The quantum Hall effect (QHE) is one of the most important effects being studied by solid-state physicists today. Measuring the limits at which it breaks down is extremely important – not only for fundamental physics but also for applying the effect as a resistance standard for redefining the kilogram. Researchers in France have now found that collective excitations of interacting electrons are responsible for the onset of the breakdown of the QHE in bilayer graphene at high electric fields and they have even calculated the “Landau velocity” at which this happens. The new result lends weight to the idea that the long-held single-electron picture is not a realistic description of the QHE. The breakdown mechanism also looks very much like what happens in superconductors at the limit at which correlated electron pairs (responsible for the supercurrent in these materials) break apart and at the point at which superfluidity collapses in systems like liquid helium.
Abstract : Breakdown of the quantum Hall effect (QHE) is commonly associated with an electric field approaching the inter-Landau-level (LL) Zener field, the ratio of the Landau gap and the cyclotron radius. Eluded in semiconducting heterostructures, in spite of extensive investigation, the intrinsic Zener limit is reported here using high-mobility bilayer graphene and high-frequency current noise. We show that collective excitations arising from electron-electron interactions are essential. Beyond a noiseless ballistic QHE regime a large super-Poissonian shot noise signals the breakdown via inter-LL scattering. The breakdown is ultimately limited by collective excitations in a regime where phonon and impurity scattering are quenched. The breakdown mechanism can be described by a Landau critical velocity as it bears strong similarities with the roton mechanism of superfluids. In addition, we show that breakdown is a precursor of an electric-field induced QHE-metal transition.
Source: W. Yang, H. Graef, X. Lu, G. Zhang, T. Taniguchi, K. Watanabe, A. Bachtold, E. H. T. Teo, E. Baudin, E. Bocquillon, G. Fève, J-M. Berroir, D. Carpentier, M. O. Goerbig, and B. Plaçais. Phys. Rev. Lett. 121, 136804 – September 25, 2018