April 13, 2024

Researchers find first experimental evidence of a graviton-like particle in a quantum material

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Light probing a chiral graviton mode in a fractional quantum Hall effect liquid. Credit: Lingjie Du, Nanjing University

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Light probing a chiral graviton mode in a fractional quantum Hall effect liquid. Credit: Lingjie Du, Nanjing University

A team of scientists from Columbia, Nanjing University, Princeton and Munster University, writing in the journal Naturepresented the first experimental evidence of collective spin excitations called chiral graviton modes (CGMs) in a semiconductor material.

A CGM appears to be similar to a graviton, a yet-to-be-discovered elementary particle best known in high-energy quantum physics for hypothetically giving rise to gravity, one of the fundamental forces of the universe, the ultimate cause of which remains mysterious.

The ability to study graviton-like particles in the laboratory could help fill critical gaps between quantum mechanics and Einstein’s theories of relativity, resolving a major dilemma in physics and expanding our understanding of the universe.

“Our experiment marks the first experimental confirmation of this concept of gravitons, postulated by pioneering work in quantum gravity since the 1930s, in a condensed matter system,” said Lingjie Du, former Columbia postdoctoral fellow and senior author of the paper.

The team discovered the particle in a type of condensed matter called fractional quantum Hall effect liquid (FQHE). FQHE liquids are a system of strongly interacting electrons that occur in two dimensions at high magnetic fields and low temperatures. They can be described theoretically using quantum geometry, emerging mathematical concepts that apply to the tiny physical distances at which quantum mechanics influences physical phenomena.

The electrons in a FQHE are subject to what is known as quantum metric, which was predicted to give rise to CGMs in response to light. However, in the decade since quantum metric theory was first proposed for FQHEs, there have been limited experimental techniques to test its predictions.

For much of his career, Columbia physicist Aron Pinczuk has studied the mysteries of FQHE liquids and worked to develop experimental tools that could probe such complex quantum systems. Pinczuk, who joined Columbia from Bell Labs in 1998 and was a professor of physics and applied physics, passed away in 2022, but his laboratory and former students around the world have continued his legacy. These alumni include paper authors Ziyu Liu, who graduated with her Ph.D. in physics from Columbia last year, and former Columbia postdocs Du, now at Nanjing University, and Ursula Wurstbauer, now at the University of Münster.

“Aron pioneered the approach to studying exotic phases of matter, including emergent quantum phases in solid-state nanosystems, through the low-altitude collective excitation spectra that are their unique fingerprints,” commented Wurstbauer, co-author of the current work.

“I’m really happy that your latest genius proposal and research idea has been so successful and is now published in Nature. However, it is sad that he cannot celebrate with us. He has always placed a strong focus on the people behind the results.”

Graviton modes and inelastic light scattering. Credit: Nature (2024). DOI: 10.1038/s41586-024-07201-w

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Graviton modes and inelastic light scattering. Credit: Nature (2024). DOI: 10.1038/s41586-024-07201-w

One of the techniques established by Pinczuk was called low-temperature resonant inelastic scattering, which measures how particles of light, or photons, scatter when they hit a material, thus revealing the material’s underlying properties.

Liu and his co-authors on the paper adapted the technique to use what is known as circularly polarized light, in which photons have a specific spin. When polarized photons interact with a particle like a CGM that also rotates, the sign of the photons’ spin will change in response in a more distinct way than if they were interacting with other types of modes.

The new paper was an international collaboration. Using samples prepared by Pinczuk’s longtime collaborators at Princeton, Liu and Columbia physicist Cory Dean completed a series of measurements at Columbia. They then sent the sample for experiments on low-temperature optical equipment that Du spent more than three years building in his new laboratory in China.

They observed physical properties consistent with those predicted by quantum geometry for CGMs, including their spin-2 nature, characteristic energy gaps between their ground and excited states, and dependence on so-called filling factors, which relate the number of electrons in the system to their magnetic field.

CGMs share these characteristics with gravitons, an as-yet-undiscovered particle predicted to play a critical role in gravity. Both CGMs and gravitons are the result of quantized metric fluctuations, Liu explained, in which the fabric of spacetime is randomly pulled and stretched in different directions.

The theory behind the team’s results could therefore potentially connect two subfields of physics: high-energy physics, which operates on the largest scales in the universe, and condensed matter physics, which studies atomic and electronic materials and interactions. that give them unique properties.

In future work, Liu says the polarized light technique should be simple to apply to FQHE liquids at higher energy levels than those explored in the current paper. It should also apply to additional types of quantum systems where quantum geometry predicts unique properties of collective particles, such as superconductors.

“For a long time, there has been this mystery about how long-wavelength collective modes like CGMs could be investigated in experiments. We have provided experimental evidence that supports the predictions of quantum geometry,” Liu said. “I think Aron would be very proud to see this extension of his techniques and a new understanding of a system he has studied for a long time.”

More information:
Jiehui Liang et al, Evidence for chiral graviton modes in fractional quantum Hall liquids, Nature (2024). DOI: 10.1038/s41586-024-07201-w

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