13C-NMR studies of the paramagnetic and charge-ordered states of the organic superconductorβ′′-(BEDT−TTF)3Cl2·2H2O under pressure

Abstract

${\ensuremath{\beta}}^{\ensuremath{'}\ensuremath{'}}$-(BEDT$\ensuremath{-}$TTF)${}_{3}$Cl${}_{2}$$\ifmmode\cdot\else\textperiodcentered\fi{}$2H${}_{2}$O [BEDT$\ensuremath{-}$TTF: $\underline{\mathrm{b}}$is-($\underline{\mathrm{e}}$thylene$\underline{\mathrm{d}}$i$\underline{\mathrm{t}}$hio)$\underline{\mathrm{t}}$etra$\underline{\mathrm{t}}$hia$\underline{\mathrm{f}}$ulvalene] is superconductive under pressures, whereas the salt exhibits metal-insulator (MI) transition under ambient pressure. The insulator phase in the salt was examined using the charge density wave (CDW) phase that was obtained from band calculation. The charge-ordered (CO) state was recently proposed as the insulator phase of the salt, and the mechanism of superconductivity intermediated by charge fluctuation was suggested. We accessed ${}^{13}$C-NMR on ${\ensuremath{\beta}}^{\ensuremath{'}\ensuremath{'}}$-(BEDT$\ensuremath{-}$TTF)${}_{3}$Cl${}_{2}$$\ifmmode\cdot\else\textperiodcentered\fi{}$2H${}_{2}$O at ambient pressure and under pressure up to 1.6 GPa. At ambient pressure, the NMR spectrum changed at approximately 100 K. Three isolated peaks appeared at low temperatures, suggesting that the CO state exists below 100 K, and spin-gap behavior was observed. By analyzing the chemical shift, the charges on the three sites were estimated as $\ensuremath{\sim}$+0.4$e$, $\ensuremath{\sim}$+0.6$e$, and $\ensuremath{\sim}$+1.0$e$. The ratio of peak intensity and unsymmetrical peak position suggest the CO state with some symmetry breaking. When pressure is applied, the splitting of the NMR peaks in the CO state is reduced. The salt finally exhibits superconductivity at 1.6 GPa, spin-gap behavior observed at ${({T}_{1}T)}^{\ensuremath{-}1}$ below 1.3 GPa suddenly disappears, whereas the NMR spectrum predicts that charge disproportionation coexists with superconductivity. The suppression of the spin-singlet formation observed in ${({T}_{1}T)}^{\ensuremath{-}1}$ at 1.6 GPa suggests the metallic state with the charge disproportionation and the CO instability with some symmetry breaking.