13 September 2019 Hugues Vincelette

Crafting new particles in the core of superconducting materials

Gaël Grissonanche

Photo : Martin Blache - U de S

The struggle to shed light on the behavior of electrons in cuprate superconductors does not only contribute to the advancement of science. Indeed, understanding the physics of cuprates could have very tangible impacts on our daily lives, in particular when it comes to electricity transport and storage. With energy and environmental issues taking a prominent place in public debates, each scientific breakthrough in the field bringing us closer to room temperature superconductivity should deserve our full attention.

The recent publication in Nature with leading author Gaël Grissonnanche, postdoctoral fellow at the Institut quantique, contributes to the advancement of knowledge in this field. The still enigmatic so-called “pseudogap phase” of cuprates is the focus of this work. “What allowed this umpteenth  study of cuprates, is an innovative experimental approach using a demanding technique that few groups master — the thermal Hall effect. To do the measurements, we need to resolve temperature with millidegree Celsius precision, which requires exceptional thermal stability” explains Gaël, who works in Professor Louis Taillefer’s group.

The thermal Hall effect is the thermal equivalent of the electrical Hall effect discovered in 1879 by American physicist Edwin Hall. This effect is the emergence of a difference in electrical potential between the side faces of a conducting bar under the influence of a magnetic field when an electric current flows through it. This phenomenon is due to the electrons in the material, that would rather move along the length of the bar (i.e. in the direction of the current), being forced to bend their trajectory towards the edges of the bar because of the magnetic field. This leads to a higher concentration of electrons on one side of the bar, creating an electrical potential perpendicular to the current.

The thermal Hall effect is simply its thermal equivalent: for a conducting bar submitted to a magnetic field and traversed by a heat current, a temperature difference develops between the lateral surfaces of this bar. This effect is also usually due to electrons (but not in the experience reported here in Nature!), because they have a charge that couples to the magnetic field and because they transport heat.

What has been observed

The way Professor Taillefer’s group has come to develop the technique of the thermal Hall effect in his laboratory is interesting. It was at a conference almost 10 years ago that Professor Taillefer was challenged to use this technique to resolve a conflict of opinions between scientists. That is what his group quickly did and they made their mark on this debate. During his PhD and postdoctoral studies, Gaël constantly developed and refined this technique, until he came to the result that is of interest today.

“What we wanted was to explore the signatures of the thermal Hall effect in different phases of cuprates. We started out in the area of the phase diagram where we knew, a priori, that cuprates behave like conventional metals. We measured the thermal Hall effect and we observed the expected signal for a metal, in this case a positive signal, reflecting that electrons are indeed the origin of the thermal Hall effect in this region of the phase diagram. Up to that point, there was nothing surprising at all.”

It was by measuring the thermal Hall effect in the pseudogap phase of cuprates, a phase which after thirty years of research has not yet revealed its true nature, that the team obtained unexpected results. “When we made the same measurements in the pseudogap phase, the thermal Hall effect suddenly changed sign— it now became negative. This sign change is dramatic and occurs immediately when we enter the pseudogap phase. It is a completely new and unexpected signature of this phase. But there was something more impressive than that. ” says the scientist.

The researcher continues his explanation. “By entering deeply into the pseudogap phase we reach an electrically insulating phase of the material, where the electrons will essentially be frozen on the spot. The deeper we went into this phase, the larger and the more negative the thermal Hall effect became. This is odd because it is conventionally the electrons that move in the magnetic field and contribute to the thermal Hall effect, but that is impossible here because they no longer travel. However, something has to move and couple to the magnetic field in order to give us this giant negative signal. Therefore, conventional particles being excluded, we must conclude that it comes from exotic particles, some would say “topological”, created within the strong interactions in the heart of the material. This means we may have observed for the first time the fundamental excitations of the pseudogap phase. It is a giant step towards deciphering the deep nature of this phase. It might have huge implications for superconductivity.”

The article is already attracting a lot of interest. Gaël has been invited to present the results numerous times in recent months and the number of articles attempting to explain the results continues to grow. “The thermal Hall effect in cuprates reveals a previously unknown aspect of this enigmatic pseudogap phase, I’m curious to see which theoretical models can explain what seemed impossible to observe until today.” An exciting project awaits theorists now.



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