New 2D Materials Exhibit Exotic Quantum Properties

A newly published study from MIT details a theoretical analysis showing that a family of two-dimensional materials exhibits exotic quantum properties that may enable a new type of nanoscale electronics.

These materials are predicted to show a phenomenon called the quantum spin Hall (QSH) effect, and belong to a class of materials known as transition metal dichalcogenides, with layers a few atoms thick. The findings are detailed in a paper appearing this week in the journal Science, co-authored by MIT postdocs Xiaofeng Qian and Junwei Liu; assistant professor of physics Liang Fu; and Ju Li, a professor of nuclear science and engineering and materials science and engineering.

QSH materials have the unusual property of being electrical insulators in the bulk of the material, yet highly conductive on their edges. This could potentially make them a suitable material for new kinds of quantum electronic devices, many researchers believe.

But only two materials with QSH properties have been synthesized, and potential applications of these materials have been hampered by two serious drawbacks: Their bandgap, a property essential for making transistors and other electronic devices, is too small, giving a low signal-to-noise ratio; and they lack the ability to switch rapidly on and off. Now the MIT researchers say they have found ways to potentially circumvent both obstacles using 2-D materials that have been explored for other purposes.

Existing QSH materials only work at very low temperatures and under difficult conditions, Fu says, adding that “the materials we predicted to exhibit this effect are widely accessible. … The effects could be observed at relatively high temperatures.”

“What is discovered here is a true 2-D material that has this [QSH] characteristic,” Li says. “The edges are like perfect quantum wires.”

The MIT researchers say this could lead to new kinds of low-power quantum electronics, as well as spintronics devices — a kind of electronics in which the spin of electrons, rather than their electrical charge, is used to carry information. Graphene, a two-dimensional, one-atom-thick form of carbon with unusual electrical and mechanical properties, has been the subject of much research, which has led to further research on similar 2-D materials. But until now, few researchers have examined these materials for possible QSH effects, the MIT team says. “Two-dimensional materials are a very active field for a lot of potential applications,” Qian says — and this team’s theoretical work now shows that at least six such materials do share these QSH properties.