Charge disproportionation with lattice distortion of α-(BEDT-TTF)2RbHg(SCN)4 observed by 13C-NMR

To explore the connection between $\ensuremath{\alpha}$-(BEDT$\ensuremath{-}$TTF)${}_{2}$I${}_{3}$ and $\ensuremath{\alpha}$-(BEDT$\ensuremath{-}$TTF)${}_{2}$$M$Hg(SCN)${}_{4}$ ($M=\text{K}$, Rb, Tl and NH${}_{4}$) and to resolve the difference between band picture and charge fluctuation of $\ensuremath{\alpha}$-(BEDT$\ensuremath{-}$TTF)${}_{2}$$M$Hg(SCN)${}_{4}$, we utilized ${}^{13}$C-NMR to examine $\ensuremath{\alpha}$-(BEDT$\ensuremath{-}$TTF)${}_{2}$RbHg(SCN)${}_{4}$ under the conditions with a large hyperfine coupling constant at each site. The temperature dependence of site susceptibility showed the development of spin disproportionation as observed in $\ensuremath{\alpha}$-(BEDT$\ensuremath{-}$TTF)${}_{2}$I${}_{3}$. We found that the linewidth of the A site only increased as temperature decreased from 200 to 100 K, a change associated with the development of lattice modulation. These findings indicate that density-wave modulation occurs during charge disproportionation instability.