Relationship between electronic inhomogeneity and bandwidth in the organic conductor κ−(BEDT−TTF)2Cu[N(CN<…

In $\ensuremath{\kappa}$-(BEDT-${\mathrm{TTF})}_{2}\mathrm{Cu}$[N(${\mathrm{CN})}_{2}]X$ ($X$ = Cl, Br) salts, where BEDT-TTF denotes bis(ethylenedithio)tetrathiafulvalene, the electronic properties can be characterized using a universal pressure-temperature phase diagram. Previous research [T. Kobayashi et al., Phys. Rev. B 100, 195115 (2019)] revealed a low-temperature insulating phase without antiferromagnetic ordering in the isostructural $X$ = I (hereafter, $\ensuremath{\kappa}$-I) salt, which cannot be understood using the universal phase diagram. In addition, the $\ensuremath{\kappa}$-I salt naturally contains electronic inhomogeneity because of the development of a superlattice structure and/or disorder of the ethylene end groups. This could lead to physical properties that are different from those described by the universal phase diagram of $\ensuremath{\kappa}$-type salts. To investigate the relationship between the electronic inhomogeneity and universal phase diagram, $^{13}\mathrm{C}$ NMR measurements were performed under pressure on this material. With increasing pressure, an increase in the bandwidth was revealed from a decrease in the Knight shift. The accompanying suppression of the inhomogeneity at low temperatures was evident from the linewidth. Once the inhomogeneity was suppressed, superconductivity was observed. The temperature dependence of the Knight shift and spin-lattice relaxation rate in the superconducting state is similar to that previously observed for $\ensuremath{\kappa}$-type salts, indicating that the symmetry of the superconducting gap is a $d$ wave. When pressure was applied and the electronic inhomogeneity was suppressed, the physical properties of the $\ensuremath{\kappa}\text{\ensuremath{-}}\mathrm{I}$ salt were found to be qualitatively identical to those of the high-pressure side on the universal phase diagram. These results indicate that the ratio of electronic inhomogeneity to bandwidth is an essential parameter for this system.