Superconductivity in Molecular Solids

The notion of synthetic metals designed to optimize the conditions for phonon-mediated pairing is often traced back to Little’s ideas related to conducting polymers. However, the ultimate realization of organic metals and superconductors took a different form, that of charge transfer salts such as (TMTSF)₂X, (BEDT-TTF)₂X. Exploration of that system and related materials revealed a rich interplay of correlation effects and dimensionality, and the emergence of superconductivity proximate to magnetic phases. Apparently irreconcilable data point to different descriptions of the order parameters, and therefore leaves open whether pairing originates with magnetic fluctuations, has some charge-fluctuation character, or is more mundane. Experimental limitations have left a number of research directions and questions unexplored: what circumstances prevent the fairly limited set of applied spectroscopies from being expanded to regularly include what is widely used in oxides and other materials, such as STM, neutron scattering, and ARPES? By what routes can the carrier density be controlled, and would parallels to cuprate physics emerge under those circumstances? Can the observations of high field superconductivity be ascribed to a transition to a Larkin-Ovchinnikov phase, or is a singlet to triplet transition the explanation for superconductivity exceeding the paramagnetic limiting field? How does proximity to a gapless spin liquid phase influence the phase diagram? Finally, what’s behind reports of a superconducting transition temperature of 120 K?