Published October 11, 2021 | Version v1
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Observation of fractional edge excitations in nanographene spin chains

  • 1. Empa - Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland
  • 2. Present address: IBM Research – Zurich, Rüschlikon, Switzerland
  • 3. International Iberian Nanotechnology Laboratory, Braga, Portugal
  • 4. University of Alicante, Sant Vicent del Raspeig, Spain
  • 5. Technical University of Dresden, Dresden, Germany
  • 6. University of the Basque Country, San Sebastián, Spain
  • 7. IKERBASQUE, Basque Foundation for Science, Bilbao, Spain
  • 8. Max Planck Institute of Microstructure Physics, Halle, Germany
  • 9. University of Bern, Bern, Switzerland

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Description

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 effect, solitons in one-dimensional conducting polymers and Majorana states in topological superconductors. 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 bulk and, remarkably, S = 1/2 edge states at the chain termini, leading to a four-fold degenerate ground state that reflects the underlying symmetry-protected topological order. This record contains data to support the result in a recent publication of ours where we use on-surface synthesis to fabricate one-dimensional spin chains that contain the S = 1 polycyclic aromatic hydrocarbon triangulene as the building block. Using scanning tunneling 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 computation.

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References

Journal reference (Manuscript where the results are discussed)
S. Mishra, G. Catarina, F. Wu, R. Ortiz, D. Jacob, K. Eimre, J. Ma, C. A. Pignedoli, X. Feng, P. Ruffieux, J. Fernández-Rossier, R. Fasel, Nature (2021), doi: 10.1038/s41586-021-03842-3

Preprint (open access version of the manuscript)
S. Mishra, G. Catarina, F. Wu, R. Ortiz, D. Jacob, K. Eimre, J. Ma, C. A. Pignedoli, X. Feng, P. Ruffieux, J. Fernández-Rossier, R. fasel, arXiv:2105.09102 [cond-mat.mes-hall] (2021)