• organic charge-transfer salts;
  • nonlinear transport;
  • Mott metal–insulator transition;
  • electronic phase separation


We present measurements of the first- and third-harmonic voltage response in ac electronic transport measurements, representing the linear (ohmic) and nonlinear resistivity, respectively, of the quasi-two-dimensional (2D) organic superconductors κ-(BEDT-TTF)2X. Nonlinear transport is a sensitive tool to probe the microgeometry of the electronic system in high-quality single crystals. For the title compounds, the normalconducting metallic state in the vicinity of the Mott metal–insulator (MI) transition and critical endpoint is known to be highly unusual. Our results reveal large current-induced intrinsic inhomogeneities, at high current densities most pronounced at the so-called T* anomaly, which characterizes the anomalous metallic state. The observed nonlinearities in the interlayer transport do not depend on frequency and cannot be ascribed to a simple Joule heating mechanism in a resistor network. Furthermore, we find evidence supporting the notion of electronic phase separation induced by the Mott critical endpoint. The observed dependence of the generated third-harmonic voltage on the current density reveals a systematic behavior suggesting that current-induced electronic inhomogeneities are more pronounced for more strongly correlated systems.