C-13 NMR study of the charge-ordered state near the superconducting transition in the organic superconductor β"-(BEDT-TTF)(4)[(H3O)Ga(C2O4)(3)]⋅C6H5NO2

The charge-ordered state in a quasi-two-dimensional organic superconductor β''−(BEDT-TTF)4[(H3O)Ga(C2O4)3]⋅C6H5NO2 has been investigated using C13 NMR spectroscopy in a high magnetic field. Enhanced resolution of the high-field spectrum allows us to estimate the site charges in the low-temperature charge-ordered state below 8.5 K. The observed strong increase in the charge imbalance suggests that superconductivity appears on the electronic state with a strong charge modulation. From the NMR intensity ratio of signals from the charge-rich and charge-poor sites, we discuss a possible charge pattern with threefold modulation, which could be stabilized by the effects of electron-electron correlations.