A new type of switch sends electrons propagating in opposite directions along the same paths – without ever colliding with each other. The switch works by controlling the presence of so-called topological kink states in a material known as Bernal bilayer graphene, and its developers at Penn State University in the US say that it could lead to better ways of transmitting quantum information.
Bernal bilayer graphene consists of two atomically-thin sheets of carbon stacked on top of each other and shifted slightly. This arrangement gives rise to several unusual electronic behaviours. One such behaviour, known as the quantum valley Hall effect, gets its name from the dips or “valleys” that appear in graphs of an electron’s energy relative to its momentum. Because graphene’s conduction and valence bands meet at discrete points (known as Dirac points), it has two such valleys. In the quantum valley Hall effect, the electrons in these different valleys flow in opposite directions. Hence, by manipulating the population of the valleys, researchers can alter the flow of electrons through the material.
This process of controlling the flow of electrons via their valley degree of freedom is termed “valleytronics” by analogy with spintronics, which uses the internal degree of freedom of electron spin to store and manipulate bits of information. For valleytronics to be effective, however, the materials the electrons flow through need to be of very high quality. This is because any atomic defects can produce intervalley backscattering, which causes electrons travelling in opposite directions to collide with each other.
A graphite/hBN global gate
Researchers led by Penn State physicist Jun Zhu have now succeeded in producing a device that is pristine enough to support such behaviour. They did this by incorporating a stack made from graphite and a two-dimensional material called hexagonal boron nitride (hBN) into their design. This stack, which acts as a global “gate” that allows electrons to flow through the device, is free of impurities, and team member Ke Huang explains that it was key to the team’s technical advance.
The principle behind the improvement is that while graphite is an excellent electrical conductor, hBN is an insulator. By combining the two materials, Zhu, Huang and colleagues created a structure known as a topological insulator – a material that conducts electricity very well along its edges or surfaces while acting as an insulator in its bulk. Within the edge states of such a topological insulator, electrons can only travel along one pathway. This means that, unlike in a normal conductor, they do not experience backscatter. This remarkable behaviour allows topological insulators to carry electrical current with near-zero dissipation.
In the present work, which is described in Science, the researchers confined electrons to special, topologically protected electrically conducting pathways known as kink states that formed by electrically gating the stack. By controlling the presence or absence of these states, they showed that they could regulate the flow of electrons in the system.
A quantized resistance value
“The amazing thing about our devices is that we can make electrons moving in opposite directions not collide with one another even though they share the same pathways,” Huang says. “This corresponds to the observation of a quantized resistance value, which is key to the potential application of the kink states as quantum wires to transmit quantum information.”
Importantly, this quantization of the kink states persists even when the researchers increased the temperature of the system from near absolute zero to 50 K. Zhu describes this as surprising because quantum states are fragile, and often only exist at temperatures of a few Kelvin. Operation at elevated temperatures will, of course, be important for real-world applications, she adds.
The new switch is the latest addition to a group of kink state-based quantum electronic devices the team has already built. These include valves, waveguides and beamsplitters. While the researchers admit that they have a long way to go before they can assemble these components into a fully functioning quantum interconnect system, they say their current set-up is potentially scalable and can already be programmed to direct current flow. They are now planning to study how electrons behave like coherent waves when travelling along the kink state pathways. “Maintaining quantum coherence is a key requirement for any quantum interconnect,” Zhu tells Physics World.
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