Quantum electrodynamics predicts that heavy atoms (Z > Zc ^~170) will undergo the process of atomic collapse where electrons sink into the positron continuum and a new family of so-called collapsing states emerges. This phenomenon has never been confirmed experimentally. The wonder material graphene has made it possible to investigate[1] similar physics in two dimensions using vacancies with tunable charge where the 'atomic' collapse occurs at a much lower critical charge (Zc ^~1). The transition from sub-critical to the supercritical regime is accompanied by trapping of electrons in quasi-bound states which are the condensed matter analogue of the long sought after phenomenon of atomic collapse in super-heavy nuclei. The quasi-bound electron-states show up as a strong enhancement of the density of states within a disc centered on the vacancy site. We find that these states are surrounded by a circular halo of hole states which are interpreted as the analogue of positron production in atomic collapse. 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 off, providing a new paradigm to confine, control and guide electrons in graphene. Recently[2], we found that a sharp STM tip is able to induce similar atomic collapse states. For large tip potentials a sub-micrometer scale p-n junction is induced that exhibits whisper gallery modes. Thus the STM tip allows us to tune a circular p-n junction in graphene from quantum confinement to optical guiding. We realized[3] a periodic buckling structure of a single graphene layer. Because of the periodic strain the electrons are subject to a periodic pseudomagnetic field that does not break time reversal symmetry. Through a detailed STM spectrum mapping and tight binding calculations, we reveal the possibility of generating a robust flat band. This buckling method should enable us to design flat bands with different superlattice symmetry which is inaccessible by the moiré superlattices method that was recently realized for magic angle twisted bilayer graphene[4].
References:
1. J. Mao et al., Nat. Phys. 12, 545 (2016).
2. Y. Jiang et al., Nat. Nanotechnology 12, 1045 (2017).
3. Y. Jiang et al, (to be published).
4. Y. Cao et al, Nature 556, 43 (2018); ibid. 556, 80, (2018).

François Peeters is a full professor and head of the "Condensed Matter Theory" group at the Department of Physics of University of Antwerp. He obtained his PhD degree from University of Antwerp in 1982. After that he conducted his postdoctoral researches at Bell Lab. He joined University of Antwerp in 1988 as a senior researcher, and promoted to Professor in 2000. He was elected the Fellow of the American Physical Society (2005), Member of the Royal Flemish Academy of Belgium (2013), and Member of the Academia Europaea (2010). He serves as a co-editor of Europhysics Letters, and associate editor of Journal of Applied Physics. His research focuses on the electrical, magnetic and optical properties of micrometer and nanometer sized materials (semiconductors and superconductors).