Interplay of charge-density waves and superconductivity in the organic conductor β″-(BEDT-TTF)₂AuBr₂

We studied the low-temperature electrical properties of the two-dimensional organic conductor β″-(BEDT-TTF)₂AuBr₂, whose electronic structure is similar to that of the pressure-induced superconductor β″ - (DODHT)₂PF₆ having a charge-ordered transition at 255 K at ambient pressure. We found that the ground state of β″-(BEDT-TTF)₂AuBr₂ at ambient pressure has a charge-density-wave phase below 6.5 K accompanied by x-ray superlattice spots. We also discovered superconductivity in the vicinity of the charge-density-wave phase by applying uniaxial strain. Based on the obtained results, we propose a unified electronic phase diagram that suggests the relationships among the charge-ordered insulating state, the superconducting phase, and the charge-density-wave phase in β″-type organic conductors. The phase diagram shows that the superconducting phase is located near the charge-density-wave phase, which suggests that spin fluctuations play a minor role in the mechanism of the superconductivity and sheds light on the possibility of charge-fluctuation-mediated superconductivity in this type of organic conductor.