Infrared and Raman studies of α-(BEDT-TTF)2MHg(SCN)4 with M=NH4 and K at low temperature: Breaking of inversion symmetry due to charge-ordering fluctuation

The infrared and Raman spectra were measured on the edge plane of the single crystals of quasi-two-dimensional organic conductors α-(BEDT-TTF)2MHg(SCN)4(M=NH4 and K) to look for experimental evidence for the charge-ordering fluctuation. In the optical conductivity spectra for K-salt, four charge sensitive ν27 modes, three of which are infrared active when inversion symmetry exists, were found at around 200 K. From the simple symmetry consideration, the finding of four ν27 modes gives evidence for the breaking of inversion symmetry, which is probably associated to charge-ordering fluctuation. In contrast, NH4-salt was found to keep the inversion symmetry down to 7.3 K.