Realization of a tunable artificial atom at a supercritically charged vacancy in graphene

Abstract

Graphene's remarkable electronic properties have fuelled the vision of a graphene-based platform for lighter, faster and smarter electronics and computing applications. One of the challenges is to devise ways to tailor graphene's electronic properties and to control its charge carriers [1][2][3][4][5] . Here we show that a single-atom vacancy in graphene can stably host a local charge and that this charge can be gradually built up by applying voltage pulses with the tip of a scanning tunnelling microscope. The response of the conduction electrons in graphene to the local charge is monitored with scanning tunnelling and Landau level spectroscopy, and compared to numerical simulations. As the charge is increased, its interaction with the conduction electrons undergoes a transition into a supercritical regime [6][7][8][9][10][11] where itinerant electrons are trapped in a sequence of quasi-bound states which resemble an artificial atom. The quasi-bound electron states are detected by a strong enhancement of the density of states within a disc centred on the vacancy site which is surrounded by halo of hole states. We further show that the quasi-bound states at the vacancy site are gate tunable and that the trapping mechanism can be turned on and o , providing a mechanism to control and guide electrons in graphene.Supercriticality in atoms occurs when the Coulomb coupling, β = Zα, exceeds a critical value of order unity, where Z is the atomic number and α ∼ 1/137 is the fine structure constant. In this regime the electronic orbitals, starting with the 1S state, sink into the Dirac continuum until Z is reduced to the critical value. This process, known as atomic collapse (AC), is accompanied by vacuum polarization and the spontaneous generation of positrons 12,13 . But accessing this new physics requires ultra-heavy nuclei which do not exist in nature. In graphene, where the effective fine structure constant, α g = α(c/ν F ) ∼ 2, is much larger, the critical coupling, β c = (Z c /κ)α g = 0.5, can be reached for a relatively modest charge [6][7][8][9][10] (c is the speed of light, ν F the Fermi velocity and κ the effective dielectric constant). The transition to the supercritical regime in graphene is marked by the emergence of a sequence of quasi-bound states which can trap electrons. However, because graphene is a good conductor it is difficult to deposit and maintain a charge on its surface. Attempts to create charge by ionizing adatoms or donors with a scanning tunnelling microscope (STM) tip 14 showed that the charge decays when the field is removed. Alternatively, ions can be deposited directly on graphene, but because the charge transfer is inefficient 15 , attaining the critical Z requires piling up many ions, which, in the absence of a controllable mechanism to overcome the Coulomb repulsion, is very challenging 11 .Here we show that a vacancy in graphene can stably host a positive charge. The charge is deposited by applying voltage pulses with an STM tip and its gradual build-up is monitored with scanning tun...

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