About the topic
Zigzag-terminated graphene nanostructures possess electronic states localized at the edge, which lead to non-trivial magnetic properties. In particular, zigzag graphene nanoribbons have been predicted to possess a ferromagnetic spin polarization along the edges and an antiferromagnetic coupling across the nanoribbon. These properties have been investigated intensively, due to their potential applications in the field of spintronics. However, in principle, there exist various effects which can undermine the stability of edge magnetism, including quantum and thermal fluctuations, edge reconstruction and passivation, and, for supported nanostructures, the interaction with the substrate. On the other hand, heavy-element substrates with strong-spin orbit coupling can also have beneficial effects, in that they induce a magnetic anisotropy that stabilizes edge magnetism against said fluctuations.
In this talk, I will present our density-functional-theory studies of zigzag graphene nanoribbons deposited on selected metallic and topological insulator substrates.
In the first part, I will consider the (111) surfaces of Ir, Au, Ag and Cu, which are commonly used to grow graphene nanostructures by chemical vapour deposition methods and/or bottom-up approaches. Both H-free and H-passivated nanostructures will be discussed. In the case of Ir(111), we do not find states localized at the nanoribbon edges. We explain this result by the presence of surface states of d character near the Fermi energy, which strongly interact with the graphene orbitals. Our simulations are in agreement with scanning tunnelling spectroscopy experiments performed on graphene islands on Ir(111). In the case of Au, Ag and Au substrates, the nanoribbons possess edge states. In spite of this, they do not exhibit significant magnetization at the edge, with the exception of H-terminated nanoribbons on Au(111), whose magnetic properties are comparable to those of free-standing nanoribbons. These findings are explained in terms of the different chemical interaction and charge transfer between the nanoribbons and the three substrates.
In the second part of the talk, I will focus on the topological insulator substrate Sb2Te3. It is known that the interplay between strong spin-orbit coupling and broken inversion symmetry can lead to chiral configurations in low-dimensional magnetic systems deposited on heavy-element surfaces. This phenomenon is due to the Dzyaloshinskii-Moriya interaction, which favours non-collinear magnetic structures. Here unpassivated nanoribbon edges are considered, which bind strongly to the substrate. It is shown that edge magnetism is preserved, in spite of the strong chemical interaction with the substrate. Furthermore, the Dzyaloshinskii-Moriya interaction is shown to lead to a twisting of the two antiferromagnetically-coupled edge states of the graphene nanoribbon. We also study the effects of this chiral magnetic configuration on the surface states of the topological insulator. It turns out that the resulting net magnetization of the nanoribbons shifts the Dirac point of the surface state and induces a small gap.
About the speaker
Prof. Riccardo Mazzarello received a PhD degree in condensed matter physics from the University of Hamburg (Germany) in 2004. He was a postdoctoral researcher at SISSA, Trieste (Italy) from 2004 to 2008 and at ETH Zurich (Switzerland) from 2008 to 2009. He was also visiting scientist at ICTP, Trieste (Italy) from 2004 to 2006. He became a junior professor in Theoretical Nanoelectronics at RWTH Aachen University (Germany) in 2009. There, he was promoted to W2 professor in Computational Solid State Theory in 2016. Since 2015, he is also adjunct professor at Xi’an Jiaotong University (China). His research interests include phase-change materials, surface physics, two-dimensional materials and topological phases of matter.