Superconductivity of Organic Charge-Transfer Salts

Forty years after the discovery of the first organic superconductor, the nature of the superconducting state in these materials is still not fully understood. Here, I present an overview on the historical developments and current knowledge on this topic for the quasi-one- and quasi-two-dimensional (2D) organic charge-transfer salts. Thereby, I focus on the prototype materials based on the donor molecules tetramethyltetraselenafulvalene (TMTSF) and bisethylenedithio-tetrathiafulvalene (BEDT-TTF or ET for short). 2D organic superconductors based on the latter molecule are found to show Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states at high magnetic fields and low temperatures. Thermodynamic and nuclear magnetic resonance data give robust evidence for the existence of this FFLO state with modulated order parameter.