Bastien Michon

Finding the crucial piece in the super-conductor puzzle

Although he has not yet reached the age of thirty, Bastien Michon has long sought explanations for the world around him and is interested in science. He chose to study physics and study in Grenoble, a city located a few hours drive from his hometown, Firminy in France. This is where his university adventure in physics begins, which will take him to the Institut quantique.

Among other things, he met Professor Thierry Klein and his team at the Institut Néel (CNRS), where he completed his first internships under the supervision of Florence Lévy Bertrand. His interest in superconductors is growing. “Through these internships, I found a passion for the subject of superconductors. I wanted to explain the “why” of these materials which are both special and amazing, in particular because of their absolutely zero electrical resistivity and levitation under the effect of a magnetic field. »

A thesis in co-supervision, a special choice?

Guided by this curiosity and the need to answer the question “why”, Bastien decided to pursue the study of superconducting materials as part of a thesis under the supervision of Professor Thierry Klein and Christophe Marcenat (CEA) in Grenoble and the supervision of Professor Louis Taillefer in Sherbrooke. A bold choice? “I made this singular choice of co-supervision because my three supervisors are internationally recognized in the field of superconductors and each, thanks to their personal expertise, brings their own perspective on the problem of superconductors. Indeed, Prof. Taillefer’s group is renowned for its expertise in the field of superconductors and transport measurements on these materials. As for the group in Grenoble, Christophe Marcenat is renowned for his expertise in specific heat measurements, particularly in high magnetic fields. By doing this co-supervision, I wanted to combine and learn from these different types of expertise. In addition, I wanted to discover Canada and especially Quebec. I chose the Université de Sherbrooke for two reasons: Professor Taillefer’s internationally known group and the teaching that this university offered me.”

“This co-supervision was a great success for two reasons,” explains Louis Taillefer. “First of all, the perfect complementarity of expertise in Sherbrooke and Grenoble. In my group, we have a long experience of cuprates and knew where to look. We had already identified the pin in the haystack… it was now necessary to measure it from every angle. In Grenoble, they have unparalleled expertise in specific heat measurements in high magnetic fields. Then, the success is due to Bastien’s exceptional qualities as a young researcher: immense motivation and great curiosity. »

Thesis project and article in Nature

These collaborations and this network of researchers with which Bastien has been associated have led him to some fine discoveries, including the publication in Nature of his thesis project entitled: Thermodynamic signatures of quantum criticality in cuprate superconductors. It is a question of understanding why cuprates are such robust superconductors.

Bastien explains his challenge by summarizing the discovery published in Nature “It is a complex but exciting puzzle to solve that requires perseverance and patience. In order to help understand these materials, I measured the specific heat. This thesis was a real experimental challenge because it was necessary to design a specific heat sensor to measure cuprates taking into account two limitations: intense magnetic fields (up to 35 teslas) and very low temperatures (up to 400 millikelvins). The measurements were carried out in collaboration with the LNCMI-CNRS intense magnetic field laboratory in Grenoble. With the help of my supervisors (notably Christophe Marcenat) we set up a specific heating technique and started collecting very interesting data on cuprates. One of my flagship projects is to look at what is happening around a particular point in the phase diagram of superconducting cuprates: the critical point of the pseudogap phase. This critical point could contain the key to superconductivity in these materials. To provide an answer to this problem, I measured the specific heat under intense magnetic fields. In this research, we have demonstrated the quantum character of the critical point of the pseudogap phase. At this point, in addition to having a perfectly linear resistivity in temperature, the specific electronic heat presents a divergence caused by strong quantum fluctuations. In cuprates, where these quantum fluctuations are at their highest, superconductivity is the most robust. This strongly suggests that the critical point of the pseudogap phase, discovered through my specific heat measurements, would be the source of the superconductivity so robust in cuprates. It should be noted that we are the first to directly highlight the existence of this quantum critical point. Indeed, technically it is very difficult to access these quantum fluctuations because the strong superconductivity of the cuprates hides their signature. However, by using a cuprate with a rather low superconductivity (of the order of 20 kelvins maximum) and strong magnetic fields to suppress superconductivity, their signature can be accessed. Here is the strategy that has been discussed during this thesis to discover these innovative results. »

“Bastien’s discovery reverses a twenty-year-old paradigm,” explains Louis Taillefer. The pseudogap phase is not simply a loss of density of states, as we all thought, but a peak in the density of states, at the quantum critical point. This peak reveals a strong interaction between electrons, which is most likely the very origin of the exceptionally robust superconductivity of cuprates.”

Fostering his curiosity

And how does Bastien want to guide his professional life? “I am currently working in Switzerland where I am doing a postdoctoral internship in Prof. van der Marel’s group in Geneva, taking optical measurements on superconducting materials there. In the long term, I would like to continue in the research that really fascinates me, because in this profession we discover something new every day that feeds our curiosity.”  According to Bastien, this kind of research requires perseverance, patience and passion, three ingredients that he has wisely brought together in this publication that reveals a central piece of the superconductor puzzle.

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