In a surprising discovery, Princeton physicists have observed an unexpected quantum behavior in an insulator made of a material called tungsten ditelluride. This phenomenon, known as quantum oscillation, is typically observed in metals rather than insulators, and its discovery provides new insights into our understanding of the quantum world. The results also suggest the existence of a completely new type of quantum particle.
The discovery challenges a long-standing distinction between metals and insulators, because in the established quantum theory of materials, insulators were not considered to be able to experience quantum fluctuations.
“If our interpretations are correct, we see a fundamentally new kind of quantum material,” said Sanfeng Wu, assistant professor of physics at Princeton University and senior author of a recent article in Nature which describes this new discovery. “We now imagine a whole new quantum world hidden in insulators. It is possible that we simply missed identifying them over the last many decades.”
Observation of quantum fluctuations has long been considered a hallmark of the difference between metals and insulators. In metals, electrons are very mobile, and resistivity – the resistance to electrical conduction – is weak. Nearly a century ago, scientists observed that a magnetic field combined with very low temperatures can cause electrons to shift from a “classical” state to a quantum state and cause fluctuations in the resistivity of the metal. In insulators, on the other hand, electrons cannot move and the materials have very high resistivity, so quantum oscillations of this kind are not expected to occur regardless of the strength of the magnetic field used.
The discovery was made when researchers studied a material called tungsten ditelluride, which they made into a two-dimensional material. They prepared the material using plain scotch tape to increasingly exfoliate or “shave” the layers down to what is called a monolayer – a single atom-thin layer. Thick tungsten ditelluride behaves like a metal. But once converted to a monolayer, it becomes a very strong insulator.
“This material has many special quantum properties,” Wu said.
The researchers then began measuring the resistivity of monolayer tungsten ditelluride under magnetic fields. To their surprise, the resistivity of the insulator, although quite large, began to oscillate as the magnetic field increased, indicating the shift to a quantum state. In fact, the material – a very strong insulator – exhibited the most remarkable quantum property of a metal.
“This came as a complete surprise,” Wu said. “We asked ourselves, ‘What’s going on here?’ We do not fully understand it yet. “
Wu noted that there are no current theories to explain this phenomenon.
Nevertheless, Wu and his colleagues have put forward a provocative hypothesis – a kind of quantum material that is neutrally charged. “Because of very strong interactions, the electrons organize themselves to produce this new kind of quantum matter,” Wu said.
But in the end, it is no longer the electrons that oscillate, Wu said. Instead, scientists believe that new particles, which they have called “neutral fermions”, are born out of these highly interacting electrons and are responsible for creating this very remarkable quantum effect.
Fermions are a category of quantum particles that include electrons. In quantum materials, charged fermions can be negatively charged electrons or positively charged “holes” responsible for the electrical conduction. If the material is an electrical insulator, these charged fermions cannot move freely. However, particles that are neutral – that is, neither negatively nor positively charged – are theoretically possible to be present and be mobile in an insulator.
“Our experimental results conflict with all existing theories based on charged fermions,” said Pengjie Wang, co-author of the paper and postdoc research assistant, “but could be explained in the presence of charge-neutral fermions.”
The Princeton team is planning further study of the quantum properties of tungsten ditelluride. They are particularly interested in finding out whether their hypothesis – about the existence of a new quantum particle – is valid.
“This is just the starting point,” Wu said. “If we’m right, future scientists will find other insulators with this surprising quantum property.”
Despite the novelty of the research and the preliminary interpretation of the results, Wu wondered how this phenomenon could be exploited in practice.
“It is possible that neutral fermions could be used in the future to encode information that would be useful in quantum computing,” he said. “In the meantime, however, we are still in the very early stages of understanding quantum phenomena like this, so basic discoveries need to be made.”
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Pengjie Wang et al., Landau quantization and highly mobile fermions in an insulator, Nature (2021). DOI: 10.1038 / s41586-020-03084-9
Provided by Princeton University
Citation: Discovery of quantum behavior in insulators suggests possible new particle (2021, January 12) retrieved January 12, 2021 from https://phys.org/news/2021-01-discovery-quantum-behavior-insulators-particle.html
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