Tunable topological phases in nanographene-based spin-1/2 alternating-exchange Heisenberg chains
arxiv(2024)
摘要
Unlocking the potential of topological order within many-body spin systems
has long been a central pursuit in the realm of quantum materials. Despite
extensive efforts, the quest for a versatile platform enabling site-selective
spin manipulation, essential for tuning and probing diverse topological phases,
has persisted. Here, we utilize on-surface synthesis to construct spin-1/2
alternating-exchange Heisenberg (AH) chains[1] with antiferromagnetic couplings
J_1 and J_2 by covalently linking Clar's goblets – nanographenes each
hosting two antiferromagnetically-coupled unpaired electrons[2]. Utilizing
scanning tunneling microscopy, we exert atomic-scale control over the spin
chain lengths, parities and exchange-coupling terminations, and probe their
magnetic response by means of inelastic tunneling spectroscopy. Our
investigation confirms the gapped nature of bulk excitations in the chains,
known as triplons[3]. Besides, the triplon dispersion relation is successfully
extracted from the spatial variation of tunneling spectral amplitudes.
Furthermore, depending on the parity and termination of chains, we observe
varying numbers of in-gap S=1/2 edge spins, enabling the determination of the
degeneracy of distinct topological ground states in the thermodynamic
limit-either 1, 2, or 4. By monitoring interactions between these edge spins,
we identify the exponential decay of spin correlations. Our experimental
findings, corroborated by theoretical calculations, present a phase-controlled
many-body platform, opening promising avenues toward the development of
spin-based quantum devices.
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