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 ${\ensuremath{\beta}}^{\ensuremath{''}}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}{\text{AuBr}}_{2}$, whose electronic structure is similar to that of the pressure-induced superconductor ${\ensuremath{\beta}}^{\ensuremath{''}}\text{\ensuremath{-}}{(\text{DODHT})}_{2}{\text{PF}}_{6}$ having a charge-ordered transition at 255 K at ambient pressure. We found that the ground state of ${\ensuremath{\beta}}^{\ensuremath{''}}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}{\text{AuBr}}_{2}$ 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 ${\ensuremath{\beta}}^{\ensuremath{''}}$-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.