Invited talk

Nanographene Spin Chains
Roman Fasel

Empa, Swiss Federal Laboratories for Materials Science and Technology, nanotech@surfaces Laboratory, Dübendorf, Switzerland

In recent years, significant advancements in on-surface synthesis have led to the development of various unprotected open-shell nanographenes, which exhibit promising potential for realizing quantum spin models. This presentation focuses on the exploration of nanographene spin chains, specifically highlighting two prototypical cases: Spin-1 Haldane chains constructed from 1D chains of [3]-triangulenes [1], and spin-½ alternating-exchange Heisenberg chains with antiferromagnetic couplings J₁ and J₂ realized from 1D chains of Clar's goblet molecules [2].

Spin-1 Haldane chains exhibit fractional edge excitations that influence the system's behavior and contribute to its topological properties. The spin-½ alternating-exchange Heisenberg (AH) chains, characterized by the ratio of J₁ to J₂ (α = J₁/J₂), display distinct gapped phases separated by a gapless phase at α = 1. The ground state degeneracy is linked to the topological properties of both models.

Here we present a nanographene-based spin chain that realizes the spin-½ alternating-exchange Heisenberg model [2]. By employing on-surface synthesis and scanning tunneling microscopy (STM), we covalently join Clar's goblets [3] – nanographenes hosting two antiferromagnetically-coupled spins. STM enables precise characterization of spin excitations using inelastic electron tunneling spectroscopy. Using hydrogenation and tip-induced dehydrogenation techniques [4], we manipulate spin chain length, parity, and termination. This allows us to explore three distinct phases characterized by ground state degeneracy and the presence or absence of edge excitations. Our experimental results, in agreement with theoretical predictions, showcase the potential of bottom-up synthesis in designing prominent spin models with large exchange interactions, raising hopes for future non-cryogenic quantum spin-based devices.

Furthermore, we report the successful on-surface synthesis and characterization of narrow zigzag graphene nanoribbons (ZGNRs) integrated with laterally fused porphyrin units along the ZGNR backbone [5]. This innovative design introduces significant alterations in the electronic band structure and enables controlled introduction of metal centers. The integration of d- and π-electron spins in ZGNR-based spin chains introduces spin-orbit coupling and magnetic anisotropy [6], expanding the possibilities for carbon nanomaterials.

In summary, this presentation illustrates the potential of on-surface synthesis techniques for constructing and investigating nanographene spin chains, revealing their unique spin properties, and opening new pathways for the development of carbon-based quantum spin systems with versatile functionalities.

[1] S. Mishra et al., Nature 598, 287 (2021).
[2] C. Zhao et al., in preparation.
[3] S. Mishra et al., Nat. Nanotechnol. 15, 22 (2020).
[4] C. Zhao et al., in preparation.
[5] F. Xiang et al., in preparation.
[6] Q. Sun et al., Adv. Sci. 9, 2105906 (2022).