New Study Resolves Mystery of Heat Flow in Quantum Material

A 2018 report of Majorana fermions in a unique insulator has been a subject of scientific debate. New research now identifies a different, novel cause for the observed effect.

The Initial Discovery and Debate

In 2018, researchers in Japan reported evidence of Majorana fermions—particles that are their own antiparticle—in the material ruthenium trichloride. Their conclusion was based on measurements of a quantized thermal Hall effect, where heat flow bends under a magnetic field.

The finding was significant because a quantized thermal Hall effect in an insulator was previously thought to be impossible, suggesting Majorana fermions were carrying the heat.

The claim was debated, with some in the scientific community attributing the signal to material defects rather than exotic quantum particles.

The New Cornell University Study

A team from Cornell University, led by associate professor Brad Ramshaw, has published a new study in Nature that clarifies the phenomenon. Their work demonstrates the effect has a different, intrinsic origin.

Using ultrasonic measurements to track phonon movement, the team found the thermal Hall effect was caused by rotating lattice vibrations known as chiral phonons, not by Majorana fermions or impurities.

Methodology and Key Finding

The experiment was designed to understand the microscopic cause of the bent heat flow. The researchers applied a magnetic field and tracked phonons—the lattice vibrations that carry heat as soundwaves.

They observed an acoustic Faraday effect, where soundwaves moved in helical paths and rotated their polarization. This rotation is driven by a property of the material called spin orbit coupling and a related quantum property known as Hall viscosity (or gravitational Hall viscosity).

"This shows the material is not 'the magic material with Majorana fermions' for quantum computing, nor is the effect due to sample impurities," said Brad Ramshaw. "It's a new intrinsic effect that nobody had ever seen before."

Significance of Hall Viscosity

The study provides the first experimental demonstration of Hall viscosity in action. Ramshaw noted that this property had been theorized as a tool for measuring new states of matter, but had never been directly shown to cause a macroscopic effect like the thermal Hall signal.

The technique developed by the team can now be used as a tool for further discoveries in quantum materials.

Research Support and Authors

Co-authors of the study include Ezekiel Horsley, Subin Kim, and Young-June Kim of the University of Toronto, who were responsible for sample growth and characterization.

Support for the research was provided 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.