In bulk semiconductors, an optically excited electron-hole pair interacts thanks to Coulomb attraction to form a bound quasi-particle, called a spatially direct exciton. Such an exciton is easy to generate but it recombines in nanoseconds. By confining electrons and holes to separate, but closely-spaced 2D quantum wells (QWs), the charge carriers are still strongly attracted but are prevented from recombining, leading to long-lived excitons. These spatially indirect excitons are predicted to behave as Bose-Einstein condensates (BECs) at temperatures much higher (around 10 K) than atomic gases (1 mK) and offer a different route to realizing superconductivity at high temperatures.

The problem is that producing exciton condensates (ECs) in electron-hole QWs has proved difficult so far because it is no easy task to fabricate high quality materials in which matched electron-hole doped layers strongly interact but are electrically isolated. This challenge can be overcome for identically doped (electron-electron or hole-hole) coupled QWs in an applied strong magnetic field. This is known as the quantum Hall effect regime.

Coulomb drag and current counterflow measurements

A team led by Cory Dean has now observed ECs in graphene double layers in this regime for the first time. The carbon material appears to be ideal for making such condensates thanks to the fact that carrier densities can be widely tuned by field-effect gating. Since the material is just a single atomic layer thick, this allows for interlayer spacing down to a few nanometres without significant electron tunnelling between the layers.

“We confirmed the presence of the EC in the quantum Hall effect regime by a combination of Coulomb drag and current counterflow measurements,” explains team member and lead author of the study Jia Li. “We also found evidence of strong interlayer coupling between the graphene layers thanks to the quantized Hall ‘drag plateau’ and a ‘re-entrance’ feature in Hall resistance measurements on the material. The vanishing Hall resistance we measured indicates the existence of charge-neutral excitons, whereas the zero-valued longitudinal resistance confirms the dissipation-less (friction-free) nature of the condensate state.”

Analogue of a superconductor

Measuring Coulomb drag is good way of studying fundamental physical phenomena like electron-electron interactions and many body effects, such as the formation of excitons. Here, the electrons carrying current in one of the layers “drag” the electrons in the other, so generating a small but measurable voltage. In the past, researchers studied Coulomb drag in traditional 2D electron systems, such as gallium arsenide quantum wells but graphene and graphene-based heterostructures allow the effect to be studied at a qualitatively new level.

“The EC we observed can be viewed as an analogue of a superconductor, with charge neutral excitons taking the place of Cooper pairs,” Li tells “Like electrons moving around without resistance in superconductors, so exciton transport in such a condensate state is also dissipation-less. Unlike superconductors, however, exciton condensates survive in strong magnetic fields and can potentially be stabilized to high temperatures, thus opening up new possibilities for making novel electronic devices.”

The research is detailed in Nature Physics doi:10.1038/nphys4140.