An artist's impression of the quantum oscillation of 'neutral fermions' in an atomically thin insulator made of tungsten ditelluride. Image designed by Kai Fu for the Wu Lab, Princeton University.
An artist's impression of the quantum oscillation of 'neutral fermions' in an atomically thin insulator made of tungsten ditelluride. Image designed by Kai Fu for the Wu Lab, Princeton University.

In a surprising discovery, physicists at Princeton University have observed an unexpected quantum behavior in an insulator made from tungsten ditelluride. This phenomenon, known as quantum oscillation, is typically observed in metals rather than insulators, and its discovery offers new insights into the quantum world. The findings also hint at the existence of an entirely new type of quantum particle.

This discovery challenges a long-held distinction between metals and insulators, because in the established quantum theory of materials, insulators were not thought to be able to experience quantum oscillations.

"If our interpretations are correct, we are seeing a fundamentally new form of quantum matter," said Sanfeng Wu, assistant professor of physics at Princeton University and senior author of a paper on the discovery in Nature. "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."

The observation of quantum oscillations has long been considered a hallmark of the difference between metals and insulators. In metals, electrons are highly mobile, and resistivity – the resistance to electrical conduction – is weak. Nearly a century ago, researchers observed that a magnetic field, coupled with very low temperatures, can cause electrons to shift from a 'classical' state to a quantum state, causing oscillations in the metal's resistivity. In insulators, by contrast, electrons cannot move and the materials have very high resistivity, so quantum oscillations of this sort are not expected to occur, no matter the strength of the applied magnetic field.

The new discovery was made when the researchers were studying tungsten ditelluride, which they produced as a two-dimensional material. This involved using standard scotch tape to increasingly exfoliate, or 'shave', the layered tungsten ditelluride down to what is called a monolayer – a single atom-thin layer. Bulk tungsten ditelluride behaves like a metal, but once it is converted to a monolayer, it becomes a very strong insulator.

"This material has a lot of special quantum properties," Wu said.

The researchers then set about measuring the resistivity of the monolayer tungsten ditelluride under magnetic fields. To their surprise, the resistivity of the insulator, despite being quite large, began to oscillate as the magnetic field was increased, indicating the shift to a quantum state. In effect, the material – a very strong insulator – was exhibiting 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 don't fully understand it yet."

According to Wu, there are no current theories to explain this phenomenon. Nonetheless, he and his colleagues have put forward a provocative hypothesis – that it is caused by a form of quantum matter that is neutrally charged. "Because of very strong interactions, the electrons are organizing themselves to produce this new kind of quantum matter," Wu said.

But it is ultimately no longer the electrons that are oscillating. Instead, the researchers believe that new particles, which they have dubbed 'neutral fermions', are born out of these strongly interacting electrons and are responsible for creating this highly 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' that are responsible for electrical conduction. In an electrical insulator, these charged fermions can't move freely. But in theory, particles that are neutral – that is, neither negatively nor positively charged – can be present and mobile in an insulator.

"Our experimental results conflict with all existing theories based on charged fermions," said Pengjie Wang, co-first author of the paper and a postdoctoral research associate, "but could be explained in the presence of charge-neutral fermions."

The Princeton team plans further investigations into the quantum properties of tungsten ditelluride. They are particularly interested in discovering whether their hypothesis – about the existence of a new quantum particle – is valid.

"This is only the starting point," Wu said. "If we're correct, future researchers will find other insulators with this surprising quantum property."

Despite the newness of the research and the tentative interpretation of the results, Wu speculated about how this phenomenon could be put to practical use. "It's possible that neutral fermions could be used in the future for encoding information that would be useful in quantum computing," he said. "In the meantime, though, we're still in the very early stages of understanding quantum phenomena like this, so fundamental discoveries have to be made."

This story is adapted from material from Princeton University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.