13C NMR Study of Superconductivity near Charge Instability Realized in $\beta"$-(BEDT-TTF)4[(H3O)Ga(C2O4)3]$\cdot$C6H5NO2

To investigate the superconducting (SC) state near a charge instability, we performed 13 C NMR experiments on the molecular superconductor $\beta"$-(BEDT-TTF) 4 [(H 3 O)Ga(C 2 O 4 ) 3 ]$\cdot$C 6 H 5 NO 2 [BEDT-TTF: bis(ethylenedithio)tetrathiafulvalene], in which charge disproportionation was reported from Raman spectroscopy at 100 K. We found an NMR spectral splitting at $T_{\text{CD}}=12$ K, which is ascribed to the low-temperature charge instability. Measurements of nuclear spin--lattice relaxation time revealed enhanced fluctuations at $T_{\text{CD}}$. We suggest the relationship between the enhanced fluctuations and superconductivity, which emerges slightly below $T_{\text{CD}}$. In the SC state, we observed a decrease in the Knight shift, which leads us to suggest a spin-singlet SC state. The possibility of an unconventional SC state is discussed.