Artificial atoms in graphene and their collapse

At a seminar held for staff, graduate students and students of the HSE, as well as for everyone, Professor Peeters told about a very interesting phenomenon – the collapse of atoms

This phenomenon was predicted on the basis of the theory of electrons proposed by Paul Dirac back in 1928. According to the equation named after him, very heavy atoms in nature become unstable – electrons fall on the atomic nucleus. This happens because due to the very strong Coulomb attraction of the nucleus, the electrons plunge into the positron continuum, causing spontaneous generation of electron-positron pairs and loss of energy. In the real world, this effect has never been observed due to the fact that it requires too much electric charge of the core. However, this phenomenon can be observed in graphene (a two-dimensional lattice of carbon atoms) for artificially created atoms, and even if the charge of the atoms is small enough. That is why graphene is a unique object where you can test the Dirac theory, created to describe elementary particles with very high energies and velocities.In his report, Professor Peeters discussed the possibility of observing the collapse of artificial atoms obtained by depositing impurities on the surface of graphene or creating defects in its crystal lattice. Such artificial atoms can capture electrons and hold them around themselves. Experimentally, this capture can be observed as spectral lines in the local density of states near the atom. As a result of such a collapse, two-dimensional atomic graphene crystals may lose stability, leading to the formation of so-called superlattices. The potential energy of such superlattices can be described as an analogue of a magnetic field with periodic modulation. The formation of superlattices can lead to the appearance of flat electronic zones. If the Fermi level falls into such a flat zone, for example, by doping, then strongly correlated states with a pseudo-gap in the electronic spectrum are formed in the system. 
Dr. Peeters presentation was based on a series of his works published previously in prestigious journals of Nature Group :
Jinhai Mao, Yuhang Jiang, Dean Moldovan, Gohong Li, Kenji Watanabe, Takashi Taniguchi, MasudRamezani Masir, Francois M. Peters and Eva Y. Andrey: Implementation of a tunable artificial atom on a charged vacancy in graphene, Nature Physics 12, 545-550 (2016).
Yuhang Jiang, Jinghai Mao, Dean Moldovan, Masoud Ramezani Masir, Gohong Li, Kenji Watanabe, Takashi Taniguchi, Francois M. Peters and Eva Y. Andrey: Setting up a circular p-n transition in graphene from quantum confinement to optical guidance, Nature Nanotechnology 12, 1045-1050 (2017).
 Mao Jinhai, Milovanovic Slavisa, Andelkovic Misa, Lai Xinyuan, Cao Yang, Watanabe Kenji, Taniguchi Takashi, Kovachi Lucian, Peters Francois, Game Andre, Jiang Yuhang, Andrey Eva: Proofs of flat bands and correlated states in curved graphene superlattices, Nature 584, 7820 (2020).