Using the cyclotron at the TRI University Meson Facility, or TRIUMF, in Vancouver, British Columbia, (CSA CSM), UCLA researchers have developed a new method to visualize topological insulators, a class of materials that could lead to improved performance in spintronics, at the nanoscale. The new method is the first use of beta-detected nuclear magnetic resonance to study the effect of the defects in topological insulators. The technique devised by UCLA researchers could help scientists better understand this potential component of next-generation electronic devices. An article highlighting the research, which was which led by Louis Bouchard, assistant professor of chemistry and biochemistry, and Dimitrios Koumoulis, a UCLA postdoctoral scholar, was published online in the Proceedings of the National Academy of Sciences.

Spintronics is considered an especially promising area of research by scientists trying to improve the semiconductors that power our electronic devices. Unlike conventional devices that use electrons’ charge to create power, spintronic devices use electrons’ spin. The technology is already used in computer hard drives and many other application, and scientists believe it could eventually be used for quantum computers, a new generation of machines that use quantum mechanics to solve complex problems with extraordinary speed.

Topological insulators are of interest in spintronics because, unlike ordinary materials that are either insulators or conductors, they function as both simultaneously—on the inside, they are insulators, but on their exteriors, they conduct electricity. However, topological insulators have certain defects that have so far limited their use in practical applications, and because they are so tiny, scientists couldn’t fully understand how the defects impact their functionality. The UCLA researchers’ new technique has allowed them to overcome this challenge.

The technique involves aiming a highly focused stream of ions at the topological insulator. To generate that beam of ions, the researchers used a large particle accelerator called a cyclotron, which accelerates protons through a spiral path inside the machine and forces them to collide with a target made of the chemical element tantalum. This collision produces lithium-8 atoms, which are ionized and slowed down to a desired energy level before they are implanted in the topological insulators.

In beta-detected nuclear magnetic resonance, ions (in this case, the ionized lithium-8 atoms) of various energies are implanted in the material of interest (the topological insulator) to generate signals from the material’s layers of interest. Bouchard said the method is particularly well-suited for probing regions near the surfaces and interfaces of different materials.

In the UCLA research, the high sensitivity of the beta-detected nuclear magnetic resonance technique and its ability to probe materials allowed the scientists to “see” the impacts of the defects in the topological insulators by viewing the electron and magnetic properties beneath the surface of the material.

Co-authors of the PNAS research were Danny King, formerly a UCLA graduate student in chemistry and biochemistry; Kang L. Wang, a UCLA professor of electrical engineering; Liang He, formerly a postdoctoral scholar in Wang’s lab; Dong Wang of the University of British Columbia; Gregory Fiete, a professor at the University of Texas, Austin; and Mercouri Kanatzidis, a professor at Northwestern University. The research was funded by the Defense Advanced Research Project Agency’s Mesodynamic Architectures program.