It was a head-spinning discovery. 

In 2018, researchers in Japan claimed to find concrete evidence of an elusive particle, a Majorana fermion, in a quantum spin liquid called ruthenium trichloride. Majoranas are highly sought-after by quantum materials scientists because when a pair are localized, or trapped, they can securely encode information and form a stable qubit – the building block of quantum computing.

Some researchers heralded the finding and used it to launch their own studies, while others believed the breakthrough – which was made by measuring what’s called the thermal Hall effect – was actually a mirage caused by defects in the material sample. 

Cornell researchers have now waded into the debate and their findings, published April 22 in Nature, show both camps were wrong. By measuring the movement of soundwaves rather than the flow of heat, the team discovered the thermal Hall effect was caused by rotating lattice vibrations called chiral phonons.

“It’s not that this is the magic material with Majorana fermions that’s going to build a quantum computer,” said Brad Ramshaw, associate professor of physics in the College of Arts and Sciences, who led the Cornell team. “But it’s also not this story of basically fancy dirt, where the samples have impurities that are bouncing the heat one way instead of another. It’s a new intrinsic effect that nobody had ever seen before.”

Majoranas are unusual in that they are their own antiparticle. While they may not ever be produced in a particle accelerator, they can emerge from complex interactions between electrons in certain quantum materials. One such candidate is ruthenium trichloride, which is notable because it is an insulator and therefore should not have a thermal Hall effect. In that phenomenon, a magnetic field is applied to a material carrying a flow of heat and the heat flow bends – behavior that was thought to be impossible for an insulator.

“Electrons are charged, and so they feel a force from the field when they move, and they know whether that force is pushing them left or pushing them right. Heat flowing through an insulator is carried by vibrations of the lattice, and the lattice doesn’t know about the field and therefore doesn’t know left from right,” Ramshaw said. “Finding a thermal Hall Effect in ruthenium trichloride was surprising. It was even more surprising that it was quantized, suggesting Majorana fermions were carrying the heat.”

That was a big claim, according to Ramshaw, but the data seemed to back it up and the quantum materials community was terribly excited. 

“Then there were troubles with reproducibility and questions about who had the better samples – all the usual arguments,” he said. “But ultimately, other people didn’t get the same answer.”

The alternate explanation, of magnetic impurities deflecting the heat, was equally difficult to prove.

“The problem is that, at the end of the day, all you’re doing is flowing heat through something and measuring a change in temperature,” Ramshaw said. “You don’t know what’s going on at the microscopic level. Heat is going one way, not another way, but you don’t know why or how. So we wanted to design an experiment that could tell you how.”

Ramshaw and the paper’s lead author, doctoral student Avi Shragai, designed a way to understand why that flow of heat bends: applying a magnetic field and following the movement of phonons, a type of lattice vibration that carries heat as it travels through the material as a soundwave – essentially the sonic equivalent of photons.

Using ultrasonic measurements that tracked how the phonons moved in a magnetic field, the researchers found the phonons had twisted paths, like a corkscrew. This so-called acoustic Faraday effect demonstrated that the sample had Hall viscosity – also called gravitational Hall viscosity – which rotates phonon polarizations and also deflects their heat currents. 

“The gravity analogy is not that far off,” Ramshaw said. “You have probably seen those images of space and time being curved by gravity from a massive star. Hall viscosity adds a `twist’ to that curvature. This doesn’t seem to happen out in the universe, but it can emerge inside a quantum material like ruthenium chloride.” 

This Hall viscosity was what produced the thermal Hall effect in ruthenium trichloride.

“When we send sound pointing in one direction into the lattice, it moves like a helix and the soundwaves actually rotate their polarization,” Ramshaw said. “Soundwaves don’t naively couple to magnetic fields, but it turns out there’s a very special property of this material, called spin orbit coupling, that lets the sound waves know left from right. That’s basically what we showed.”

Researchers previously theorized Hall viscosity could be used to measure new and elusive states of matter, according to Ramshaw, but this is the first time it’s been demonstrated.

“This technique can now be used to make new discoveries,” he said. “I mean, essentially what we have here is a very elaborate null result on someone else’s bold claim. Going forward, we can use this technique to make bold claims of our own.”

Co-authors include Ezekiel Horsley, Subin Kim and Young-June Kim of University of Toronto, who conducted sample growth and characterization. 

The research was supported by the U.S. Air Force Office of Scientific Research, the Canadian Institute for Advanced Research, the Natural Sciences and Engineering Research Council of Canada, the Canada Foundation for Innovation and the Ontario Research Fund.