In a surprising discovery, physicists at Princeton University observed unexpected quantum behavior in an insulator made of tungsten ditelluride. This phenomenon is called quantum oscillation and is usually observed in metals rather than insulators. Its discovery provides new insights into our understanding of the quantum world. These findings also hint at the existence of a whole new kind of quantum particle.
This discovery challenges the long-standing distinction between metals and insulators, because in the established quantum theory of materials, insulators are believed to be unable to undergo quantum oscillations.
Wu Sanfeng, assistant professor of physics at Princeton University and senior author of a paper recently published in the journal Science, said: “If our explanation is correct, then we will see a fundamentally new form of quantum matter. ” nature Describe this new discovery in detail. “We are now imagining a new quantum world hidden in an insulator. It is likely that we have missed the opportunity to identify them in the past few decades.”
For a long time, people have believed that observing quantum oscillations is a sign of the difference between metals and insulators. In metals, electrons have high mobility and resistivity (resistance to conductivity) is very weak. Nearly a century ago, researchers observed that the combination of a magnetic field and extremely low temperature would cause electrons to change from a “classical” state to a quantum state, causing oscillations in metal resistivity. In contrast, in an insulator, electrons cannot move, and the resistivity of the material is very high, so this quantum oscillation does not occur regardless of the strength of the applied magnetic field.
This discovery was made when researchers were studying a material called tungsten ditelluride, which they made into a two-dimensional material. They used standard scotch tape to peel off or “shave” these layers to so-called monolayer-monoatomic thin layers to prepare this material. Thick tungsten ditelluride behaves like metal. But once it is converted to a single layer, it will become a very strong insulator.
Wu said: “This material has many special quantum properties.”
Then, the researchers set about measuring the resistivity of a single layer of tungsten ditelluride under a magnetic field. To their surprise, although the resistivity of the insulator is large, it starts to oscillate with the increase of the magnetic field, indicating that it has transformed into a quantum state. In fact, this material-a very strong insulator-exhibits the most significant quantum properties of metals.
Wu said: “It was a complete surprise.” “We asked ourselves,’What’s going on?’ We don’t fully understand yet.”
Wu Bangguo pointed out that there is no theory to explain this phenomenon.
Nevertheless, Wu and his colleagues put forward a provocative hypothesis-a quantum substance with a neutral charge. Wu said: “Due to very strong interactions, electrons are organizing themselves to produce this new type of quantum matter.”
Wu said, but in the end it is no longer the electron oscillating. On the contrary, the researchers believe that the new particles they call “neutral fermions” are produced by these strongly interacting electrons and are responsible for producing this very excellent quantum effect.
Fermions are a class of quantum particles including electrons. In quantum materials, charged fermions can be negatively charged electrons or positively charged “holes”, which are responsible for conducting electricity. That is, if the material is an electrical insulator, these charged fermions will not be able to move freely. However, in theory, neutral particles (neither negatively nor positively charged) may exist and move in the insulator.
The paper’s first author and post-doctoral research associate Wang Pengjie said: “Our experimental results contradict all existing theories based on charged fermions, but can be explained in the presence of charge-neutral fermions.”
The Princeton University team plans to further study the quantum properties of tungsten ditelluride. They are particularly interested in discovering whether their hypothesis (about the existence of new quantum particles) is valid.
Wu Bangguo said: “This is just the starting point.” “If we are correct, future researchers will discover other insulators with such surprising quantum properties.”
Although this research is novel and provides a preliminary explanation of the results, Wu is still speculating how to put this phenomenon into practice.
He said: “Neutral fermions may be used in the future to encode information useful for quantum computing.” “However, at the same time, we are still in the early stages of understanding such quantum phenomena, so basic discoveries must be made.”
Lossless conduction at the edges of topological insulators
Wang Pengjie Wang et al. Landau quantification and high mobility fermions in insulators, nature (2021). DOI: 10.1038 / s41586-020-03084-9
Courtesy of Princeton University
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