Copper Salts of Rigid N,N-Dicyanoquinone Diimine Derivatives

I describe some of the phenomena encountered in charge-transfer salts that make them very attractive for condensed-matter physicists. These materials exhibit many interesting electronic properties, including reduced dimensionality, strong electron-electron and electron-phonon interactions and the proximity of antiferromagnetism, insulator states and superconductivity. A wide variety of low-temperature groundstates have been observed in the salts; frequently, one is able to move between these states by applying magnetic field, temperature, pressure or "chemical pressure". In spite of this complex behavior, the charge-transfer salts possess very simple electronic bandstructure which it is often possible to measure in great detail. Hence, one can use the salts as "model systems" in which tractable theoretical calculations for phenomena such as superconductivity are compared directly with experiment.