23 April 2021 Jessica Blakeney
Chiral Phonons in Cuprates

Marie-Eve Boulanger, First Author of a Publication in Nature Communications

Marie-Eve Boulanger

Photo : Institut quantique

Cuprates were discovered over 30 years ago, but the mechanisms responsible for high-temperature superconductivity in these materials are not yet well understood by the scientific community. “Our study concerns cuprates in their Mott insulator state, that is, insulating materials whose electrons are localized by strong interactions. We thought we understood this state well, but our knowledge was turned upside down by the 2019 study by my colleague Gaël Grissonnanche, postdoc in our team”, shares Marie-Eve Boulanger, doctoral student in physics at the Université de Sherbrooke, who is part of Professor Louis Taillefer’s team.

Published in Nature in July 2019, the article Giant thermal Hall conductivity in the pseudogap phase of cuprate superconductors observes a negative thermal Hall effect in cuprates up to the Mott insulator. In an insulator, electrons are stationary, so observing a thermal Hall effect—normally caused by the moving electrons of a metal—suggests something new and forces us to rethink the world of cuprates. In July 2020, the Taillefer team published a second article in Nature Physics, Chiral phonons in the pseudogap phase of cuprates, in which it is shown that the thermal Hall effect is in fact caused by phonons, the acoustic vibrations of the material—a surprise, since phonons are not usually sensitive to magnetic fields. The fact that phonons have chirality, that is, they are deflected by a magnetic field, has awakened the curiosity of several researchers, and opened the door to new research.

A Logical Continuation

As part of her doctorate at the Université de Sherbrooke, Marie-Eve Boulanger’s research concerns the thermal Hall effect in cuprates, a logical continuation of the team’s two articles. Although previous research has shown that phonons are responsible for the large thermal Hall effect in these insulators, the mechanism by which cuprate phonons acquire chirality in a magnetic field is still unknown. Marie-Eve’s research in Professor Louis Taillefer’s group focuses on this issue, and its importance has earned a publication in Nature Communications: Thermal Hall conductivity in the cuprate Mott insulators Nd2CuO4 and Sr2CuO2Cl2.

“Once we confirmed that phonons were the source of the thermal Hall effect in cuprates, the main question we asked ourselves was: how do phonons become chiral? In other words, how are they deflected by the magnetic field?” shares Marie-Eve.

“Marie-Eve has become the thermal Hall effect expert in our team. This is the most difficult transport measurement to perform—only four or five groups in the world currently master this technique,” says Professor Taillefer. “This expertise opens up several avenues of research in the vast field of quantum materials, including spin liquids, for example.”

The Exclusion of Several Hypotheses

The research proceeded by a comparative study of two insulating cuprates of Mott, Nd2CuO4 and Sr2CuO2Cl2. A measurement of thermal conductivity shows that phonons conduct heat much better in Nd2CuO4, and that the thermal Hall effect is even greater in Nd2CuO4. This correlation is further proof that it is the phonons that are the heat vectors responsible for the thermal Hall response. “The technique of measuring the thermal Hall effect is a challenge itself, since the signal we measured is extremely weak. When we take a thermal Hall effect measurement, we resolve a signal of the order of millikelvin. Very high sensitivity and precision are therefore required to measure this signal,” adds Marie-Eve.

Marie-Eve’s exhaustive study also invalidated several hypotheses that could be at the origin of this signal in Mott insulators, linked to the structure and the magnetic order of the different materials, thus approaching a better understanding of phonon chirality in cuprates.

“Our comparative study reveals that the details of the materials’ crystal structure are not essential to the mechanism of chirality. We even excluded two potential mechanisms, namely the diffusion of phonons by impurities of rare-earth magnets and by structural or antiferromagnetic domains. Our results suggest that the process is rather intrinsic,” adds Marie-Eve.

The Next Steps

“This work is part of my doctoral project and sets the stage for the rest of my project, which will be to work on other cuprates. In our publication in Nature Communications, we mention that the chiral mechanism could come from a coupling of acoustic phonons to the intrinsic excitations of the CuO2 planes. This will be verified with the study of electron-doped cuprates.”

In their exploration of the thermal Hall effect, Marie-Eve Boulanger and the other postdoctoral students of the Taillefer team benefit from several collaborations, both within the Institut quantique and through CIFAR’s quantum materials program and the Laboratoire de Circuits et Matériaux Quantiques, an international associated laboratory (LIA) of the CNRS in France.

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