Foreword: Prizes of the French Academy of Sciences 2020

jvillain@infonie.fr This special issue of Comptes Rendus Physique brings together articles by three winners of prizes awarded by the Academy of Sciences in 2020. The goal of this foreword is to make them more accessible by non experts, especially students. The first article, by Philippe Bourges (“Science et innovation” CEA prize), Dalila Bounoua and Yvan Sidis, is part of the history of superconductivity, fertile in unexpected twists for 110 years. It was in 1911 that the Dutch scientist Kamerlingh Onnes, having succeeded in achieving very low temperatures which allowed him to liquefy helium, had the very natural curiosity to study the properties of various elements at these temperatures. He was surprised to find that some of them had zero electrical resistance. An electric current could thus circulate for days in a ring of mercury. What was the explanation for this perpetual motion? It was not found until 46 years later by Bardeen, Cooper and Schrie ff er, in 1957. These three scientists showed that electrons can form pairs (Cooper pairs). In a simplified description, we can say that these pairs are bosons and that at very low temperatures these bosons undergo Bose condensation, which leads to superconductivity. It is obvious that a vanishing electric resistance has a considerable technological interest, but this interest was considerably reduced by the need for very low temperatures. The critical temperature of the elements culminates at about ten kelvins in niobium and can reach higher values in compounds, but it was not expected to exceed the 23 K which correspond to Nb 3 Ge. However, in 1986 in Zürich, Müller and Bednorz discovered a family of superconductors whose critical temperature was higher, and a little later exceeded the temperature of liquid