High-Temperature Charge and Structure Anomalies in β"-(BEDT-TTF)4[(H3O)Ga(C2O4)3]•C6H5NO2

We have performed specific heat, X-ray diffraction, electron spin resonance (ESR), and NMR experiments on the organic superconductor beta"-(BEDT-TTF)(4)[(H3O)Ga(C2O4)(3)]center dot C6H5NO2 (beta"-Ga) salt at high temperatures to reveal the origin of the resistivity minimum observed around 100 K. Since splitting of the Raman spectrum was observed at a charge-sensitive vibration mode, the anomaly around 100 K has been ascribed to a charge instability with fractional charge imbalance. In addition, another charge instability was detected at 8.5 K in a previous NMR experiment. The origin of these anomalies observed at different temperatures should be investigated to uncover the electronic state in which superconductivity occurs. Our results suggest that the beta"-Ga salt undergoes a thermodynamic charge transition at T-CO = 90 K, which has a different origin from the charge instability observed at 8.5 K in the NMR experiment. We discuss the possible connection between these charge instabilities by comparing the present results with those obtained for other BEDT-TTF salts with two charge-ordering transitions.