String theory: the link between black holes and superconductivity
In 2009, Leiden physicist Koenraad Schalm and his colleagues Zaanen and Cubrovic demonstrated that string theory may have a practical application. He used it to better understand the mysterious behaviour of electrons in high-temperature superconductors. Thanks to a VICI grant, Schalm can now proceed with his research.
The article that the Leiden physicists published in the journal Science in 2009 represented a breakthrough for string theory, because for the first time, a direct link was made between a string theory calculation and the physical reality of an experiment.
Schalm and his colleagues demonstrated that string theory can be used to understand the mysterious collective behaviour of electrons in special copper oxides which already display superconductivity far above the absolute zero point.
In order to do this, Schalm took a detour via the universe. He used a component of string theory which makes it possible to translate certain problems in solid state physics to physical phenomena in the gravity field around black holes. This mathematical conversion trick is called the AdS/CFT correspondence.
Schalm: ‘Even though it sounds strange, sometimes it’s easier to make calculations on the basis of the gravity field around black holes than on the basis of the behaviour of electrons in a piece of metal. It’s quite easy to calculate the interaction of one electron with another electron in a grid of regular metal atoms, but the phenomenon of high-temperature superconductivity occurs when all the electrons collectively begin to display a very different behaviour. In this kind of collective you can actually no longer view them as individuals, but we still don’t quite understand how we are supposed to view them.’
The research project for which Schalm received the VICI grant of 1.5 million euro will be building on this discovery from 2009. ‘We now understand a number of examples of the AdS/CFT correspondence, but we don’t yet know how it applies to all situations.’
Once you do know, does this automatically result in a recipe for materials which act as superconductors at even higher temperatures? After all, ‘high temperature’ in superconductivity at present still means a temperature of at least 135 degrees below zero. Schalm: ‘It’s rather unlikely that I will reach this point with my research. And even if it did work, a second conversion would be required to calculate the material properties, and a third would be needed to improve these materials.’ This does not bother him. ‘Every bit of understanding is progress. Other people can then build on it for further gain.’
String theory was and remains his source of inspiration. ‘It is to date the only candidate for a theory that unites quantum mechanics and the theory of relativity. There are no loose ends in string theory, you cannot add or remove anything: the electromagnetic forces, the gravitational forces and all the other forces must already be included.’
But string theory is not yet complete; it is more a concept than a full theory. How can theoretical physicists make any progress then? Schalm: ‘You have an idea, and at the end of the day the calculations show whether the idea is valuable. But real ideas don’t come while you are sitting alone in your attic. They have to be bounced back and forth. The article in Science represented a year of collaboration. Once that fell into place, it was really as if someone had placed a gold-coated carrot in front of our noses. We made the first breakthrough; with the VICI grant we are going for the next one. The goal is to turn this direct contact between string theory and experiment into a real connection.’
(12 March 2013)
Fundamentals of Science is one of the six themes for researech at Leiden University.