Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

The two-dimensional organic conductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Br_xCl_1−x undergoes a transition from an insulator to a superconductor upon substituting Cl by Br. We have performed in and out-of-plane electric-transport measurements on the alloyed series with x = 20%, 40%, 70%, 80%, 85%, and 90% as a function of temperature in order to explore the bandwidth-controlled phase transition between the Mott insulator and the Fermi-liquid. All crystals exhibit a similar semiconducting behavior of ρ(T) from room temperature down to 100 K. Below approximately 50 K, a metal-to-insulator transition is found for compounds with x < 70%. Out of this Mott insulating state, magnetic order develops below T_N ≈ 25 K. The Br-rich samples cross a bad-metal regime before they become coherent metals and eventually superconducting at T_c ≈ 12 K. For these systems the resistivity at T_c ≤ T ≤ T₀ reveals a ρ(T) ∝ T² dependence associated with a strongly correlated Fermi-liquid, limited by some characteristic temperature T₀. The conclusions are corroborated by data from microwave, magnetic and optical experiments.