Superconductivity: Striking a balance

08/31/2015

A new metallic state is discovered in an unconventional superconductor based on buckyballs

A schematic depiction of the lattice structure of alkali-metal fulleride superconducting materials.
A schematic depiction of the lattice structure of alkali-metal fulleride superconducting materials.

© 2015 Kosmas Prassides

Superconductors conduct electricity without resistance and hence do not dissipate energy as heat. As it is exceedingly difficult to make superconductors that work at relatively high temperatures, increasing the critical temperature at which this intriguing phenomenon occurs is a very active research area.

Unlike typical superconductors, which consist of regular arrangements of atoms, molecular superconductors are characterized by periodic arrangements of molecules. Molecular superconductors that have an ordered lattice of fullerene molecules (C60, also known as ‘buckyballs’), and alkali-metal atoms (see image) currently boast the highest critical temperature (38 kelvin) of all known molecular superconductors.

An international team led by Kosmas Prassides of the AIMR at Tohoku University has investigated one such molecular superconductor, cesium fulleride (Cs3C60)1. By replacing some of its cesium atoms with smaller rubidium atoms the researchers were able to vary the distance between adjacent fullerene molecules within the periodic structure. This substitution of smaller atoms mimics the effect of increasing the hydrostatic pressure, because it forces the fulleride molecules to pack more closely together. The researchers found that critical temperature has a dome-like variation with the density of fulleride molecules and that the peak of this dome occurs precisely at the point where the molecular and extended lattice features of the electronic structure are optimally balanced.

This material exhibits a wide range of phases: it is an insulator at ambient pressure but becomes superconducting under hydrostatic or chemical pressure; in addition, it has metallic and magnetic phases. The scientists have now identified a new metallic phase, which they term ‘a Jahn–Teller metal’ because delocalized, metallic electrons coexist with electrons localized on the fullerene molecules.

“We have shown that this new state, which gives access to the highest critical temperature, has its origins in the electronic structure of the C60 molecule,” says Prassides. “This study will allow theorists to pinpoint how the competing insulating and superconducting ground states are connected, and experimentalists to modify the materials to control the transition and perform detailed measurements to further elucidate how the electronic ground states are related.”

These molecular materials are exciting because by tailoring the synthetic method and starting materials, chemists will be able to control the chemical and electronic structure of the molecular components. High-temperature superconductivity could be achievable by optimizing their design. “This research direction is not possible in the atom-based analogs that dominate most known families of superconducting materials,” notes Prassides. “It could eventually make superconductors viable for widespread use and hence, greatly increase electrical efficiency.”

References

  1. Zadik, R. H., Takabayashi, Y., Klupp, G., Colman, R. H., Ganin, A. Y., Potočnik, A., Jeglič, P., Arčon, D., Matus, P., Kamarás, K. et al. Optimized unconventional superconductivity in a molecular Jahn–Teller metal. Science Advances 1, e1500059 (2015). | article

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.