In a surprising discovery, physicists at Princeton University observed the unexpected quantum behavior of 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.
“If our explanation is correct, then we will see a fundamentally new form of quantum matter,”
For a long time, the observation of quantum oscillations has been regarded as 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 studied a material called tungsten ditelluride, which they made into a two-dimensional material. They prepare the material by gradually peeling off or “shaving” each layer to a single layer (single layer) using standard scotch tape. atom-Thin layer. Thick tungsten ditelluride behaves like metal. But once it is converted to a single layer, it will become a very strong insulator.
“This material has many special quantum properties,” Wu said.
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.
“It’s really surprising,” Wu said. “We asked ourselves,’What’s going on?’ We don’t fully understand yet.”
Wu pointed out that there is no theory to explain this phenomenon.
Nevertheless, Wu and his colleagues put forward a provocative hypothesis-a neutral form of quantum matter. Wu said: “Because the interaction is very strong, electrons are organizing themselves to produce this new type of quantum matter.”
Wu said, but in the end it was no longer the electrons 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) can exist and move in an insulator.
Pengjie Wang, the first author of the paper and an associate post-doctoral researcher, said: “Our experimental results contradict all existing theories based on charged fermions, but can be obtained in the presence of charged neutral fermions. Explanation.”
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.
“This is just a starting point,” Wu said. “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.
“In the future, neutral fermions may be used to encode useful information, Quantum computing“,” he said. “But, at the same time, we are still in the early stages of understanding this quantum phenomenon, so basic discoveries must be made. “
Reference: “Landau quantization and highly mobile fermions in an insulator” by Pengjie Wang, Guo Yu, Yanyu Jia, Michael Onyszczak, F. Alexandre Cevallos, Shiming Lei, Sebastian Klemenz, Kenji Watanabe, Takashi Taniguchi, Robert J. Cava, Leslie M . Schoop and Sanfeng Wu, nature.
DOI: 10.1038 / s41586-020-03084-9
In addition to Wu and Wang, the research team also includes the first author, Guo Yu, a graduate student in electrical engineering, and Jia Yanyu, a graduate student in physics. The other main contributor to Princeton University is Leslie Schoop, an assistant professor of chemistry. Robert Cava, Russell Wellman Moore Professor of Chemistry; Physics graduate student Michael Onyszczak; and three former post-doctoral research assistants: Lei Shimin, Sebastian Clay Menz and F. Alexander Cevalos, who also received his PhD from Princeton University in 2018. Alumni. Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Japan also contributed.
Wang Pengjie, Guo Yu, Jia Yanyu, Michael Onyszczak, F. Alexandre Cevallos, Shiming Lei, Sebastian Klemenz, Watanabe Kenji, Takashi Taniguchi, Robert J. Cava, Leslie M. Schoop and Wu Sanfeng were published in the magazine on January 4 nature (DOI: 10.1038 / s41586-020-03084-9).
This work was mainly supported by the National Science Foundation (NSF) through the Princeton University Materials Research Science and Engineering Center (DMR-1420541 and DMR-2011750) and a career award (DMR-1942942). Early measurements were performed at the National High Magnetic Field Laboratory (supported by the NSF Cooperation Agreement (DMR-1644779)) and Florida. Additional support was provided by the “Basic Strategy Initiative” initiated by the Ministry of Education, Culture, Sports, Science and Technology of Japan (JPMXP0112101001), the KAKENHI program of the Japan Association for the Advancement of Science (JP20H00354) and the CREST program of the Japan Science and Technology Agency. (JPMJCR15F3). Further support came from the US Army Research Office Topological Insulator Multidisciplinary University Research Program (W911NF1210461), the Arnold and Maybel Beckman Foundation through the Beckman Young Investigator Fund, and the Gordon and Betty Moore Foundation (GBMF9064) .