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Multichannel ballistic transport observed in graphene nanoribbons

By Kendra Redmond January 31, 2019

By mapping current across graphene nanoribbons (GNR) grown on the sidewalls of silicon carbide mesa structures, researchers have revealed a multichannel electron transport system. The channels are spatially separated and current travels through them with negligible electrical resistance. As reported in a recent issue of Nature Communications, the unique electronic properties of the system stem from the morphology of the GNR edges and have implications for quantum information devices.

Multichannel ballistic transport - see caption
Sketch of graphene nanoribbons grown on silicon carbide sidewalls. Credit: Nature Communications and Aprojanz et al.

A research group led by Christoph Tegenkamp at Technische Universität Chemnitz in Germany previously showed that GNRs grown on the sidewalls of mesa structures of silicon carbide demonstrate room-temperature ballistic transport, electron transport with essentially zero resistance, at unusually long length scales. In this new research, the group collaborated with researchers from other European institutions to characterize the current flow in detail and study the transport mechanism.

To map current flow in the nanoribbon, the researchers used an in situ two-point probe measurement system that combines scanning tunneling microscopy (STM) and scanning electron microscopy techniques. They placed an STM probe with a blunt tip (radius > 50 nm) on the GNR so that the tip covered the width of the ribbon, and then scanned across the ribbon at a fixed distance with a second STM probe. The second probe had a narrower tip (40 nm), and an initial contact area with the ribbon of 15 nm. This technique enabled the researchers to spatially map the current across the width of the ribbon. Using probe-to-probe spacing as small as 70 nm, the researchers were able to explore more channels than in previous experiments.

Their results showed a single ballistic channel running along the lower edge of the sidewall-grown ribbon, as expected based on previous characterizations done with larger probe separations. However, as the second probe moved inward, contrary to the steady increase in conductance they anticipated, the researchers saw quantized increases.

“The step-like feature is completely unexpected in normal graphene nanoribbons, where the individual channels tend to overlap,” says Stephen Power from Trinity College Dublin, who together with colleagues from ICN2 (Spain) and the Technical University of Denmark (DTU) developed the theoretical model for this study. “The step-like behavior instead suggests that these channels are localized, so that a sudden jump appears each time new channels are contacted,” he says.

The suggestion was confirmed by atomic force microscopy images, taken by colleagues from the University of Twente (the Netherlands) and Lund University (Sweden), that revealed multiple ballistic transport channels parallel to but spatially isolated from the channel at the lower edge of the ribbon. “[T]his means that electrons travelling in different regions across the ribbon width do not influence each other, so that a nanoribbon wire could in principle be used to carry multiple independent signals,” Power says.

By modeling the system in quantum transport simulations, the research team showed that the multichannel structure results from the combined influence of asymmetry between the morphology of the upper and lower edges of the ribbon and the zigzag edge magnetization near the lowest channel.

“The key realization was that this [quantized] behavior could be introduced by an asymmetry between the top and bottom edges of the ribbon,” Power says. “This is exactly what occurs for sidewall nanoribbons: the top and bottom edges connect differently to the silicon carbide substrate, and once this is taken into account, the theory and experiment are in excellent agreement.”

Moving forward, the team plans to characterize the robustness and length scales of the newly identified channels. They also hope to exploit the multiple independent channels simultaneously, which could present new opportunities for information transfer and logic devices.

Peter Bøggild, an expert on two-dimensional materials at DTU who was not associated with this work, was impressed by the team’s ability to map the position of individual current channels in the nanoribbon and determine the role of edge morphology in their distribution. “The powerful two-probe measurement methodology demonstrated in the paper could be the key to unlock the secrets of the many other quantum transport systems with localized current channels: edge states, topological currents, and hot spots,” Bøggild says. “I am confident that we have only seen the beginning.”

Read the article in Nature Communications