Piecing together the superconducting jigsaw

Researchers from the universities of Liverpool and Durham have used ISIS muons to show how structural control of superconductivity can be used to uncover the mechanism behind high-temperature superconductors.

Next year marks the 100th anniversary of the discovery of superconductivity, the ability of certain materials to conduct electricity without resistance at a low temperature. Historically, the ‘critical’ temperature required to induce superconductivity is so low – around 4 Kelvin (-269oC) – that expensive liquid helium cooling is required to achieve it. This has restricted application of superconductors to a few specialist industries where the benefits outweigh the significant costs, namely MRI machines and particle accelerators and detectors.

In 1986, it was discovered that some materials have comparatively high critical temperatures above 90 Kelvin (-183oC). These temperatures can be achieved using liquid nitrogen cooling. As nitrogen is a far more abundant resource than helium the whole process is much cheaper. With potential savings for industry, academic interest into superconductivity was renewed.

Whilst considerable progress has been made in the development of high-temperature superconductors, an understanding of the mechanism driving their superconductivity remains elusive. However, new research using football shaped carbon-60 molecules may shed light on high temperature superconductivity, its determining factors, and what the maximum critical temperatures might be.

A research group headed by Prof Matthew Rosseinsky from the University of Liverpool and Prof Kosmas Prassides from Durham University reacted C60 molecules and caesium to form a high temperature superconducting alkali metal fulleride (Cs3C60) with a critical temperature of around 35 Kelvin (-238ËšC) under applied pressure. Cs3C60 is special among fulleride superconductors because it incorporates the largest alkali metal ions. Last year a particular variant was discovered with a body-centred cubic crystal structure (bcc). Using the latest synthetic techniques, Rosseinsky and Prassides have isolated a second variant with the same composition but arranged in a face-centred cubic (fcc) structure.

These two compositionally identical but structurally different compounds both display superconductivity under applied pressure. Detailed analysis of their individual properties and behaviour provides a new window into the world of high-temperature superconductivity.

At ambient pressure, both variants behave as magnets where the negatively charged outer electrons are localised on the C60 molecules by their repulsion. However, once additional pressure is applied, their crystal structure shrinks allowing the electrons to overcome their mutual repulsion, pair up and travel through the material without resistance. This was shown to occur in both variants independent of their structural differences – pointing to the superconductivity being governed by the correlations between the electrons.

 “This research suggests that there is a universal trend in high temperature superconducting materials, which is a great step forward in understanding the fundamental nature of superconductivity,” explains Dr Peter Baker, scientist at ISIS who worked alongside Rosseinsky and Prassides on the project.

The fcc structure of Cs3C60 also presents a separate curiosity. The magnetic interactions between C60 molecules compete against each other to avoid magnetic order. This is an example of magnetic frustration and suppresses the magnetic ordering transition from 46 Kelvin in the un-frustrated bcc structure to 2 Kelvin in the fcc material.

To probe the magnetic behaviour of their sample Rosseinsky and Prassides chose muons, a positively charged subatomic particle. Muons can be uniquely powerful in investigating a wide variety of magnetic systems because they can act as a microscopic magnetometer, measuring the strength and direction of the magnetic field at the atomic level.

The team carried out their analysis at the Science and Technology Facilities Council’s ISIS facility in Oxfordshire. As well as a world leading research facility for neutron scattering, ISIS is the world's most intense source of pulsed muons for condensed matter research.

Obtaining a pure sample of fcc Cs3C60 material has been the holy grail of fullerene superconductivity, but eluded all synthetic efforts to make it until the work of Rosseinsky and Prassides.

The team experienced great difficulty producing their fulleride compound and they explain that any commercial application of its superconductivity is unlikely due to its still comparatively low critical temperature. However, analysis of its properties offers significant insights into the mechanism of superconductivity and will inform future work on all high temperature superconductors.

Armed with a more accurate understanding, future research on high temperature superconductors will move to optimise their performance and value in existing commercial usage and could open up applications in new sectors of industry.
Optimisation of performance is a possibility for MRI machines, many of which currently use superconducting magnets working at very low temperature. If the magnets could work at even 173 Kelvin (-100ËšC) then MRI scanners would be significantly cheaper to run and smaller.

More practical superconductors would also have significant impacts on the energy industry. Having zero resistance would remove the problems of transmission loss whilst increasing the amounts of energy that could be stored in a magnetic field.

“Once we know the mechanism of high-temperature superconductivity it will be easier to develop materials with specific properties, opening the door to new applications and ultra efficient energy transmission” explains Dr Baker.

If the trends uncovered for the C60 based materials can be applied to other materials, it will inform the theoretical understanding of all high temperature superconductors. Despite this breakthrough, questions remain over some of the properties of Cs3C60: how does the magnetic state disappear as superconductivity emerges? Are there ways of inducing superconductivity in these materials at ambient conditions? And what is the relationship to the highest temperature copper oxide superconductors. To answer these questions will be a challenge for both the synthetic chemistry and the physical probes of these materials.

Whatever future analysis reveals, it is clear that research into superconductivity and the prospects for high temperature superconductors will gather pace as new industrial applications emerge. Rosseinsky and Prassides hope their research will enable the development of new higher temperature superconductors and refinement of the science which underpins them.