Thanks to the ever-increasing power of computers, it is nowadays possible to solve numerically quantum physics equations using computer algorithms and so design, from theory and simulations, materials according to the properties we wish them to have. The team of Theoretical Materials Physics at the University of Liège recently presented, in two major publications, two original materials whose properties will interest two different technology sectors. One of them, even though it is based on a metal that is not usually magnetic, has very unusual... magnetic properties! It might get involved in the development of the electronics of tomorrow – the spintronics. The other one is showing very astonishing thermoelectric performance, which is of interest to many industrial sectors seeking, in vain up until now, to transform into electricity the vast quantities of heat lost during certain industrial processes.

“We're acting like architects at the atomic scale”. An image that perfectly summarises the work of Professor Philippe Ghosez and his team in the unit of Theoretical Materials Physics at the University of Liège. Their goal? To build new materials with completely original properties. An approach that is certainly as old as materials science itself except that, in this case, it is the method to achieve the goal that has radically changed. Philippe Ghosez: “For a long time, knowledge of materials was macroscopic and empirical. We tested one then the other; we proceeded through trials and errors and, every now and then, we came across a suitable compound: one was piezoelectric, another was ferroelectric, etc. We now have simulation tools that allow us to predict the properties and understand their origin at a microscopic scale and this has speed up research progress."  These recent tools are based on quantum mechanics and owe their existence to the development of computers. While quantum mechanics is a fundamental tool, it is the increase of computing power that has really made it operational insofar as today’s computers allow us to effectively solve quantum physics’ equations for increasingly complex systems using computer algorithms.

“This is what is known as first-principles calculations", Professor Ghosez explains. "By solving the equations of Schrödinger and of the electromagnetism, we can predict the behaviour of a material by taking into account the interactions between the atoms composing this material. It is no longer material X or Y which is considered globally, but each of the nuclei and electrons composing it.” This means that all the materials can be dealt with on a similar footing. Physicists are therefore able to adapt the equations to the composition and external conditions (superimpose two layers of different atoms, insert an X atom into a layer of Y atoms, apply an electrical field, a mechanical load, etc.) and predict the properties of the new materials that has been designed. Of course, the results are theoretical, but if an interesting property is identified, then other teams, such as that of Professor Jean-Marc Triscone (University of Geneva), with whom Professor Ghosez often works, are trying to grow the most promising compounds, also atom by atom and characterize them. An experimental step that is essential to validate the predictions of the theoretical models. “For us theoreticians”, Professor Ghosez adds, “this experimental verification is very important because it validates our approximations and our approach. When it has been validated, we can use it to try to understand the mechanisms that produce the effects and further finely tune our study: are there other materials that can produce the same effect? What are the restricted conditions required for this to happen? And can we try to design a new material whereby this mechanism will be amplified or particularly favoured? This really allows knowledge to develop on a fundamental level, with the possibility of ultimately leading to concrete applications.”