The charge-ordered state in a quasi-two-dimensional organic superconductor ${\beta }^{\prime \prime }\text{−}\left(\text{BEDT-TTF}\right){}_4\left[\right({\mathrm{H}}_3\mathrm{O}\left)\text{Ga}\right({\mathrm{C}}_2{\mathrm{O}}_4\left){}_3\right]·{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{NO}}_2$ has been investigated using ${}^{13}\mathrm{C}$ 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.

• Received 26 June 2014
• Revised 3 September 2014

DOI:https://doi.org/10.1103/PhysRevB.90.121106