Geometric frustration forces graphene to be magnetic

In 1972 organic chemist Erich Clar predicted that a bowtie-shaped polycyclic aromatic hydrocarbon—a flake of graphene, essentially—would be magnetic. At first glance, graphene is an unlikely material in which to find magnetism because its electrons tend to pair up into strong covalent bonds. Each C atom has three neighbors, with which it forms alternating single or double bonds. But in the molecule Clar envisioned, C₃₈H₁₈, shown here and known as Clar’s goblet, the graphene takes on a shape that makes it impossible for every electron to pair up. Indeed, two electrons (represented as dots)—one on each end—remain unpaired, a phenomenon caused by geometric frustration (see the article by Roderich Moessner and Art Ramirez, Physics Today, February 2006, page 24 ).

Doctoral students Shantanu Mishra of the Swiss Federal Laboratories for Materials Science and Technology (Empa) and Doreen Beyer of the Technical University of Dresden, their advisers Roman Fasel and Xinliang Feng, respectively, and colleagues have now synthesized the elusive molecule. To achieve the complex synthesis, the researchers first prepared organic precursor molecules and sublimated them onto a gold surface that acted as both a substrate and a catalyst. They then heated the gold, which transformed the precursors into Clar’s goblet. The presence of the lone electrons makes the molecule so reactive that the synthesis had to be performed under ultrahigh vacuum.

Using a low-temperature scanning tunneling microscope (STM), the team confirmed the molecule’s structure and its magnetic properties. Although the unpaired electrons are spatially separated, their magnetic moments couple antiferromagnetically in the ground state. The team also used the STM to excite the molecule into its ferromagnetic state—just 23 meV higher in energy. That energy difference is just below the thermal energy at room temperature, and the hope is to use similarly geometrically frustrated molecules as spin logic gates, because their states can be controllably switched in ambient conditions. (S. Mishra et al., Nat. Nanotechnol., 2019, doi:10.1038/s41565-019-0577-9 .)