Three-dimensional (3D) topological insulators, a new class of materials predicted by physicists just over a decade ago, have garnered excitement from researchers recently because these materials are seen to be promising candidates for quantum computing. But there have been few studies that examine in detail how the materials behave.

Researchers at the University of Utah have now shown that if the material is made thin enough, the top and bottom surfaces—which are typically metallic in a 3D topological insulator—start to couple and lose their metallic properties. The work is part of a bigger effort “to really understand how this material behaves,” says Vikram Deshpande, a professor of physics and astronomy at the University of Utah, who published the results in the journal Physical Review Letters.

The possibility to fine-tune the surfaces paves the way for exploring exotic quantum phenomena such as topological exciton condensation, says Kang Wang, a professor of electrical engineering at the University of California, Los Angeles, who was not involved in the work. Excitons are bound electron-hole pairs that can form an exotic state of matter known as a Bose-Einstein condensate. Such condensates are predicted to occur in double-layer materials, Desphande explains, and a topological insulator is a naturally occuring double layer. “A topological Bose-Einstein Condensate, if realized in topological insulator samples such as ours, would be an entirely new state of matter never observed before,” Desphande says.

Finding appropriate materials for qubits, the basic units of a quantum computer, is a big challenge for materials scientists and engineers. Three-dimensional topological insulators, which are insulating in their bulk interiors but metallic at the top and bottom surfaces, are promising candidates because the conductive surfaces make them insensitive to decoherence so they retain their quantum logic state.

Deshpande and his colleagues studied the 3D topological insulator bismuth antimony tellurium selenide (BSTS). They made devices by stacking five layers that are each a few atoms thick. The stack has BSTS flakes in the middle sandwiched between graphite and hexagonal boron nitride (hBN) layers on each side. The BSTS flakes varied in thickness from 89 nm down to 10 nm. The top and bottom graphite and hBN layers serve as the gate electrode and dielectric, respectively.

The researchers exposed the devices to a perpendicular magnetic field of 18 Tesla. They measure the longitudinal and transverse conductivities of the samples versus the top and bottom gate voltages. This maps the data in a checkerboard pattern, showing the pathways by which electrical current move on the surface. The grid represents the quantized conductivities versus the voltages at the two gates. As the material became thinner, the checkerboard grew warped. Finally at a thickness of around 16 nm for the BSTS layer, the checkerboard split into two, with an energy bandgap appearing in the middle, which shows that the surfaces are coupled and no longer conductive. 

“As you make the materials thinner, the surfaces will couple and that destroys the conductivity of the surfaces,” Deshpande says. “So in some sense you could say we’re trying to look at limits of this material and say at this point you cease to call it a 3D topological insulator.”

Previous studies in 2010 and 2012 had observed the energy gap on the metallic surfaces as the insulating material gets thinner. But those experiments used scanning tunneling microscopy and found that the energy gap appeared at materials that were 5-nm thick.

This new device-based work gives “clear evidence of how two independent surfaces interact under gate control,” Wang says. This surface coupling is known in theory and has been studied in part, he says, but “this work shows the details of the interactions by carefully controlling the top and bottom gates.”

Read the abstract in Physical Review Letters.