C13–NMR study of charge fluctuations in α–(BEDT–TTF)MX salts (M=alkali metal; X=S, Se)

Abstract

Optical studies of the density wave (DW) salt of $\ensuremath{\alpha}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{KHg}(\mathrm{SCN})}_{4}$ revealed a pseudogap due to a charge fluctuation below ${T}^{*}\ensuremath{\simeq}$ 200 K. Moreover, $^{13}\mathrm{C}$-NMR studies of the DW salt of $\ensuremath{\alpha}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{RbHg}(\mathrm{SCN})}_{4}$ and of the superconductivity (SC) salt of $\ensuremath{\alpha}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{NH}}_{4}\mathrm{Hg}{(\mathrm{SCN})}_{4}$ 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, $^{13}\mathrm{C}$-NMR measurements were obtained of the metallic salt of $\ensuremath{\alpha}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{TlHg}(\mathrm{SeCN})}_{4}$, and its electronic structure was compared with those of the DW and SC salts. The NMR line width of the A site and ${\ensuremath{\chi}}_{\mathrm{B}}/{\ensuremath{\chi}}_{\mathrm{C}}$ in Rb(SCN) and $\mathrm{NH}{}_{4}$(SCN) increased and deviated from that in Tl(SeCN) below ${T}^{*}$, suggesting the development of the instability of horizontal stripe modulation in Rb(SCN) and $\mathrm{NH}{}_{4}$(SCN) and that ${T}^{*}$ anomalies could be due to the instability.