Vibrational spectra of BEDT-TTF based 2D charge ordering systems

We measured the polarized Raman spectra of θ-(BEDT-TTF)2RbZn(SCN)4 (RbZn) and (BEDT-TTF)3CuBr4 (CuBr) to investigate the detailed property of charge ordering (CO), and discussed an approach for analyzing the vibrational spectra of CO systems. The Raman spectra of the two compounds showed multiple peaks around the CC stretching region. With the help of isotope-shift measurements, the multiple peaks of RbZn were assigned to two symmetric CC stretching modes, called ν2 and ν3. We used the vibronic signals of ν3 to characterize the spatial symmetry of charge distribution, and employed the peak frequencies of ν2 with weaker vibronic coupling effect to the estimation of charge disproportionation ratio. Similar spectral analysis was applied to CuBr salt to verify whether the approach can be applied to other systems.