Graphene is a form of carbon with enough interesting electronic properties to merit the 2010 Nobel Prize in Physics, which was awarded to Andre Geim and Konstantin Novoselov for their work in isolating and characterizing graphene. Although its very simplicity—it's a sheet of carbon a single atom thick—can limit its applications, Rahul Nandkishore, L. S. Levitov, and A. V. Chubukov propose a way to create an elusive type of superconductor out of graphene.

Chiral superconductivity may have been seen in strontium ruthenate (Sr 2 RuO 4 ), but the phenomenon hasn't really been thoroughly studied experimentally. It differs from both standard superconductivity and high-critical-temperature superconductivity by being one-way: the resistance-free current flows through the material in one direction, but not the opposite way. This effect breaks time-reversal symmetry, and could be useful in constructing quantum computers, among other applications.

A new modeling paper suggests that graphene's electronic properties may allow it to exhibit chiral superconductivity with the addition of impurities, a process familiar from semiconductor technology, where it's known as doping.

Graphene is a two-dimensional lattice of carbon atoms arranged in a hexagonal, honeycomb pattern. Graphite, commonly used in pencils and mechanical lubricants, is composed of stacked sheets of graphene; carbon nanotubes are cylinders of graphene.

Under normal conditions, graphene is a strong semiconductor—it carries an electrical current with very little resistance despite not being a metal. (Metals share electrons between atoms, so current flows easily under a voltage bias; semiconductors require some energy input to boost electrons into a conductive state.) The conductive properties are direction-specific: some routes across the hexagons have lower resistance than other directions.

By doping graphene with impurities (as we do with other semiconductors), more electrons can become available for conduction, making a stronger current possible. Nandkishore, Levitov, and Chubukov have made a model of doped graphne that shows that doping can lead to superconductivity.

This superconductivity only occurs at low temperatures, as with other materials, but its mechanism is completely different: high conductivity is enabled by the mutual electrical repulsion between electrons. Typically, low temperatures enable lattice vibrations (sound waves) to make electrons appear to be positively charged to each other, so they attract and form what are known as Cooper pairs. Within doped graphene, the particular directional nature of the electronic structure may allow certain currents to flow freely even without these lattice effects.

In the researchers' model, this superconducting ability depends on what's called chirality. Many organic molecules exist in two forms, known as left- or right-handed in analogy with human hands: they contain the same number of atoms in nearly the same configuration, but are mirror-images of each other.

According to the researchers' model, the interactions between electrons in doped graphene produce something known as d-wave excitations, which are chiral in character. These are not like ripples in a pond (as with lattice vibrations in normal superconductors), but are lobe-like and have strong directional components: passing one way around a hexagon in the lattice has a different effect than traveling the other direction. Hence, d-wave excitations produce chiral superconductivity: currents would flow one way through the graphene sheet, but not the other.

As with any theory, the key will be to look for chiral superconductivity in the laboratory. Graphene has been successfully doped using calcium and potassium atoms without wrecking the lattice structure, so testing the predictions should be fairly straightfoward. Other effects may be stronger than superconductivity and make it difficult to detect. However, if chiral superconductivity is apparent, the relative simplicity of the graphene material should make it extremely useful in applications such as quantum computers or anything else where a strongly directional current is needed.

Nature Physics, 2012. DOI: 10.1038/nphys2208 (About DOIs).