The results are the latest in a series of breakthroughs concerning the alkali fulleride class of superconductors. By adding caesium ions to carbon-60 (fullerene) molecules, the researchers had recently synthesized body-centred cubic (bcc) Cs3C60, an alkali fulleride with a superconducting temperature of 38 Kelvin, the highest within its class. Now, they have found that its structural variant, based on the face-centred cubic (fcc) structure, is also superconducting, suggesting the superconductivity in this material is independent of the packing.
By comparing the properties of the fullerides containing different alkali atoms, the scientists also observed a universal trend, which identifies the proximity to a transition between metallic and insulating states as the key parameter controlling the superconducting behaviour. This indicates that the correlations between electrons are crucial to the superconducting mechanism.
“The alkali fullerides have traditionally been thought of as conventional Bardeen-Cooper-Schrieffer (BCS) superconductors”, explains Dr. Peter Baker, from the Science and Technology Council's ISIS facility in the UK, and who probed the magnetic behaviour of fcc Cs3C60 using a sophisticated technique called muon spin rotation. “Our work shows that the superconductivity in these materials emerges from an antiferromagnetic insulating state, and therefore highlights a close similarity with the more exotic high-temperature superconductors, such as the copper oxide materials”.
Copper oxide-based superconductors display the highest known superconducting transition temperatures.
However, they can only be made with one structure – a two-dimensional square motif. The alkali fullerides, on the other hand, can be made with different high symmetry structures, allowing for the effects of structural changes to be probed. Thus, says professor Kosmas Prassides, from the department of chemistry at Durham University, “we have demonstrated that the alkali fullerides are essentially three-dimensional members of the high-temperature superconductivity family".
The scientists hope their work will help to guide theoretical models of other families of high-temperature superconductors, leading to an understanding of its underlying mechanism. “The role of magnetic fluctuations appears to be very important in fcc caesium fulleride”, says Prassides. “It may therefore provide a model platform to study the proposed mechanisms giving rise to high-temperature superconductivity”.
Professor Stephen Blundell, from the University of Oxford, agrees: “The researchers have shown that superconductivity in the alkali fullerides is by no means a finished subject. They may in fact be a new paradigm for seeing high-temperature superconductivity arising in new geometries”.