15 December 2020 Jessica Blakeney
He is nominated Fellow of the APS for his significant contribution to the field

Claude Bourbonnais and organic superconductors

Professor Claude Bourbonnais

Photo : Michel Caron - UdeS

Since the early 1980’s, Professor Claude Bourbonnais has devoted his research to organic superconductors. He made his first contributions to the field during his doctorate in co-supervision at the Solid States Physics Laboratory (LPS) in Orsay, France, and at the Université de Sherbrooke. His research led to a new understanding of the mechanism of superconductivity in the presence of magnetism. The impact of this research was not limited to organic conductors: it is now applied to a series of new materials. Professor Bourbonnais’ dedication earned him a nomination as a Fellow of the American Physical Society (APS), more specifically for his pioneering contributions to the field of low-dimensional conductor and superconductor theory. As part of this prestigious appointment, the researcher, professor, and co-director of the Department of Physics tells us about his field.

His Contributions to the Field

Organic conductors belong to a new class of materials that emerged during the 1970’s. These molecular materials can form crystals through the regular stacking of large planar organic molecules that make them approximate realizations of one-dimensional systems. “We would be tempted to believe that organic materials are rather perishable or even pliable, as in new generations of screens, but they can also be crystalline and be excellent metals presenting a quite remarkable variety of states of matter like superconductivity, that is to say to conduct the electric current without losses. Organic superconductivity was discovered at the end of 1979 by Professor Denis Jérôme’s group at the LPS. My contributions in this field are at the theoretical modelling level, aiming at a better understanding of the experiments carried out on these materials,” shares Professor Bourbonnais.

“As the APS is a large association of American physicists bringing together tens of thousands of members around the world, the fact that my work is recognized tells me that it has had repercussions not only in the field of low-dimensional organic conductors, but beyond, in other equally unconventional classes of superconductors which were subsequently discovered, but which in many ways present a similar problem,” he adds.

Organic superconductors are among the very first material categories that forced physicists to change the way they think about superconductivity. Indeed, the closeness observed between a certain form of magnetism, known as antiferromagnetism, and superconductivity was something that shattered previous ideas and had never been seen before: “It was thought that these two states of matter were mutually exclusive, and they could not tolerate each other. And in these low-dimensional conductors, superconductivity appeared side by side with antiferromagnetism, to everyone’s surprise. It was later discovered that this kinship of states of matter was found in several series of new superconducting materials, both organic and inorganic.”

Organic Superconductors’ Field Evolution

The field of organic superconductors has evolved rapidly. In the beginning, these materials were extremely popular as they were among the first to show unconventional properties. We also discovered a remarkable number of new phases, new materials, etc. When the superconducting oxides, the cuprates, were discovered in the second half of the 1980’s, the field experienced a certain migration of researchers attracted by the high critical temperature of the latter.

However, the field of organic superconductors continued to be a forerunner in condensed matter physics with contributions often ahead of other series of materials: “We were discovering new materials and with them certain new apparent characteristics, while they were often already known in organic superconductors. Most of my career has been spent interpreting experimental data that was unusual, so-called unconventional effects or physical properties not visible in more traditional materials like ordinary metals where they are well understood. It is this questioning raised by the data from experience that has constantly motivated me to develop new theoretical approaches and thus contribute to a better understanding of these systems over the years.” These interpretations of the experimental data have been the subject of many publications.

More Than 160 Published Papers

Professor Bourbonnais has more than 160 published papers, but a series of them makes him particularly proud. The latter, published in 1986 during his postdoctoral fellowship at the LPS, in collaboration with several experimental researchers and theorists, including Professor Laurent Caron from the Université de Sherbrooke, set the stage for more than twenty years of research that followed. This series of work proposed a new formulation of the theoretical approach called the “renormalization group”, which could provide a unified understanding of the mechanisms behind the wide variety of phases observed in organic superconductors, including superconductivity and antiferromagnetism.

“Essentially, we predicted that a quantum entanglement between magnetism and superconductivity was possible, provided that the pairing of electrons, responsible for the phenomenon of superconductivity, takes place in an unusual pattern. It was also found that the proposed renormalization group technique gave the possibility of establishing a very clear link between all the phases observed in these organic materials.”

Thus, while superimposing several explanatory aspects, this series of articles proposed a new formalism applicable to experimental data. “Basically, what matters to me are the results of the experiment, which is what motivated the creation of this new formulation of the renormalization group. Its application to these materials was successful.” Today, similar approaches are used in several other areas of the physics of so-called strongly correlated systems. This recognition as an APS Fellow comes from this pioneering contribution at the time, a proposal recognized today as a reliable approach particularly suited to the description of low-dimensional superconductors.

The Metallic Phase Mystery

Professor Bourbonnais continues his research on the problem of organic superconductors while focusing on the conductive metallic state which precedes the superconducting state.

“In the metallic phase preceding the onset of superconductivity, organics exhibit rather strange electrical resistivity behaviour, but completely similar to that observed in high critical temperature superconducting cuprates. What we suspect to be at the origin of this strange behaviour is this interaction between magnetism and superconductivity, which would influence the processes of collisions responsible for the electrical resistivity at high temperature in the metallic phase. It is a difficult problem that interests many researchers around the world, and in particular at the Université de Sherbrooke! Theorists like André-Marie Tremblay and David Sénéchal, and the experimentalists of Louis Taillefer’s and Patrick Fournier’s groups of are actively interested in this problem of the metallic normal state of superconductors at high critical temperature. From my point of view, organics can do well since they are, by their almost one-dimensional nature, relatively simpler to deal with mathematically and therefore to model, so that we can potentially learn faster from organic materials,” he shares.

Despite the progress made and the successes encountered, several gray areas remain in these materials. Professor Bourbonnais wishes to unravel the mystery of the metallic phase, a theme that will be part of his research for the coming years.

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