Quantum Phenomenon

A team of physicists from the United States and Japan has experimentally observed a phenomenon called quantum oscillation in a two-dimensional topological insulator — monolayer tungsten ditelluride (WTe₂). The team’s findings, published in the journal Nature, hint at the existence of an entirely new type of quantum particle — a ‘neutral fermion.’

Cartoon illustration of the device design, where a thin hBN layer (blue) with selectively etched areas is used to avoid contact of the Pd electrodes (yellow) with the edges of the monolayer WTe₂. The stack of graphite (gray)/hBN (blue)/monolayer WTe₂ (red), shown in the cross-sectional view, is further stacked onto the bottom part with the electrodes, as indicated by the arrow. Inset: an image of device 1; the dashed red line highlights the monolayer edges and the white squares denote the contact regions. Image credit: Wang et al., doi: 10.1038/s41586-020-03084-9.

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 is weak. Nearly a century ago, physicists 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 magnetic field applied.

“If our interpretations are correct, we are seeing a fundamentally new form of quantum matter,” said senior author Dr. Sanfeng Wu, a researcher in the Department of Physics at Princeton University.

“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.”

For the research, Dr. Wu and colleagues prepared tungsten ditelluride by using standard scotch tape to increasingly exfoliate the layers down to what is called a monolayer.

Thick tungsten ditelluride behaves like a metal. But once it is converted to a monolayer, it becomes a very strong insulator.

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 into 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. We asked ourselves, ‘What’s going on here?’ We don’t fully understand it yet,” Dr. Wu said.

‘There are no current theories to explain this phenomenon.”

Nonetheless, the scientists have a provocative hypothesis — 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. But it is ultimately no longer the electrons that are oscillating,” Dr. Wu said.

Instead, the authors 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.

In quantum materials, charged fermions can be negatively charged electrons or positively charged ‘holes’ that are responsible for the electrical conduction.

Namely, if the material is an electrical insulator, these charged fermions can’t move freely. However, particles that are neutral — that is, neither negatively nor positively charged — are theoretically possible to be present and mobile in an insulator.

“Our experimental results conflict with all existing theories based on charged fermions, but could be explained in the presence of charge-neutral fermions,” said first author Dr. Pengjie Wang, a postdoctoral researcher in the Department of Physics at Princeton University.

_____

P. Wang et al. Landau quantization and highly mobile fermions in an insulator. Nature, published online January 4, 2021; doi: 10.1038/s41586-020-03084-9

This article is based on text provided by Princeton University.