Nature 483, 306–310 (2012)

Graphene — a single layer of carbon atoms — has a variety of intriguing properties including the fact that its electrons can behave as if they have no mass. Such electrons, which are best described using the relativistic Dirac equation rather than the Schrödinger equation, are known as massless Dirac fermions and are the result of the honeycomb-lattice structure of graphene. However, the arrangement of carbon atoms is rigid and so the properties of the Dirac fermions cannot be controlled. Hari Manoharan and colleagues at Stanford University and the Instituto de Ciencia de Materiales de Madrid have now created a 'molecular graphene' in which the effect of minute variations in lattice geometry can be probed.

The graphene analogue was made by positioning carbon monoxide molecules in a hexagonal pattern on a copper surface using the tip of a scanning tunnelling microscope. The molecules repel the surface electrons and cause them to travel in a honeycomb track. Using structures prepared from around 100 to 1,000 molecules, Manoharan and colleagues first confirmed that the electrons were massless Dirac fermions by measuring their conductance spectrum. They then showed that by adjusting the position of the carbon monoxide molecules, the Dirac fermions could be given a tunable mass or the electrons could be made to act as if they were in an electric or magnetic field.

This is not the only approach available for controlling Dirac fermions: Tilmann Esslinger and colleagues at ETH Zurich have also recently explored the use of potassium atoms trapped in an adjustable honeycomb optical lattice (Nature 483, 302–305; 2012).