Scaling scientific heights to overturn a long-held theory of the 'peak effect' in superconducting materials

3D model to illustrate the concept of vortices in superconductors

Superconductors – substances that conduct perfectly at a critical, low temperature – have the ability to generate large electrical currents. Powerful superconducting electromagnets are used in, among other things, Maglev trains, magnetic resonance imaging (MRI) and particle accelerators such as the one at CERN, which has superconducting coils around the 60km-long ring.

But even as scientists explore the possibility of a superconductor capable of operating at room temperature, some superconducting phenomena are still not fully understood. Dr Mark Laver, a Birmingham Fellow and Senior Lecturer in the School of Metallurgy and Materials, has made a significant breakthrough by looking at what’s called the ‘peak effect’.

In an international collaboration with four colleagues from Denmark, France and Switzerland, he carried out a raft of experiments, the results of which have overturned an accepted theoretical picture of more than 40 years’ standing.

The result is a paper entitled ‘Decomposing the Bragg glass and the peak effect in a Type-II superconductor’, which was published in Nature Communications. It was named the winner of the College of Engineering and Physical Science’s Paper of the Month award.

‘A superconductor is no longer a superconductor at high temperatures or high magnetic fields,’ explains Mark, whose research explores the nature and development of functional materials, including superconductors. ‘If you have a high current, this generates its own field, which’ll eventually destroy the superconductivity. So, therefore, you might think that if you apply increasing temperatures or magnetic fields, you’ll progressively suppress the critical current that the superconductor can take. But, in fact, you get a peak in the critical current, known as the peak effect.’

The way scientists have tended to think about the peak effect works like this: As you keep increasing the magnetic field, eventually the field goes through the superconductor, but in the form of vortices. Each vortex carries one quantum of magnetic flux.

‘So, we have these vortices, but unfortunately they cause problems: vortices try to move when you apply a current,’ says Mark. ‘The movement causes resistance and therefore heat, which you don’t want. So ideally you would either create a situation where they can move through the material in a frictionless way, or they are pinned to points and can’t move.’

Now, vortices in superconductors like to arrange themselves in 2D lattices. What scientists have thought happens in the peak effect is that these vortex lattices become disordered. Mark and his colleagues decided to test this idea, carrying out a series of experiments at international facilities in France, Switzerland and the US, using equipment – namely scattering instruments at neutron sources – that 40 years ago weren’t nearly as good as they are today. Neutrons were used to probe inside the sample, as they have a weak interaction with matter and, moreover, a magnetic spin, so the magnetic field profile inside materials can be imaged.

To the researchers’ surprise, the vortices didn’t disorder at the peak effect.

‘We found there was a vortex lattice and that it became disordered, as you would expect in a peak effect, but the disordering was in a different place to where the peak effect was observed,’ explains Mark. ‘People have thought they should coincide for decades, but we found that they didn’t do so. That’s quite a big thing. Hence there’s now an open question, and we’ve raised more questions than we’ve answered.

‘But what this essentially means is that the theorists will have to come up with a new model for the peak effect. Eventually we hope we can envisage a situation where you can know exactly what drives such effects and hence engineer the microstructure of superconductors to meet application needs in a smart way.’