Robert Cava, the Russell Wellman Moore Professor of Chemistry
N. Phuan Ong, the Eugene Higgins Professor of Physics and director of the Princeton Center for Complex Materials
Ali Yazdani, professor of physics
Invention: Topological insulators for improved electronics
What It Is: Crystalline materials that feature highly electrically conductive surfaces but interiors that act as insulators. Electrons are able to flow very quickly on the surface but not through the middle of the crystal. This high surface conductivity could be harnessed for use in electronic devices that are smaller, faster, and less heat-generating than today's electronics.How It Works: Topological insulators are highly unusual because the electrons that travel on their surfaces are much less susceptible to the normal scattering that happens when electrons encounter imperfections or defects in a crystal. In most materials, electrons bounce away from these defects, in energy-losing collisions, causing loss of conductivity and the loss of energy as heat. On the surface of a topological insulator, however, electrons that encounter imperfections in the material continue in the direction of travel rather than scattering in different directions. "We found very interesting evidence that they were actually penetrating through barriers," said Yazdani, who has imaged these electrons using scanning tunneling microscopy.
In crystalline materials capable of conducting electrical current, the electrons are characterized by two important properties, their spin and momentum. The spin specifies how the electron rotates about an axis (either up or down), while the momentum specifies the direction of its velocity. In topological insulators, the spin and momentum of the surface electrons are firmly locked together, so that for example a forward-moving electron can only have its spin axis pointing to the right whereas a backward-moving electron can only have its spin axis pointing to the left. This unusual spin-locking property appears to prevent the scattering of electrons from the forward to backward direction because they cannot do that without changing their spin direction, which is not possible due to its high energy cost.
Topological insulators are typically made using bismuth and other heavy metals. Some of these materials are already in commercial use for their thermoelectric properties (their ability to cool or heat due to the passage of electrical current), but new topological insulators are being created at Princeton for use in electronic applications. A team led by N. P. Ong was among the first to demonstrate that the surface current on a topological insulator can be measured and manipulated to make it useful in electronic applications. "Now that we have observed the surface current, we can improve the material for possible future applications in electronics," said Ong.
Applications: The non-scattering behavior of electrons on the surfaces of topological insulators has many potential advantages in electronic applications. As electronic components continue to shrink, the wire connections between transistors, known as 'interconnects,' are also shrinking. These thinner and smaller wires lose more energy through heating and are more delicate and therefore likely to become damaged during fabrication and use. Replacing the conventional metal interconnects with topological materials could help improve their conductivity and reduce the amount of heat generated by consumer devices. The conductivity could be used in other electronic applications and lead to entirely new classes of electronics based on the manipulation of spin rather than electronic charge.
Unlike other exotic electronic states of matter, these crystals demonstrate their unusual properties at room temperature under standard conditions. "These materials do not require extreme cold or pressure conditions – the surface states exist at room temperature and in the air," said Robert Cava, who is an expert in growing the crystals.
Inspiration: The study of topological insulators is a new sub-field in physics and materials science that has grown rapidly world-wide. The pioneering experiments, confirming the predictions of theoretical physicists at the University of Pennsylvania, were performed in 2008 by Princeton's M. Zahid Hasan, professor of physics, together with Cava on the material Bi-Sb alloy. Cava, Ong and Yazdani have carried out further investigations of these novel materials. The team has since found several new topological insulator materials with improved properties through close feedback between materials preparation and physical measurements.
Collaborators: This work was supported by the National Science Foundation Materials Research Science and Engineering Center (NSF-MRSEC) program, the Defense Advanced Research Projects Agency (DARPA) and the Eric and Wendy Schmidt Transformative Technology Fund. Additional collaborators are postdocs and student Yew San Hor, Shuang Jia, and Huiwen Ji in the Department of Chemistry; Jun Xiong, Yuehaw Khoo, Yongkang Luo and Stephen Rowley in the Department of Physics; and Pedram Roushan, Junpil Seo and Haim Beindenkopf in the Department of Physics.
Commercialization Status: A patent application has been filed.