Invited talk

Magnetism in nanographenes: Neel vs Bethe
Joaquin Fernández Rossier^1,2

1 International Iberian Nanotechnology Laboratory, Braga, Portugal
2 Departamento de Física Aplicada, Universidad de Alicante, Spain

In this talk I will address the fundamental question of whether broken symmetry magnetism takes place in nanographene-based structures. This goes back to the early days of quantum mechanics, when two conflicting views of antiferromagnetism were proposed. The first, derived from the quantum mechanical solutions of Bethe for spin chains, or even the simple singlet wave functions of molecular hydrogen. The second, the broken-symmetry paradigm proposed by Neel, initially rejected on account of its apparent incompatibility with the first. I will discuss how this conundrum is solved, both in general, and specifically in the context of nanographenes. I will focus on how the well-known limitations of conventional density functional theory methods and mean-field approximations can misguide our understanding of this problem. I will discuss how, so far, experiments show that magnetism in 0D [1,2] and 1D nanographene structures [3] fall into the quantum paradigm, and how broken-symmetry magnetism may appear in two-dimensional nanographene lattices [4].

[1] Collective All Carbon Magnetism in Triangulene Dimers, S. Mishra, D. Beyer, K. Eimre, R. Ortiz, J. Fernández-Rossier, R. Berger, O. Gröning, C. A. Pignedoli, R. Fasel, X. Feng, P. Ruffieux, Angewandte Chemie International Edition 59, 12041-12047 (2020)
[2] Giant magnetic exchange coupling in rhombus-shaped nanographenes with zigzag periphery, S. Mishra, X. Yao, Q. Chen, K. Eimre, O. Groening, R. Ortiz, M. Di Giovannantonio, JC Sancho-Garcia, J. Fernandez-Rossier, CA Pignedoli, K. Muellen, P. Ruffieux, A. Narita, R. Fasel, Nature Chemistry 13, 581–586 (2021)
[3] Observation of fractional edge excitations in nanographene spin chains, Shantanu Mishra, Gonçalo Catarina, Fupeng Wu, Ricardo Ortiz, David Jacob, Kristjan Eimre, Ji Ma, Carlo A. Pignedoli, Xinliang Feng, Pascal Ruffieux, Joaquín Fernández-Rossier, Roman Fasel, Nature 598, 287 (2021)
[4] Broken-symmetry magnetic phases in two-dimensional triangulene crystals, G. Catarina, J. C. G. Henriques, A. Molina-Sánchez, A. T. Costa, J. Fernández-Rossier In preparation (arXiv, June 2023)