Graphene has attracted an ongoing interest in the scientific community since its outstanding properties were unveiled: from bulletproofing to nanotechnology, studying how graphene behaves in certain situations is opening the way to new applications of this material. Similarly, quantum contacts, discovered at the end of the 20th century, are important systems than can work as extremely sensitive charge detectors, which make them useful in certain quantum computing architectures.
An international team of researchers, including an IFISC researcher (UIB-CSIC), has published a paper in Physical Review Letters exploring the behavior of these quantum contacts built with graphene. To do this, they grew three samples using two layers of graphene attached to metallic terminals. They tested how the device design affects its properties by selecting different separations between the metallic contacts for each of the quantum contacts.
By applying a voltage to the bilayer graphene system, a gap is generated in the energy spectrum. This effect is characteristic of semiconductor materials and is key to the working principle of field effect transistors, which form the basis of modern integrated circuit technology.
The team then measured the variation of the conductance of the graphene quantum point contact as a function of the voltage applied to the metallic terminals and observed that this variation was not completely linear: it presented certain flat areas (plateaus) where an increase or decrease of the voltage difference did not affect the conductance. The appearance of these plateaus is a spectacular manifestation of quantum physics governing this graphene device.
The team then applied a magnetic field and observed that for large values of the field the conductance was less sensitive to variations of the terminal voltage. In addition, for sufficiently large magnetic fields the electrons experience a magnetic confinement rather than an electric one. This is the so called quantum Hall regime. Measuring the conductance as a function of both the magnetic field and the applied voltage, the researchers found out a pattern that could be reproduced, with slight variations, for all the investigated devices. Numerical simulations revealed the fundamental role played by the lattice structure of bilayer graphene in the generation of the observed pattern.
This paper represents a step forward in the experimental
characterization of graphene nanostructures, which might have a considerable
impact on future electronic components for quantum computers.
Overweg, H., Knothe, A., Fabian, T., Linhart, L., Rickhaus, P., Wernli, L., Watanabe, K., Taniguchi, T., Sánchez, D., Burgdörfer, J., Libisch, F., Fal'ko, V. I., Ensslin, K., i Ihn, T. Physical Review Letters 121, 257702 (1-6) (2018). doi: 10.1103/PhysRevLett.121.257702.