Abstract
Quantum point contacts are cornerstones of mesoscopic physics and central building blocks for quantum electronics. Although the Fermi wavelength in high-quality bulk graphene can be tuned up to hundreds of nanometres, the observation of quantum confinement of Dirac electrons in nanostructured graphene has proven surprisingly challenging. Here we show ballistic transport and quantized conductance of size-confined Dirac fermions in lithographically defined graphene constrictions. At high carrier densities, the observed conductance agrees excellently with the Landauer theory of ballistic transport without any adjustable parameter. Experimental data and simulations for the evolution of the conductance with magnetic field unambiguously confirm the identification of size quantization in the constriction. Close to the charge neutrality point, bias voltage spectroscopy reveals a renormalized Fermi velocity of ∼1.5 × 106 m s−1 in our constrictions. Moreover, at low carrier density transport measurements allow probing the density of localized states at edges, thus offering a unique handle on edge physics in graphene devices.
Highlights
Quantum point contacts are cornerstones of mesoscopic physics and central building blocks for quantum electronics
In this work we report on the observation of size quantization and localized trap states in ballistic transport through graphene constrictions approximating quantum point contacts
We prepared four-probe devices based on high-mobility graphene–hexagonal boron nitride sandwiches on SiO2/Si substrates and use reactive ion etching to pattern narrow constrictions with widths ranging from WE230 to 850 nm, connecting wide leads (Fig. 1a–c)
Summary
Quantum point contacts are cornerstones of mesoscopic physics and central building blocks for quantum electronics. With further progress in fabrication technology, graphene nanoribbons and constrictions are expected to evolve from a disorder-dominated[12,13,14,15] transport behaviour to a quasi-ballistic regime where boundary effects, crystal alignment and edge defects[16,17] govern the transport characteristics. In this work we report on the observation of size quantization and localized trap states in ballistic transport through graphene constrictions approximating quantum point contacts. Close to the Dirac point, deviations from ballistic behaviour allow for probing the density of localized trap states
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