Charge Order and Superconductivity in Low-Dimensional Organic Conductors

Molecular solids provide the opportunity to create materials of desired properties and function­nalities by tailoring the constituents and tuning their interactions. The interplay of electronic, magnetic and lattice degrees of freedom allows us to tackle fundamental questions of competing interactions. A slight variation of the constituents and proper arrangement, for instance, causes localization of the conduction electrons, drives a Mott insulator superconducting or establishes magnetic order. The exemplary collaboration of chemists, materials scientists, experimental and theoretical physicists has advanced our understanding of organic con­ductors enormously in the last years, albeit the potential of molecular solids is far from being fully explored.

Organic charge-transfer salts are a well-established class of strongly-correlated electron sys­tems; many of them are subject to ordering phenomena in the spin or charge sector. Some of the two-dimensional quarter- filled BEDT-TTF salts are superconductors, while some of them remain metallic down to low temperatures; others undergo a sharp metal to insulator transition. Why do these materials behave electronically so differently although they are similar in structure? Optical spectro­scopy complemented by magnetic investi­gations reveals that these compounds are subject to charge order to a different degree. The interplay of charge order and superconductivity suggest superconductivity mediated by charge fluctuations.