Exploring Electronic States in BEDT-TTF Organic Superconductors

Much like inorganic metals, organic compounds can exhibit superconductivity at low temperatures where electrons move through the material without any resistance. However, unlike conventional inorganic metals, the superconductivity is due to strong electron-electron interactions.

To reveal the electronic states responsible for the material’s superconducting behavior, a review published in Journal of the Physical Society of Japan summarizes the electronic states inherent in a BEDT-TTF or bis(ethylenedithio)tetrathiafulvalene type of organic superconductor.

κ-type BEDT-TTF compounds have a layered structure composed of anion layers and BEDT-TTF layers. These molecules form a distinctive triangular lattice, resulting in a hole-like Fermi surface at half-filling, where strong electron correlations hold electrons at the lattice sites, leading to a Mott insulating state.

The review highlights that, under pressure, the compound with a half-filled electronic configuration undergoes a metal-insulator transition due to a shift in the occupancy of electronic states. This is accompanied by the Mottness transition which is the tendency or degree of prohibited double occupation. However, when additional charge carriers are introduced into the material through doping, the excess electrons or holes make the material conductive even under the prohibition of double occupancy.

In such cases, two distinct metallic states are formed: a conventional Fermi liquid at high pressures and a non-Fermi liquid state at low pressures. The high-pressure state exhibits typical Bardeen-Cooper-Schrieffer superconductivity due to Cooper pairing of conventional Fermi particles. As the material passes through the Mottness transition at low pressures, it behaves as a quantum spin liquid, exhibiting Bose-Einstein Condensate-like superconductivity.

These findings underscore the significance of pressure and cooling in effectively managing electron interactions, that lead to the formation of a superconducting state.

The insights gained may contribute to understanding similar phenomena in complex electron systems and hold potential value in the information industry, where efficient electron control is essential for energy conservation.