in terms of a localized spin model, where spin-1/2 antiferromagnetic dimer chains are weakly coupled with each other.