Have Physicists Discovered a New Particle?
What, another subatomic particle?
Just when I was beginning to get my head around quarks, gluons, leptons, and fermions, another particle shows up?
Recently, physicists from Princeton observed what they described as ‘unexpected quantum behavior’ in an insulator constructed of tungsten ditelluride. The surprising part is that the observed activity, which is called quantum oscillation, is generally something they see in metals instead of insulators.
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This observation provides completely new insight regarding the way scientists view the quantum world. The discovery also suggests the existence of a new kind of quantum particle — with a new characteristic.
A new definition of metals and insulators?
This event is now challenging a long understood belief of how insulators and metals differed. In the well-established quantum theory of materials, insulators were believed not to experience any quantum oscillation.
“If our interpretations are correct, we see a fundamentally new form of quantum matter,” claimed Sanfeng Wu, who is an assistant physics professor at Princeton University and also the senior author of a recent paper published in Nature describing the discovery. “We are now imagining a wholly new quantum world hidden in insulators. It’s possible that we simply missed identifying them over the last several decades.”
Long-held quantum beliefs shattered
The presence of quantum oscillations has long been seen as the stark distinction between insulators and metals. Electrons are extremely active in metals with very low resistivity — meaning low resistance to electrical conduction.
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It was almost a century ago when scientists confirmed that a magnetic field, in low temperatures, causes electrons to move into a quantum state from a ‘classical’ state — meaning there were oscillations in the resistivity of the metal.
Conversely, electrons cannot move in insulators because of their high resistivity. Therefore, quantum oscillations are not expected to happen in insulators, regardless of the magnetic field’s strength or intensity.
Researchers used a material called tungsten ditelluride, which they had made into a two-dimensional material when the discovery was made. They formed the material with standard scotch tape to ‘shave’ the layers down to what’s known as a monolayer — or a single atom-thin layer. While multi-layered tungsten ditelluride will behave like a metal, it becomes a strong insulator as a monolayer.
“This material has a lot of special quantum properties,” Wu added.
No explanation for this observation
The new observation prompted researchers to then measure the resistivity of their monolayer tungsten ditelluride in magnetic fields. They were shocked to see that the insulator’s resistivity, which was very large, started oscillating when the magnetic field was increased. This indicates a definite shift into the quantum state.
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This meant that a powerful known insulator clearly exhibited the quantum property of a metal.
“This came as a complete surprise,” Wu summarized. “We asked ourselves, ‘What’s going on here?’ We don’t fully understand it yet.”
Wu pointed out that no existing theories account for this new phenomenon.
As with any good physicist, Wu and his colleagues have offered their own hypothesis — which implies a type of quantum matter with a neutral charge.
“Because of very strong interactions, the electrons are organizing themselves to produce this new kind of quantum matter,” Wu concluded. “But it is ultimately no longer the electrons that are oscillating.”
This research team believes that a new particle, which they are calling ‘neutral fermions,’ are spawning from these strong electron interactions and are inducing this incredible quantum effect.
A new fermion?
Fermions are a type of quantum particle classification that also include electrons. In the world of quantum materials, a charged fermion can exist as a negatively charged electron or a positively charged ‘hole’ that participates in the electrical conduction.
When the quantum material happens to be an electrical insulator, a charged fermion cannot move freely. But when a particle is neutral, meaning it has neither a negative nor positive charge, then it is theoretically able to exist and move within an insulator.
“Our experimental results conflict with all existing theories based on charged fermions,” stated Pengjie Wang, who was co-first author of the research paper, and a postdoctoral research associate, “but could be explained in the presence of charge-neutral fermions.”
As of now, the Princeton physics team intends to conduct more studies on the quantum properties of tungsten ditelluride. Obviously, they are quite determined to confirm the validity of their new hypothesis.
“This is only the starting point,” Wu replied. “If we’re correct, future researchers will find other insulators with this surprising quantum property.”
U.S. Department of Energy. (January 2, 2021). Science Made Simple: What Are Quarks and Gluons? https://scitechdaily.com/science-made-simple-what-are-quarks-and-gluons/.
The University of Basel. (May 26, 2020). Topological insulators feature lossless conduction at the edges. https://phys.org/news/2020-05-topological-insulators-feature-lossless-edges.html.
Tom Garlinghouse, Princeton University. (January 12, 2021). Discovery of quantum behavior in insulators suggests possible new particle. https://phys.org/news/2021-01-discovery-quantum-behavior-insulators-particle.html.