Spin excitations in the quasi-two-dimensional charge-ordered insulatorα−(BEDT−TTF)2I3probed viaC13NMR

The spin excitations from the nonmagnetic charge-ordered insulating state of $\ensuremath{\alpha}\ensuremath{-}{(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF})}₂{\mathrm{I}}₃$ at ambient pressure have been investigated by probing the static and low-frequency dynamic spin susceptibilities via site-selective nuclear magnetic resonance at $¹³\mathrm{C}$ sites. The site-dependent values of the shift and the spin-lattice relaxation rate $1/{T}₁$ below the charge-ordering transition temperature (${T}_{\mathrm{CO}}\ensuremath{\approx}135\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) demonstrate a spin density imbalance in the unit cell, in accord with the charge-density ratio reported earlier. The shift and $1/{T}₁$ show activated temperature dependence with a static (shift) gap ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{S}}\ensuremath{\approx}47\ensuremath{-}52\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$ and a dynamic ($1/{T}₁$) gap ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{R}}\ensuremath{\approx}40\phantom{\rule{0.16em}{0ex}}\mathrm{meV}$. 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.