A multinational team of scientists led by the University of Minnesota discovered that a specific superconducting metal is more resilient when used as a very thin layer. The research is the first step toward a larger goal of better understanding uncommon superconducting states in materials, which could one day be used in quantum computing.
The collaborators include physicists from Cornell University and other institutions, as well as Professor Rafael Fernandes, Associate Professor Vlad Pribiag, and Assistant Professors Fiona Burnell and Ke Wang from the University of Minnesota’s School of Physics and Astronomy. The findings of the study were reported in the journal Nature Physics.
Niobium diselenide (NbSe2) is a superconducting metal, which means it can conduct electricity without resistance or move electrons from one atom to another. When materials are very small, it is typical for them to act differently, yet NbSe2 has certain potentially useful qualities. The scientists found that the material in 2D form (a very thin substrate only a few atomic layers thick) is a much more resilient superconductor than thicker samples of the same material because it has a two-fold symmetry.
Pribiag and Wang began investigating atomically thin 2D superconducting devices after being inspired by Fernandes and Burnell’s theoretical prediction of exotic superconductivity in this 2D material. Like a snowflake, Wang and his colleagues expected it to have a six-fold rotational pattern. In the experiment, it only displayed two-fold behavior despite the six-fold structure. Pribiag stated that this was one of the first times [this phenomenon] was noticed in a real substance.
The mixing of two similarly competing kinds of superconductivity, namely the conventional s-wave type — typical of bulk NbSe2 — and an unconventional d- or p-type mechanism which arises in few-layer NbSe2 is attributed to the newly discovered two-fold rotational symmetry of the superconducting state in NbSe2. In this system, the energy of the two forms of superconductivity is quite comparable. As a result, they engage and compete with one another.
Physicists at Cornell University, according to Pribiag and Wang, were evaluating the same physics using various sampling techniques, name quantum tunneling measurements. They decided to publish a full report by combining their findings with the Cornell research.
Burnell, Pribiag, and Wang plan to build on these preliminary findings by understanding the properties of atomically thin NbSe2 in combination with other exotic 2D materials, which could probably result in the use of unconventional superconducting states like topological superconductivity to construct quantum computers.
On the atomic scale, what Pribiag and his team want is a perfectly flat interface. They hoped that this technology will provide them with a better platform for studying materials to utilize them in quantum computing applications,” says the team.
Pribiag, Fernandes, Burnell, and Wang collaborated with University of Minnesota physics graduate students Alex Hamill, Brett Heischmidt, Daniel Shaffer, Kan-Ting Tsai, and Xi Zhang; Cornell University faculty members Jie Shan and Kin Fai Mak, as well as graduate student Egon Sohn; and Helmuth Berger and László Forró of the Ecole Polytechnique Fédérale de Lausanne in Switzerland; Xiaoxiang Xi, a professor at Nanjing University in China, and Alexey Suslov, a researcher at the National High Magnetic Field Laboratory in Tallahassee, Fla.
The National Science Foundation (NSF) funded the majority of the University of Minnesota’s research through the Materials Research Science and Engineering Center (MRSEC). The National Science Foundation (NSF) funded the Cornell research and the office of Naval Research (ONR). The Swiss National Science Foundation funded the research in Switzerland.
Main paper: Alex Hamill et al, Two-fold symmetric superconductivity in few-layer NbSe2, Nature Physics (2021). DOI: 10.1038/s41567-021-01219-x