Researchers in Australia and Singapore are the first to have measured the electronic properties of topological Dirac semimetal thin films, which are 3D analogues of graphene. These films, made from sodium bismuth, have high charge carrier mobilities and could be used to make new electronic devices like topological transistors.

Graphene boasts a wealth of fascinating electronic properties, many of which arise from the fact that it is a semiconductor with a zero-energy gap between its valence and conduction bands. Near where the two bands meet (the Dirac point), the relationship between the energy and momentum of the electrons in the material becomes linear and it is described by the Dirac equation. The energy-momentum relationship of the electrons in graphene in fact resembles that of a photon (which has no mass), and the bands through which the electrons travel (called Dirac cones) enable the electrons to travel through the carbon sheet at extremely high speeds. This means that electronic devices, like transistors, made from the material could be faster than any that exist today.

The electrons in topological Dirac semimetals also travel at these extremely high speeds, but in all three dimensions, not just two, and are thus said to possess a “bulk” Dirac cone. This is why we think of them as 3D graphene.

Measuring the Dirac point energy

The researchers obtained their results at a unique, new combined low-temperature ultra-high vacuum scanning tunnelling microscope and magneto-transport facility at the Monash Centre for Atomically Thin Materials at Monash University. This facility allowed them to grow a thin film of sodium bismuth (Na₃Bi), a TDS first discovered in 2014, on single-crystal insulating alumina substrates in ultrahigh vacuum using molecular-beam epitaxy. They then measured the electrical proerties of the films while observing them in a scanning tunnelling microscope without removing them from the vacuum. This is important, says team member Mark Edmonds at Monash, since Na₃Bi rapidly oxidizes in air.

“We characterized our films with scanning tunnelling microscopy to measure the atomic structure and size of the Na₃Bi islands that formed,” he says. “We then used scanning tunnelling spectroscopy to measure the Dirac point energy in the films relative to the Fermi energy. Next, low-energy electron diffraction was used to confirm that the film was of a high quality across the entire 1 cm² sample area.”

High charge-carrier mobilities

Finally, the team used low-temperature magneto-transport measurements to determine the carrier density and mobility in the films.

“We found that these films have high charge-carrier mobilities exceeding 6000 cm²/V/s and carrier densities below 10¹⁸/cm³,” Edmonds tells nanotechweb.org. “These values are comparable to, if not better than, those of the best single crystals. Since the mobility characterizes the electronic quality of a material, and the mobility of our Na₃Bi films is very similar to that of the first graphene samples studied on insulators, our results bode well for future studies of Na₃Bi.”

Towards topological transistors

The work should open up numerous possibilities for TDS materials since it has shown that it is possible to grow them on an insulating substrate, adds Edmonds. “This means in that in the future, we could make TDS structures whose electronic properties could be modulated with an applied voltage. This could lead to new electronics devices in which the topological state could be tuned to make topological transistors.”

The films might also be interfaced with superconductors to produce fundamental and exotic particles such as Marjorana fermions (particles that are their own antiparticles). These fermions are predicted in high-energy physics but have yet to be observed in particle physics experiments. If they were to be found in a condensed-matter setting, like a TDS, they might be used to build so-called topological qubits – which could be used to make a “fault-tolerant” quantum computer.

The team, led by Michael Fuhrer and which includes researchers from the National University of Singapore, the Australian Synchrotron in Melbourne and Curtin University in Perth, is now trying to reduce the thickness of the films so that the electrons in the material are confined to the nanoscale. Here, the topological properties could be tuned with thickness or with an electric field, as mentioned. “Another important thing to do is to try and make air-stable capping layers for the films so that they can survive in air, which of course would be an important step towards real-world technological applications.”