If the discovery of two-dimensional materials as graphene has brought its share of new physical phenomena, the incredible transport properties of these materials also allow to shed a new light on phenomena already observed and supposed understood. In the framework of a collaboration with two theoretical researchers from ENS Lyon and LPS Orsay, LPA researchers have revisited the quantum Hall breakdown by observing the intrinsic limit in high mobility graphene using high frequency shot noise measurements. They propose a radically different explanation of the currently known process.
In the quantum regime, the application of a magnetic field on a metallic material has a well known effect : it exchanges its continuous energy band by a discrete series of Landau levels by making the sample insulating in the bulk. Applying a weak electric field on the material results in a current without loss only on the edges... until a certain limit of voltage, where the material undergoes a brutal phenomenon of breakdown, giving rise to an avalanche of excited electrons.
In the case of bilayer graphene, researchers have explored this mechanism using high frequency signals ( 5 GHz). Significantly more relevant than the electrical current, the electronic noise enables to detect precisely the appearance of the breakdown. The exceptional quality of the bilayer graphene samples makes it possible to obtain breakdown fields up to 106 V / m, closely approaching the fundamental limit.
The breakdown is generally understood using a mechanism involving single electrons (the Zener mechanism) : electrons can jump via quantum tunnel effect from one Landau level to the next, triggering an avalanche. And yet, the situation is very different in the case of high quality graphene bilayer : the researchers have shown that it is collective excitations - known as magneto-excitons - that trigger the phenomenon. These excitations can involve a large number of electrons (as indicated by a Fano factor of 20 to B = 7T). In the image of the avalanche, the latter is not made of individual snowflakes but aggregates like snowballs. Again, the electronic noise is richer in information than the current itself, and confirms the collective nature of electronic transport.
This study, which was published in Physical Review Letters, therefore proposes a new interpretation of the breakdown of the quantum Hall effect as a collective phenomenon due to magneto-excitons. The minima (called "rotons") of collective excitations having been introduced as part of the understanding of superfluid helium. The article makes a bridge between quantum Hall breakdown phenomenon and normal / superfluid transition.
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Collaborators : David Carpentier (Ens Lyon) and Mark Goerbig (LPS Orsay)
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