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.