Superconductivity: Missing piece of jigsaw found

09/28/2015

A long-standing debate has been resolved with the observation of superconductivity in BaC6, shedding light on its superconducting mechanism

The crystal structure of BaC6, where the small black spheres represent the carbon atoms of the two-dimensional graphite layers and the large red spheres represent barium atoms, which are intercalated between the graphite layers.
The crystal structure of BaC6, where the small black spheres represent the carbon atoms of the two-dimensional graphite layers and the large red spheres represent barium atoms, which are intercalated between the graphite layers.

© 2015 Satoshi Heguri

Superconductivity in the compound BaC6 has been observed for the first time by AIMR researchers1. While its low critical temperature of 65 millikelvin means that BaC6 is unlikely to find much application as a superconductor, the discovery is of deep significance for gaining a refined understanding of the superconducting mechanism of ‘conventional’ superconductors.

BaC6 belongs to a group of materials known as graphite intercalation compounds (GICs), so-called because they consist of two-dimensional graphite sheets with metal atoms sandwiched, or intercalated, between them. Many GICs are considered to be conventional superconductors — that is, they conduct electricity without resistance below a certain critical temperature because their electrons form Cooper pairs, which can travel through the crystal lattice without being scattered by it.

Interest in conventional superconductors has been revived by the recent discovery of high-temperature conventional superconductivity in pressurized hydrogen sulphide. GICs are valuable materials for studying conventional superconductivity because of their layered structure and because the superconducting critical temperature varies greatly with the distance between their layers. But one important observation for this family had been missing for many years — that of BaC6 (see image). Indeed, there had been a long-standing debate about whether it was superconducting or not — theory predicted it should be, but experiments had failed to confirm these predictions.

Satoshi Heguri and Katsumi Tanigaki of the AIMR at Tohoku University and collaborators at the University of Hyogo have succeeded in observing superconductivity in BaC6.

“This finding is important because it provides a complete picture of GIC superconductors, which is crucial for understanding the mechanism of their superconductivity,” says Heguri. In particular, the measurement finally allows full descriptions of how the critical temperature varies with the distance between adjacent layers, thus how superconductivity is controlled.

“Historically, the superconducting mechanism of GIC superconductors has been understood in the framework of the conventional electron-pairing mechanism,” explains Heguri. “However, our results suggest that some other factors should be considered for a complete description. We anticipate that this finding will advance our understanding of two-dimensional superconductivity.”

The researchers had tried for about a year to observe superconductivity in BaC6. Heguri attributes their success to “improved sample quality, a specially designed measurement cell and the performance of our dilution refrigerator.”

The team intends to continue investigating the parameters that demonstrably affect superconductivity in GICs. In addition, they also want to investigate another material about which there has been much debate regarding whether it is superconducting — metal-decorated graphene.

References

  1. Heguri, S., Kawade, N., Fujisawa, T., Yamaguchi, A., Sumiyama, A., Tanigaki, K. & Kobayashi, M. Superconductivity in the graphite intercalation compound BaC6. Physical Review Letters 114, 247201 (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.