C-13-NMR study of charge fluctuations in α-(BEDT-TTF)MX salts (M = alkali metal; X = S, Se)

Optical studies of the density wave (DW) salt of α-(BEDT-TTF)(2)KHg(SCN)(4) revealed a pseudo- gap due to a charge fluctuation below T* similar or equal to 200 K. Moreover, C-13-NMR studies of the DW salt of α-(BEDT-TTF)(2)RbHg(SCN)(4) and of the superconductivity (SC) salt of α-(BEDT-TTF)(2)NH4Hg(SCN)(4) suggested that the NMR line width of the A site increased and the charge disproportionation in the A column developed below T*. These phenomena may be due to the development of a horizontal stripe modulation of the local charge density. However, whether the horizontal stripe modulation develops or not and the relationships among T* anomalies, the pseudogap, and line broadening, were unclear. In this study, C-13-NMR measurements were obtained of the metallic salt of α-(BEDT-TTF)(2)T1Hg(SeCN)(4), and its electronic structure was compared with those of the DW and SC salts. The NMR line width of the A site and chi(B)/chi(C) in Rb(SCN) and NH4(SCN) increased and deviated from that in T1(SeCN) below T*, suggesting the development of the instability of horizontal stripe modulation in Rb(SCN) and NH4 (SCN) and that T* anomalies could be due to the instability