In a paper published recently in the journal Nature Communications, the first author of which is Andrea Gamucci, the researchers report on a new class of compound electronic structures in which single or bi-layer graphene is set in close proximity to a quantum well made from gallium arsenide.
A quantum well, formed from a semiconductor with discrete energy values, confines charged particle motion to a two-dimensional plane. Combining graphene with a quantum well results in a heterostructure formed from two different two-dimensional materials, and such a compound assembly may be used to investigate the interaction of electrons and electron holes. A hole is formed when an electron is excited into a higher energy state, leaving in its wake a quasi-particle which behaves as if it were a ‘missing’ electron, or an electron with positive rather than negative charge. Note that electron holes are not the same thing as the physically real anti-particles, known as positrons.
Superfluidity in 2D heterostructures
In the case of the graphene-GaAs heterostructures reported in the Nature Communications paper, the Coulomb drag measurements are consistent with strong interactions between the material layers, with the attractive electrostatic force between electrons and holes in solid-state devices predicted to result in superfluidity and Bose-Einstein condensation. In other words, the strong interaction between material layers leads to quantum effects manifest in large ensembles of electrons and holes confined within micrometre-sized devices.
“We show that such effects may happen when electrons are confined in a thin well made of gallium arsenide, with holes confined in monolayer or bilayer graphene,” says Polini. “Electrons and holes separated by a few tens of nanometres attract each other through one of the strongest forces exhibited in nature – the electrical force. At sufficiently low temperatures, our experiments reveal the possible emergence of a superfluid phase, in which opposite currents flow in the two separate two-dimensional systems.” Pellegrini continues: “Such currents flow with minimal dissipation, and may make possible a number of coherent electronic devices which dissipate little energy.” Ferrari adds:“This is an another example of cutting edge results enabled by the deterministic assembly of graphene and other two-dimensional structures, which is precisely the overall target of the Graphene Flagship.”