On the phonon’s trail

Étienne Lefrançois, a recent PhD graduate, has made his mark in quantum physics by studying the elusive α-RuCl3 material, an insulator at the very center of cutting-edge quantum material research. α-RuCl3 is believed to harbor a rare and exotic particle, or so it is theorized…

A natural path towards quantum physics

It was through Etienne’s first internship in mentor Louis Taillefer’s group that he was attracted to this research. As he puts it, a second internship, a master’s degree followed by a doctorate were “only natural”. Étienne focused on a central research question: how does heat propagate in quantum materials? It’s a bit like heating one end of a small metal plate and looking at the heat moving to the other end, only to find that it’s explained by the movement of electrons. However, in quantum physics, thermal transport is more challenging to grasp, particularly in certain materials such as α-RuCl3.

α-RuCl3 and the elusive Majorana fermion

The magnetic insulator α-RuCl3 is attracting the scientific community’s interest because of a special feature: It is a liquid potential of quantum spins. In this state of matter, electron spins do not align to form a magnetic order at low temperatures. Instead, they remain in a state of dynamic “vortex“. This quantum characteristic is particularly interesting since α-RuCl3 could potentially harbor the Majorana fermion. This elusive particle, which is its own antiparticle, has the potential to revolutionize the field of quantum computing by making quantum computers more robust and less error prone.

The Thermal Hall Effect to probe α-RuCl3

The thermal Hall effect, a lesser-known cousin of the electrical Hall effect, occurs when a transverse temperature gradient in a solid is generated by a magnetic field applied perpendicular to the direction of the thermal power flow. It was postulated that this probe could detect Majorana’s famous fermion signature. Étienne therefore immersed α-RuCl3 in an intense magnetic field, induced a heat current in the sample and measured the resulting Hall thermal gradient.

This was easier said than done: Measuring the thermal Hall effect is a delicate task, requiring extremely stable laboratory conditions, very low temperatures, strong magnetic fields and several custom-designed devices. “It’s an experimental challenge – measuring the thermal Hall effect is very difficult because the signals are extremely weak, so it’s a nice experimental challenge,” says Étienne.

Prof. Louis Taillefer praised Étienne’s determination: “I really enjoyed having Étienne on my research team. Firstly, for his courage in tackling the greatest challenges, and his perseverance in seeing things through to the end. He showed exceptional willpower. But also for his team spirit. He was always ready and happy to welcome and train new people who joined us. He was a pillar of our team – he will be sorely missed!”

When phonons take center stage

Rather than detecting the signature of the Majorana fermion, Étienne discovered that phonons, quasiparticles associated with the vibrations of the material, were at the origin of the thermal Hall effect in α-RuCl3. A counter-intuitive revelation, since, as Étienne reminds us, “a phonon has no charge, so it shouldn’t respond to the magnetic field“. But in α-RuCl3, phonons are strongly coupled to the magnetic environment and are responsible for heat transport, which explains the thermal Hall effect in this insulating material.

“When I first started the project, I didn’t know much about the thermal Hall effect in magnetic insulators… and even less about the thermal Hall effect in phonons! I was very happy to do my PhD on this subject because it’s an exciting, emerging field. It’s cool to be part of a subject we know very little about at the moment – so the PhD was a completely exploratory course!” says Étienne, underlining the importance of this research in today’s field.

From lab to finish line

Outside the lab, Étienne Lefrançois is no different. A challenge enthusiast, he runs ultramarathons in the mountains and volunteers his time to help high-school students with academic difficulties. “It’s difficult, of course, but I’m always looking for a challenge”. – a state of mind that characterizes him perfectly, on the trails as well as in quantum research.