Ultrathin Materials Combo in QD Enables Tunable Quantum Systems

A new type of quantum dot (QD) has been developed that allows for more accurate and more widely tunable energy levels of confined electrons. The advance in QD technology combines graphene (a conductive single atomic layer of carbon atoms) and hexagonal boron nitride (h-BN), a single layer of material similar to graphene except that it is insulating. These QDs could allow control of individual electrons by fine-tuning electrons’ energy levels directly.

New kinds of quantum bits: extremely small nanostructures that allow delicate control of individual electrons by fine-tuning their energy levels. Courtesy of TU Wien.

“Efficiently manipulating the information stored in the quantum state of the electrons requires perfect control of the system parameters. An ideal system allows for continuous tuning of the energy difference from zero to a large value,” said professor Florian Libisch of Vienna University of Technology (TU Wien).

The graphene QD, induced by the tip of a scanning tunneling microscope, was used to demonstrate valley splitting that was tunable from −5 to +10 meV by <10-nm displacements of the QD position, boosting the range of controlled valley splitting by about one order of magnitude.

Like graphene, h-BN forms a honeycomb lattice, but the honeycombs in graphene and h-BN are not of equal size.

“If you carefully put a single layer of graphene on top of hexagonal boron nitride, the layers cannot perfectly match. This slight mismatch creates a superstructure over distances of several nanometers, which results in an extremely regular wave-like spatial oscillation of the graphene layer out of the perfect plane,” said Libisch.

Researchers from TU Wien, RWTH Aachen University and the University of Manchester performed simulations to show that the oscillations in the graphene on h-BN could form a platform for controlling electron energies. Depending on the exact position of the tip of the scanning tunneling microscope, the energy levels of the electronic states inside the QD were found to change.

Florian Libisch, Vienna University of Technology (TU Wien). Courtesy of TU Wien.

“A shift by a few nanometers allows for changing the energy difference of two neighboring energy levels from −5 to +10 meV with high accuracy — a tuning range about 50× larger than previously possible,” said Libisch.

As a next step, researchers say the tip of the scanning tunneling microscope could be replaced by a series of nanoelectronic gates. According to the team, this would allow the QD state of graphene on h-BN to be exploited for scalable quantum technologies, such as valleytronics.

The research was published in Nature Nanotechnology (doi:10.1038/s41565-018-0080-8).