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      Screening model of metallic non-ideal contacts at integer quantized Hall regime

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          Abstract

          In this work, we calculate the electron and the current density distributions both at the edges and the bulk of a two dimensional electron system, focusing on ideal and non-ideal contacts. A three dimensional Poisson equation is solved self-consistently to obtain the potential profile in the absence of an external magnetic field considering a Hall bar defined both by gates (contacts) and etching (lateral confinement). In the presence of a perpendicular magnetic field, we obtain the spatial distribution of the incompressible strips, taking into account the electron-electron interactions within the Thomas-Fermi approximation. Using a local version of Ohm's law, together with a relevant conductivity model, we also calculate the current distribution. We observe that the incompressible strips can reside either on the edge or at the bulk depending on the field strength. Our numerical results show that, due to a density poor region just in front of the contacts, the incompressible strips do not penetrate to the injection region when considering non-ideal contact configuration. Such a non-ideal contact is in strong contrast with the conventional edge channel pictures, hence has a strong influence on transport. We also take into account heating effects in a phenomenological manner and propose a current injection mechanism from the compressible regions to the incompressible regions. The model presented here perfectly agrees with the local probe experiments all together with the formation of hot-spots.

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          Crossover from mesoscopic to universal phase for electron transmission in quantum dots

          Measuring phase in coherent electron systems (mesoscopic systems) provides ample information not easily revealed by conductance measurements. Phase measurements in relatively large quantum dots (QDs) recently demonstrated a universal like phase evolution independent of dot size, shape, and occupancy. Explicitly, in Coulomb blockaded QDs the transmission phase increased monotonically by pi throughout each conductance peak, thereafter, in the conductance valleys the phase returned sharply to its base value. Expected mesoscopic features in the phase, related to spin degeneracy or to exchange effects, were never observed. Presently, there is no satisfactory full explanation for the observed phase universality. Unfortunately, the phase in a few-electron QDs, where it can be better understood was never measured. Here we report on such measurements on a small QD that occupy only 1-20 electrons. Such dot was embedded in one arm of a two path electron interferometer, with an electron counter near the dot. Unlike the repetitive behavior found in larger dots we found now mesoscopic features for dot occupation of less than some 10 electrons. An unexpected feature in this regime is a clear observation of the occupation of two different orbital states by the first two electrons - contrary to the recent publications. As the occupation increased the phase evolved and turned universal like for some 14 electrons and higher. The present measurements allowed us to determine level occupancy and parity. More importantly, they suggest that QDs go through a phase transition, from mesoscopic to universal like behavior, as the occupancy increases. These measurements help in singling out potential few theoretical models among the many proposed.
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            Realistic modelling of quantum point contacts subject to high magnetic fields and with current bias at out of linear response regime

            The electron and current density distributions in the close proximity of quantum point contacts (QPCs) are investigated. A three dimensional Poisson equation is solved self-consistently to obtain the electron density and potential profile in the absence of an external magnetic field for gate and etching defined devices. We observe the surface charges and their apparent effect on the confinement potential, when considering the (deeply) etched QPCs. In the presence of an external magnetic field, we investigate the formation of the incompressible strips and their influence on the current distribution both in the linear response and out of linear response regime. A spatial asymmetry of the current carrying incompressible strips, induced by the large source drain voltages, is reported for such devices in the non-linear regime.
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              Author and article information

              Journal
              2010-03-30
              Article
              10.1103/PhysRevB.82.165308
              1003.5963
              38029ebc-f0a8-47c7-baf4-df8cb8dd98f4

              http://arxiv.org/licenses/nonexclusive-distrib/1.0/

              History
              Custom metadata
              Physical Review B (2010)
              7 pages 4 Figures. Regular article
              cond-mat.mes-hall

              Nanophysics
              Nanophysics

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