Strange materials with potential for innovation in technology and energy
PHYSICS
Following in the footsteps of Professor Jan Zaanen, visiting fellow Louk Rademaker is exploring quantum effects in so-called strange materials. His research is paving the way for new materials that could be used in emerging technologies.
Rademaker always keeps an eye out for surprising results that emerge from his calculations on quantum materials, and from the experiments that follow his theoretical work. ‘Does our material behave as expected, or do we see something unusual happening? Every time you calculate something, you encounter new questions. It’s a fun and endless search.’
Following in the footsteps of Professor Jan Zaanen
Rademaker is no stranger to Leiden. He moved to the city at the age of nine and went on to study Mathematics and Astronomy for his undergraduate degrees. He then completed his Master's in Theoretical Physics and earned his PhD under Professor Jan Zaanen at the Lorentz Institute. It seems like his family has a tradition: Rademaker is the fourth generation to do PhD research at Leiden University. He has also been very involved in the city’s community, serving as a city council member from 2006 to 2014.
After completing his PhD, Rademaker worked for several years at the University of California Santa Barbara, the Perimeter Institute for Theoretical Physics in Canada, and the University of Geneva. Following the passing of Jan Zaanen in 2024, a position opened at LION, which Rademaker applied for: Associate Professor in Theoretical Quantum Matter. Although his full-time move to Leiden is taking a little longer for practical reasons, Rademaker looks forward to settling in Leiden and collaborating more closely with the city’s physicists.
Materials science combined with fundamental physics
Rademaker talks about his research: ‘From how we generate energy to how we communicate and store information, modern technology relies on the electrical and/or magnetic properties of materials. We study these properties too, but in so-called strange materials. Strange materials are those that behave differently from what we would expect based on conventional quantum theories. These materials offer a lot of potential for new applications precisely because they react differently from ordinary materials.’
Strange materials, an example
One example of a strange material is copper oxide mixed with rare earth metals such as lanthanum, strontium, yttrium, or barium.
Rademaker explains the challenge of researching strange materials on the quantum level: ‘In most materials, you can ignore the interaction and entanglement of electrons. You can work with an assumption we call the ‘band structure,’ which is a very successful theory. But with the strange materials we study, this trick doesn’t work. You must describe the behaviour of each electron separately. This is almost an impossible task because you’re dealing with 1023 electrons per mole, and these electrons all interact with each other.’
Unraveling Strange Materials
We want to uncover not just the secrets, but also the promises of strange materials. We investigate the temperature at which superconductivity occurs — the phenomenon where a material has no electrical resistance. In ‘normal’ materials, this only happens at temperatures below 10 Kelvin (-263°C), but in the ‘strange material’ copper oxide, the critical temperature for superconductivity is remarkably higher: 120 Kelvin.
Creating Strange Materials
Rademaker is also researching ultra-thin layers of materials, layers that are so thin that they can be described as flat or two-dimensional. They are only one or a few atoms thick. We then stack two of these layers on top of each other, not exactly aligned, but at a small angle. ‘This is how you create moiré materials with geometric patterns. In this way, we transform ordinary materials into strange materials,’ Rademaker explains.
‘We can change the material in a very controlled way from something we understand to something we don’t understand. There’s still a lot to discover in these moiré materials,’ says Rademaker. ‘We found in graphene layers – and also in molybdenum ditelluride with tungsten diselenide sandwiches – that we get all sorts of new electronic phases that we haven’t seen before. Electrons no longer move in a straight line in these materials; they only travel along the edge of the material. Pretty remarkable, don’t you think?!’
From Theory to Practice
Rademaker: ‘Our starting point is always the quantum theory that’s already well-known, like the Schrödinger equation. We assume that this theory is correct, and then we try to make smart assumptions about the behaviour of the materials we’re looking at. We calculate the consequences of these assumptions, and we keep searching until our model makes good predictions. Once we have a new theory, we want to have it tested in experiments.’ Rademaker is eager to carry out this work in Leiden with colleagues such as Sense Jan van der Molen, Tjerk Oosterkamp, and Semonti Bhattacharyya.