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<p>Antiferromagnetic Heisenberg chains exhibit two distinct types of excitation spectrum: gapped for integer-spin chains and gapless for half-integer-spin chains. However, in finite-length half-integer-spin chains, quantization induces a gap, requiring precise control over sufficiently long chains to study its evolution. Here we create length-controlled spin-1/2 Heisenberg chains by covalently linking Olympicenes—Olympic-ring-shaped magnetic nanographenes. With large exchange interactions, tunable lengths and negligible magnetic anisotropy, this system is ideal for investigating length-dependent spin excitations, probed via inelastic electron tunnelling spectroscopy. We observe a power-law decay of the lowest excitation energy with length <i>L</i>, following a 1/<i>L</i> dependence in the large-<i>L</i> regime, consistent with theory. For <i>L</i> = 50, a V-shaped excitation continuum confirms a gapless behaviour in the thermodynamic limit. Additionally, low-bias current maps reveal the standing wave of a single spinon in odd-numbered chains. Our findings provide evidence for the realization of a one-dimensional analogue of a gapless spin liquid within an artificial graphene lattice.</p>

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Spin excitations in nanographene-based antiferromagnetic spin-1/2 Heisenberg chains

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年3月14日

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  • Chenxiao Zhao,
  • Lin Yang,
  • João C. G. Henriques,
  • Mar Ferri-Cortés,
  • Gonçalo Catarina,
  • Carlo A. Pignedoli,
  • Ji Ma,
  • Xinliang Feng,
  • Pascal Ruffieux,
  • Joaquín Fernández-Rossier,
  • Roman Fasel

摘要

Antiferromagnetic Heisenberg chains exhibit two distinct types of excitation spectrum: gapped for integer-spin chains and gapless for half-integer-spin chains. However, in finite-length half-integer-spin chains, quantization induces a gap, requiring precise control over sufficiently long chains to study its evolution. Here we create length-controlled spin-1/2 Heisenberg chains by covalently linking Olympicenes—Olympic-ring-shaped magnetic nanographenes. With large exchange interactions, tunable lengths and negligible magnetic anisotropy, this system is ideal for investigating length-dependent spin excitations, probed via inelastic electron tunnelling spectroscopy. We observe a power-law decay of the lowest excitation energy with length L, following a 1/L dependence in the large-L regime, consistent with theory. For L = 50, a V-shaped excitation continuum confirms a gapless behaviour in the thermodynamic limit. Additionally, low-bias current maps reveal the standing wave of a single spinon in odd-numbered chains. Our findings provide evidence for the realization of a one-dimensional analogue of a gapless spin liquid within an artificial graphene lattice.