The behavior of electrons and other particles depends on their environment. In particular, the interactions inside materials can alter the collective properties of the material's electrons, producing what are effectively new "particles"—known as quasiparticles—with correspondingly new behaviors. The surfaces of solids are fertile ground for quasiparticles, since they are two-dimensional; as we've seen in a number of other experiments, the loss of the third dimension can lead to exciting new physics.

A new experiment involving a graphene-like material has shown that it's possible to perform some spectacular manipulations of the properties of these quasiparticles. The work is described in a Nature letter by Kenjiro Gomes, Warren Mar, Wonhee Ko, Francisco Guinea, and Hari C. Manoharan. The team arranged carbon monoxide molecules to form the same hexagonal pattern found in graphene, except that they could change the spacing slightly.

This produced an environment where the material's electrons behave remarkably like relativistic particles, with a "speed of light" that they can adjust. Additionally, the researchers could change the spacing between molecules in a way that the masses of the quasiparticles changed, or cause them to behave as though they are interacting with electric and magnetic fields—without actually applying those fields to the material. This setup will potentially help us explore new physics that may arise in these environments.



Ordinary graphene is a hexagonal lattice of carbon atoms; stacks of multiple layers of graphene sheets are known as graphite, and cylindrical rolls of graphene are carbon nanotubes. Graphene has exhibited a number of exotic physical phenomena, including Ordinary graphene is a hexagonal lattice of carbon atoms; stacks of multiple layers of graphene sheets are known as graphite, and cylindrical rolls of graphene are carbon nanotubes. Graphene has exhibited a number of exotic physical phenomena, including superconductivity , due to its unusual electronic properties.

The molecular graphene used in this experiment was constructed by placing individual carbon monoxide (CO) molecules in a hexagonal pattern on an ultraclean copper surface at a temperature of 4.2 Kelvin. This process simulates many of the properties of ordinary graphene (see sidebar for more) while providing a more tunable set of electronic properties, since the spacing between the molecules is adjustable. By moving molecules individually, the researchers were able to produce a special environment for their electrons, allowing greater control over quasiparticle properties.

The particular quasiparticle excitation they achieved is known as a Dirac fermion, which is analogous to free relativistic particles first described by Paul Dirac in 1928. A major difference between relativistic and non-relativistic systems is in how their velocity relates to their momentum: relativistic systems have a speed limit, so velocity cannot grow indefinitely. In materials, the speed limit is not the speed of light (as it is for free relativistic particles); instead, it's just a property of the electronic structure of the system at hand. Dirac fermion quasiparticles travel at this effective "speed of light" if they are massless excitations, but are unable to reach it if they have mass.

By tweaking the properties of the carbon monoxide lattice, it's possible to set both the speed of light and the effective mass of the quasiparticles, which is typically less than the free electron mass. The researchers achieved a transition between massless and massive quasiparticle states by distorting the lattice, a process that is analogous to how the Higgs field acts in particle physics.

Additional manipulation of the lattice made the quasiparticles behave like they were under the influence of a strong magnetic field, even though no such field was applied to the material. Specifically, the quantum state of the quasiparticles is similar to that observed in the quantum Hall effect, where the Dirac fermions can exhibit even stranger behavior, such as a fractional electric charge. Ordinarily this phenomenon requires huge magnetic fields, but the exotic electronic configuration in this graphene-like structure produced it without any applied magnetic field at all.

The implications for future experiments are profound. By manipulating graphene-like structures on a surface, quasiparticle properties can be tuned in new ways, exhibiting behaviors typically seen only under very different environmental conditions.

Nature, 2012. DOI: 10.1038/nature10941 (About DOIs).