Field-induced breakdown of the quantum Hall effect

A numerical analysis is made of the breakdown of the quantum Hall effect caused by the Hall electric field in competition with disorder. It turns out that in the regime of dense impurities, in particular, the number of localized states decreases exponentially with the Hall field, with its dependence on the magnetic and electric field summarized in a simple scaling law. The physical picture underlying the scaling law is clarified. This intra-subband process, the competition of the Hall field with disorder, leads to critical breakdown fields of magnitude of a few hundred V/cm, consistent with observations, and accounts for their magnetic-field dependence \propto B^{3/2} observed experimentally. Some testable consequences of the scaling law are discussed.Comment: 7 pages, Revtex, 3 figures, to appear in Phys. Rev.

Scanned Potential Microscopy of Edge and Bulk Currents in the Quantum Hall Regime

Using an atomic force microscope as a local voltmeter, we measure the Hall voltage profile in a 2D electron gas in the quantum Hall (QH) regime. We observe a linear profile in the bulk of the sample in the transition regions between QH plateaus and a distinctly nonlinear profile on the plateaus. In addition, localized voltage drops are observed at the sample edges in the transition regions. We interpret these results in terms of theories of edge and bulk currents in the QH regime.Comment: 4 pages, 5 figure

Déterrer, démêler et réarticuler les corps du génocide au Rwanda

This paper is concerned with the mass graves and exhumed bodies of victims of the Rwanda genocide and war of the 1990s. A government-led programme of exhumation of mass burials and individual graves has taken place over the last decade. The exhumation of mass graves has been undertaken, in the main, by Tutsi genocide survivors who work under the supervision of state officials. Post-unearthing, these bodies are unravelled, and the remnants of soft flesh, clothing, personal possessions and bones are separated from each other. Skeletal structures are fully disarticulated and the bones pooled into a vast collective, for placement within memorials. The outcome of these exhumations is that remains almost always lack individual identity at the point of reinterring. A productive analytical comparison is found in examining exhumations of Spanish Civil War graves, where the fates of individual dead are closely entangled with the lives of survivors. Here there is a clear contrast with exhumations in Rwanda, in the possible re-articulation of identities with specific human remains. But a similarity is also critical: in both cases the properties of human remains, as unsettling materials, garner specific 'affects', which drive forward national political projects that aim to consolidate particular collective memories of conflict, albeit that this kind of 'material agency' is mobilized to very different ends in each case