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 () and Roman Fasel
Additional contact information
Shantanu Mishra: Empa—Swiss Federal Laboratories for Materials Science and Technology
Gonçalo Catarina: International Iberian Nanotechnology Laboratory
Fupeng Wu: Technical University of Dresden
Ricardo Ortiz: University of Alicante
David Jacob: University of the Basque Country
Kristjan Eimre: Empa—Swiss Federal Laboratories for Materials Science and Technology
Ji Ma: Technical University of Dresden
Carlo A. Pignedoli: Empa—Swiss Federal Laboratories for Materials Science and Technology
Xinliang Feng: Technical University of Dresden
Pascal Ruffieux: Empa—Swiss Federal Laboratories for Materials Science and Technology
Joaquín Fernández-Rossier: International Iberian Nanotechnology Laboratory
Roman Fasel: Empa—Swiss Federal Laboratories for Materials Science and Technology

Nature, 2021, vol. 598, issue 7880, 287-292

Abstract: Abstract Fractionalization is a phenomenon in which strong interactions in a quantum system drive the emergence of excitations with quantum numbers that are absent in the building blocks. Outstanding examples are excitations with charge e/3 in the fractional quantum Hall effect1,2, solitons in one-dimensional conducting polymers3,4 and Majorana states in topological superconductors5. Fractionalization is also predicted to manifest itself in low-dimensional quantum magnets, such as one-dimensional antiferromagnetic S = 1 chains. The fundamental features of this system are gapped excitations in the bulk6 and, remarkably, S = 1/2 edge states at the chain termini7–9, leading to a four-fold degenerate ground state that reflects the underlying symmetry-protected topological order10,11. Here, we use on-surface synthesis12 to fabricate one-dimensional spin chains that contain the S = 1 polycyclic aromatic hydrocarbon triangulene as the building block. Using scanning tunnelling microscopy and spectroscopy at 4.5 K, we probe length-dependent magnetic excitations at the atomic scale in both open-ended and cyclic spin chains, and directly observe gapped spin excitations and fractional edge states therein. Exact diagonalization calculations provide conclusive evidence that the spin chains are described by the S = 1 bilinear-biquadratic Hamiltonian in the Haldane symmetry-protected topological phase. Our results open a bottom-up approach to study strongly correlated phases in purely organic materials, with the potential for the realization of measurement-based quantum computation13.

Date: 2021
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DOI: 10.1038/s41586-021-03842-3

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