It was a dizzying discovery.
In 2018, Japanese researchers claimed to have found concrete evidence of an elusive particle, Majorana fermions, in a quantum spin liquid called ruthenium trichloride. Majoranas are very popular among quantum materials scientists because, when the pairs are localized or trapped, they can safely encode information and form stable qubits, the building blocks of quantum computing.
While some researchers published this discovery and used it in their own research, others believed that the breakthrough obtained by measuring the so-called thermal Hall effect was actually a mirage caused by a defect in the material sample.
Researchers at Cornell University are now weighing in on this debate and their findings. Published in Nature on April 22ndshowing that both sides were wrong. By measuring the movement of sound waves rather than heat flow, the researchers discovered that the thermal Hall effect is caused by rotating lattice vibrations called chiral phonons.
“It’s not like this is some magic material containing Majorana fermions that builds quantum computers,” he said. brad ramshawan associate professor of physics in the College of Arts and Sciences, led the team at Cornell University. “But we’re not talking about basically fancy dirt, where impurities in the sample reflect heat in one direction and back again. This is a new, unique effect that no one has seen before.”
Majorana is unusual in that it is itself an antiparticle. Although they may never be produced in particle accelerators, they can be produced from complex interactions between electrons within certain quantum materials. One such candidate is ruthenium trichloride. It is noteworthy that since it is an insulator, it has no thermal Hall effect. In this phenomenon, a magnetic field is applied to a material carrying heat flow, causing the heat flow to bend. This is a behavior that was thought to be impossible with insulators.
“Because the electrons are electrically charged, they feel a force from the field as they move, and they know whether that force is pushing them to the left or to the right. The heat flowing through the insulator is carried by the vibrations of the lattice, and the lattice doesn’t know left or right because it doesn’t know about the field,” Ramshaw said. “We were surprised to find a thermal Hall effect in ruthenium trichloride. Even more surprising was that it was quantized and Majorana fermions were conducting heat.”
It was a big claim, Ramshaw says, but the data seemed to support it, and the quantum materials community was very excited.
“Then questions arose about reproducibility and who has the better samples. These are all common discussions,” he said. “But in the end, other people didn’t get the same answer.”
Another explanation, that magnetic impurities deflect heat, has been similarly difficult to prove.
“The problem is, at the end of the day, you’re just passing heat through something and measuring the change in temperature,” Ramshaw said. “We don’t know what’s going on at the microscopic level. Heat travels in one direction and not the other, but we don’t know why or how. So we wanted to design an experiment that would allow us to find out how it happens.”
Ramshaw and the paper’s lead author, doctoral student Avi Shragai, have devised a method to understand why heat flow bends. The idea is to apply a magnetic field to follow the movement of phonons (essentially the acoustic equivalent of photons), a type of lattice vibration that carries heat as they travel through materials as sound waves.
Using ultrasound measurements to track how phonons move in a magnetic field, the researchers discovered that phonons have twisted paths, like a corkscrew. This so-called acoustic Faraday effect demonstrated that the sample has a Hall viscosity (also called gravitational Hall viscosity) that rotates the polarization of phonons and deflects heat flow.
“The gravity analogy is not that far-fetched,” Ramshaw says. “You’ve probably seen images of space and time being bent by gravity from massive stars. Hall viscosity puts a ‘twist’ on that curvature. This doesn’t seem to happen in the universe, but it can appear inside quantum materials like ruthenium chloride.”
This Hall viscosity caused the thermal Hall effect in ruthenium trichloride.
“If you send sound pointing in one direction through the grating, the sound moves in a spiral, and the sound waves actually rotate the polarization,” Ramshaw says. “Sound waves don’t simply couple with magnetic fields, but it turns out that this material has a very special property called spin-orbit coupling that makes the sound waves know left and right. That’s basically what we showed.”
Ramshaw said researchers had previously theorized that Hall viscosity could be used to measure elusive new states of matter, but this is the first time this has been demonstrated.
“We can now make new discoveries using this technology,” he said. “So, essentially, what we’ve got here is a very sophisticated null result for someone else’s bold claim. You can use this technique in the future to make your own bold claims.”
Co-authors include Ezekiel Horsley, Subin Kim, and Young-June Kim from the University of Toronto, who grew and characterized the samples.
This research was supported by the U.S. Air Force Office of Scientific Research, the Canadian Institute for Advanced Study, the Natural Sciences and Engineering Research Council of Canada, the Canadian Foundation for Innovation, and the Ontario Research Fund.
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