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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.

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