Raman spectroscopic evidence for the charge disproportionation in a quasi-two-dimensional organic conductor theta-(BDT-TTP)(2)Cu(NCS)(2)

We present spectroscopic evidence for the charge disproportionation in the crystal $\ensuremath{\theta}\ensuremath{-}{(\mathrm{B}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{P})}_{2}{\mathrm{Cu}(\mathrm{NCS})}_{2}$ $(\mathrm{B}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{P}=2,5\ensuremath{-}\mathrm{b}\mathrm{i}\mathrm{s}{(1}^{\ensuremath{'}}{,3}^{\ensuremath{'}}\ensuremath{-}\mathrm{dithiol}{\ensuremath{-}2}^{\ensuremath{'}}\ensuremath{-}\mathrm{ylidene})\ensuremath{-}1,3,4,6\ensuremath{-}\mathrm{t}\mathrm{e}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{h}\mathrm{i}\mathrm{a}\ensuremath{-}\mathrm{p}\mathrm{e}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{e}\mathrm{n}\mathrm{e}).$ $\ensuremath{\theta}\ensuremath{-}{(\mathrm{B}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{P})}_{2}{\mathrm{Cu}(\mathrm{NCS})}_{2}$ is a highly correlated organic conductor with a quasi-two-dimensional electronic structure. In the electrical resistance of this compound, we found a second-order phase transition at 250 K. Below this phase-transition temperature, the optical absorption in the midinfrared region exhibits a high-frequency shift, and the Raman-active $\mathrm{C}\mathrm{=}\mathrm{C}$ stretching modes ${\ensuremath{\nu}}_{2},$ ${\ensuremath{\nu}}_{3},$ and ${\ensuremath{\nu}}_{4},$ which are sensitive to the charge on BDT-TTP, split into 12 bands at 5 K. Based on the excitation-light dependence of the Raman spectrum, we propose that a disproportionation of charge occurs below the phase-transition temperature.