Graphite is composed of several layers of carbon atoms, stacked upon each other and weakly interacting. This weak cohesion is responsible for its crumby aspect, so that exfoliation of this material is quite an easy matter. Recently, a very simple exfoliation technique has been discovered that allow the isolation of one single atomic plane of carbon atoms, named graphene, onto an insulating substrate. Such a system displays remarkable electronic properties linked on one hand to its bi-dimensional character, and on the other hand to the honeycomb lattice of carbon atoms that makes graphene.
The most spectacular effect observed in graphene is certainly the quantum Hall effect. When charge carriers (electrons or holes) are subjected to a magnetic field perpendicular to their confinement plane, The Hall conductance Gxy is quantized at multiple values of G0=e2/h. This effect is also observed in graphene, but the Hall plateaus lie at value Gxy=4.e2/h.(n+1/2).(n+1/2) where n is an integer, which is distinctive to any other conventional bi-dimensional systems.
This particular Hall conductance quantization is directly linked to the nature of charge carriers which, in graphene, are both electrons and holes (in other words, the charge carrier wave function is a linear combination of electronic and hole-like states). Such quasi-particles also have zero effective mass ! To our knowledge, no other electronic systems displaying such peculiarities have been discovered up to now!
In the laboratory, we are reproducing the quantum Hall effect in graphene with the use of very intense magnetic field, likely to produce new phenomena that have not been explored yet! Indeed, when the magnetic field is high enough, the quantization of the energetic spectrum into “Landau levels” allows the exploration of the most fundamental quantum states, towards a better understanding of the electronic properties of this system.
Quantum Hall effect in graphene. The longitudinal resistance oscillates as un function of magnetic field while the Hall resistance shows a step-like behaviour
When graphene is etched into a small stripe of a few tens of nanometers in width (we are dealing with a graphene nano-ribbon), its electronic properties are greatly modified because of the lateral confinement. A gap develops in its energetic spectrum that is likely to be exploited for the realisation of tomorrow’s electronic nano-devices. There again, the combined use of low temperature and intense magnetic field can address the fundamental electronic states of this under-explored nano-object.
To be updated soon
Former members of the laboratory involved in this activity :
A. Shulka ; R. Kumar ; A. Balan (UPMC - Paris - France)
V. Bellani ; M. Amado (University of Salamanca - Spain)
K.S. Novoselov (University of Manchester - U.K.)
S. Roche (CEA- Grenoble - France)
Selected publications :
To be updated soon