Influence of randomness on the Mott transition in κ-(BEDT-TTF)₂X

We present a detailed transport study in the organic superconductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Br that is irradiated by X-rays. Weak molecular disorder introduced by X-ray irradiation induces the Anderson-type localization insulating state from the strongly correlated metallic/superconducting state. The hydrostatic pressure to the localization insulator restores the metallic properties. These observations indicate that the stronger electron correlations upon approaching the Mott transition enhance Anderson-type electron localization due to disorder introduced by X-ray irradiation. In the metallic state with weak disorder or under pressure, the resistivity shows a T² dependence, which suggests that the dominant electron scattering comes from the electron-electron correlations of a Fermi-liquid metal. The coefficient A of the T² term, however, seems not to follow the Fermi-liquid theory fully as the origin of the T² dependence in a case of the disorder effect. Further studies of the competition and/or cooperation of the Mott and Anderson transitions close to the Mott critical point are important for understanding the critical behavior of the electrons in real materials.