Superconductivity in quasi-two-dimensional organic charge-transfer salts

Since the discovery of the first organic superconductor, more than 30 years ago, these materials revealed many fascinating properties and allowed to study fundamental low-dimensional physics. Besides superconductivity, the organic metals show a wealth of different ground states such as antiferromagnetic, spin-Peierls, spin-density-wave, and charge-density-wave phases. These ground states are accessible by tuning the structure, counter anion, magnetic field, temperature, and pressure. The study of these fertile phase diagrams has led to new theoretical concepts; however, a solid understanding of some of these states still remains a challenge and especially the nature of the superconducting state is a controversially discussed issue since many years. Even the normal metallic phase of these electronically low-dimensional metals reveals unusual properties sometimes not in line with conventional Fermi-liquid theory. Here, a review on selected normal-state and superconducting properties of the layered quasi-two-dimensional organic superconductors will be given. Thereby, the focus will be laid on the superconducting properties of the charge-transfer salts based on bisethylenedithio-tetrathiafulvalene, BEDT-TTF or ET for short, the building block of most of the to-date known organic superconductors. Some basic features of the crystallographic structure, the highly anisotropic electronic properties, and of the superconductivity in the organics will be highlighted. In more detail the recently reported evidence for the existence of a Fulde Ferrell Larkin Ovchinnikov state for exactly aligned in-plane magnetic fields will be presented