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The family of materials (EDO-TTF)2XF6 represents quasi-one-dimensional ¼ filled systems exhibiting insulator-to-metal (I-M) phase transition at thermal equilibrium. (EDO-TTF)2PF6 is known to undergo a photoinduced I-M conversion with cooperative response to light excitation. Here we use femtosecond pump-probe experiments to study the photoresponse of (EDO-TTF)2SbF6 made of a larger counter-anion SbF6 compared to the well studied (EDO-TTF)2PF6. In the early stage of the photoinduced process, we reveal a multi-component coherent oscillating feature. The evolution of this feature with excitation density and temperature points to the local nature of the photoswitching in (EDO-TTF)2SbF6. At longer timescale, we did not detect the features associated with the transformation to the M phase, albeit observed in the PF 6 derivative. We propose a scenario whereby the bigger size of the counter-anion in (EDO-TTF)2SbF6 hinders the establishment of this transformation at macroscopic scale.

We have examined time-averaged charges of the molecular donors around the phase boundary between the insulating, superconducting, and metal phases for ${\ensuremath{\beta}}^{\ensuremath{''}}$-type ET [$\mathrm{ET}=\text{bis}$-ethelenedithio-tetrathiafulvalene] salts with one (two) hole(s) per two (three) molecules by using vibrational spectroscopy. Around the phase boundary, the site charges are neither those expected for a well-developed charge-ordered (CO) state nor a homogeneous distribution. The molecular charges exhibit an inhomogeneous distribution just above the insulator-superconductor transition temperature. We have analyzed the distribution of the site charges from the viewpoint of the alternation of intersite Coulomb interactions along the stacking direction. The closeness in the energy of several CO patterns mostly contributes to the inhomogeneous distribution, whereas a large difference (no difference) in the energy levels contributes to the CO (homogeneous) state. Our observations indicate that the instability due to the closeness in the energy of several distributions correlates with the insulator-superconductor transition in nondimerized or weakly dimerized molecular conductors.

We present the temperature-dependent infrared and Raman spectra of ${\ensuremath{\beta}}^{\ensuremath{''}}\text{\ensuremath{-}}{(\mathrm{ET})}_{3}{X}_{2}$ ($X=\mathrm{H}\mathrm{S}{\mathrm{O}}_{4}$ and $\mathrm{Cl}{\mathrm{O}}_{4}$; $\mathrm{ET}=\mathrm{bis}$-ethylenedithio-tetrathiafulvalene) compounds which undergo a metal-insulator transition. The infrared-active ${v}_{27}$ mode discontinuously split into two bands at the metal-insulator transition temperature of $127\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the $X=\mathrm{H}\mathrm{S}{\mathrm{O}}_{4}$ salt, whereas, the corresponding mode continuously changed across the metal-insulator transition temperature of $\ensuremath{\sim}170\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the $X=\mathrm{Cl}{\mathrm{O}}_{4}$ salt. In both compounds, the charge-sensitive ${v}_{27}$ mode split into two in the insulating phase. This drastic spectral change indicates that the metal-insulator transition originated in the charge ordering. Employing the frequencies of the split ${v}_{27}$ bands, the site charges are estimated to be $+{0.3}_{4}$ and $+{0.8}_{1}$ for $X=\mathrm{H}\mathrm{S}{\mathrm{O}}_{4}$ and $+{0.3}_{6}$ and $+{0.8}_{1}$ for $X=\mathrm{Cl}{\mathrm{O}}_{4}$. We also estimated the site charges using the Raman-active ${v}_{2}$ mode and obtained consistent results. Examining the selection rule of the ${v}_{3}$ mode, we reached the conclusion that the unit cell with space group $P1$ has a pseudoinversion center in the charge-ordered phase. Based on the symmetry, we propose a charge-ordering pattern for the $X=\mathrm{H}\mathrm{S}{\mathrm{O}}_{4}$ and $X=\mathrm{Cl}{\mathrm{O}}_{4}$ salts, which is quite different from that of the $X=\mathrm{Re}{\mathrm{O}}_{4}$ salt reported previously. We qualitatively discuss the differences in the charge-ordering pattern considering the anisotropic intersite Coulomb interaction.

We present the infrared and Raman spectra of ${\ensuremath{\beta}\phantom{\rule{0.1em}{0ex}}}^{\ensuremath{''}}{\text{\ensuremath{-}}(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{3}{{(\mathrm{ReO}}_{4})}_{2}$ $(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF}=\mathrm{bis}\text{\ensuremath{-}}\text{ethylenedithio}\text{\ensuremath{-}}\text{tetrathiafulvalene})$ measured at various temperatures. We investigated the three $\mathrm{C}\mathrm{C}$ stretching modes, ${\ensuremath{\nu}}_{2},{\ensuremath{\nu}}_{27}$, and ${\ensuremath{\nu}}_{3}$. The infrared and Raman spectra discontinuously changed at $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Below $80\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, for example, the doublet ${\ensuremath{\nu}}_{2}$ bands split into three peaks, the doublet ${\ensuremath{\nu}}_{27}$ also split into three, and a mutual exclusion rule for the vibronic ${\ensuremath{\nu}}_{3}$ modes in the infrared and Raman spectra was broken. This symmetry lowering was consistent with the result of x-ray crystal structure analyses conducted at $100\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $22\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The site charges in the unit cell estimated from the splitting of ${\ensuremath{\nu}}_{2}$ were $+{0.7}_{3},+{0.7}_{3}$, and $+{0.5}_{3}$ in the metallic phase, and they were $+0.2,+{0.8}_{5}$, and $+{0.9}_{5}$ in the insulating phase. The metal-insulator transition of this compound was characterized as the charge-ordering transition originated from the localization of charge due to Coulomb interactions.