Record-breaking critical current densities in a stoichiometric superconductor

3 September 2018

Researchers from Oxford’s Centre for Applied Superconductivity (CfAS) in Oxford Physics have uncovered the impressive properties of a novel superconductor, marking it out as the best of its kind with huge technological potential.

Superconductors can conduct electricity with no resistance, which means they can carry very large currents without any loss or heating. However, they will only do so when three criteria are fulfilled. The most well-known is the critical temperature – superconductors must be very cold in order to operate. They can also carry current only up to a critical current density, beyond which they stop superconducting. Finally, they must be in a magnetic field that is less than the critical field of the material. Only when all three of these are satisfied will a superconductor demonstrate its unusual and useful properties. There are many technologies that already use superconductors, including MRI scanners, MagLev trains and some generators and motors, but there are numerous possibilities for applications with the advent of better superconducting materials.

The novel material in question, CaKFe4As4, was first synthesised only a couple of years ago both in Japan and Ames Laboratory, USA, and the investigation of its superconducting properties has recently been undertaken by Oxford’s CfAS researchers in Oxford Physics. Remarkably, its critical current density in single crystals was found to exceed that of all known iron-based superconductors, with a value of around 107 A/cm2. This is over ten times larger than that of current state-of-the-art (Ba,K)Fe2As2-122 superconductors. A high critical current density is important for industrial applications, as it broadens the possibilities for superconducting cables and is crucial for producing high magnetic field superconducting magnets, such as those found in MRI scanners. Their measurements showed in addition that CaKFe4As4 has a relatively high critical temperature of 35K and a large critical field of exceeding 80T.

Moreover, it is particularly interesting that CaKFe4As4 has a high critical temperature, high critical field and high critical current density in what is known as its stoichiometric form, whereby all the chemical compounds that constitute it are present in integer ratios. In contrast, many superconductors need to be doped in order to achieve a superconducting state: some fraction of one element is swapped out for another, and this facilitates the superconductivity. That CaKFe4As4 demonstrates better superconducting properties in its stoichiometric form is highly unusual and does not yet have a theoretical underpinning.

Furthermore, generating impurities uniformly within a superconductor that create the pinning centres for high critical currents is challenging. By avoiding the need for doping, CaKFe4As4 should be easier to synthesise consistently. The CfAS group plans to now devote its efforts to developing CaKFe4As4 in both wire and powder forms, as this material has the potential to overturn existing applied superconductor research and provide a new and exciting class of superconducting technologies. The results of this research have been recently published in Phys. Rev. Materials 2, 074802 (2018) and they are also posted on the open access archive.