Quantum graphene

The fractional quantum Hall effect has been observed in bilayer graphene and shown to be tunable with an electric field, which might allow this material to be used in components of a quantum computer.

Researchers at Columbia University, New York, have spent the last few years studying the fractional quantum Hall effect whereby electrons confined to a thin layer of material and exposed to a large magnetic field display collective behavior. In 2009, they observed the effect in a single graphene layer and then showed in 2011 that they could measure this effect over large ranges of electron density. However, bilayer graphene has much greater potential where two metal gate electrodes (above and below) should allow independent tuning of the charge density in each layer. This opens up the possibility of manipulating the fractional states in new ways, perhaps even leading to exotic 'non-abelian' states that could be used for quantum computation.

"We knew that we could fabricate very clean bilayer graphene structures, but we suffered from our inability to make good electrical contact since bilayer graphene develops an electronic 'band-gap' under the high magnetic fields and low temperatures required for our experiments," explains team member Cory Dean. The breakthrough came when the team came up with a new design that allowed them to tune the charge density of the contact regions independently from the rest of the device. "Once we had this new device structure the results were spectacular," he adds. [Dean et al., Science (2014)]

In bilayer graphene the question of spin states among the collections of electrons in each layer is rather complicated by the numerous degrees of symmetry at play. Moreover, polarization effects can arise spontaneously in one layer relative to the other. This complexity could be exploited in devices but makes the results all the more impressive and, the team says, provides an interesting new phase space to explore for new and unusual effects.

The team has now shown for the first time that tweaking an applied electric field triggers a phase transition although the exact characteristics of the different phases involves is not yet known. Their findings support the theoretical expectation that the ground state order is tunable. The next stage in their research will attempt to pin down the exact nature of this ordering in the bilayer. "The implications for this result could be far reaching," Dean adds, "While we do not yet see any evidence of non-abelian states, the fact that we are able to modify the nature of the fractional quantum Hall effect by electric fields is a really exciting first step."

"We are now working on applying these techniques to pursue the existence of non-abelian quasi-particles," Dean told Materials Today. "Pushing the device technology to yet cleaner limits, and working with the National High Magnetic Field laboratory to explore these materials at even higher magnetic fields will be the crucial next steps."

David Bradley blogs at sciencebase.com and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".