Topological frustration induces unconventional magnetism in a nanographene

The chemical versatility of carbon imparts manifold properties to organic compounds, where magnetism remains one of the most desirable but elusive. Polycyclic aromatic hydrocarbons, also referred to as nanographenes, show a critical dependence of electronic structure on the topologies of the edges and the π-electron network, which makes them model systems with which to engineer unconventional properties including magnetism. In 1972, Erich Clar envisioned a bow-tie-shaped nanographene, C38H18, where topological frustration in the π-electron network renders it impossible to assign a classical Kekulé structure without leaving unpaired electrons, driving the system into a magnetically non-trivial ground state. In this record we include data needed to support our recent work where we demonstrate the experimental realization and in-depth characterization of this emblematic nanographene, known as Clar’s goblet. Scanning tunnelling microscopy and spin excitation spectroscopy of individual molecules on a gold surface reveal a robust antiferromagnetic order with an exchange-coupling strength of 23 meV, exceeding the Landauer limit of minimum energy dissipation at room temperature. Through atomic manipulation, we realize switching of magnetic ground states in molecules with quenched spins. Our results provide direct evidence of carbon magnetism in a hitherto unrealized class of nanographenes, and prove a long-predicted paradigm where topological frustration entails unconventional magnetism, with implications for room-temperature carbon-based spintronics.

碳的化学多样性赋予有机化合物极为丰富的性质，其中磁性始终是最令人向往却又难以捉摸的目标之一。多环芳烃（polycyclic aromatic hydrocarbons），又称纳米石墨烯（nanographenes），其电子结构高度依赖边缘拓扑与π电子网络的构型，因此成为调控磁性等非常规性质的理想模型体系。1972年，Erich Clar构想了一种领结形纳米石墨烯C₃₈H₁₈，其π电子网络中的拓扑阻挫（topological frustration）使得无法在不遗留未成对电子的前提下为其指派经典凯库勒结构，从而驱使该体系进入具有非平凡磁性的基态。本数据集收录了支撑我们近期研究所需的全部数据，该研究实现了这一被称为克拉杯（Clar’s goblet）的标志性纳米石墨烯的实验制备与深度表征。通过在金表面上单分子层面开展扫描隧道显微镜（scanning tunnelling microscopy）与自旋激发光谱表征，我们观测到体系存在稳定的反铁磁有序，交换耦合强度达23 meV，其强度超过室温下兰道尔极限（Landauer limit）所设定的最小能量耗散阈值。借助原子操控手段，我们实现了自旋被淬灭的分子的磁性基态切换。本研究为此前尚未实现的纳米石墨烯类别中的碳磁性提供了直接实验证据，验证了长期以来预言的"拓扑阻挫催生非常规磁性"范式，该发现对室温碳基自旋电子学具有重要参考价值。