Mechanisms of new type quantum Hall breakdown

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There are two types of quantum Hall effect(QHE) breakdown ; one is that the breakdown current scales linearly with the channel width (linear type), and the other is that the breakdown current increases sub-linearly with the channel width (sub-linear type). To investigate the origin of sub-linear type breakdown, we have studied the QHE breakdown by using samples ; (1)conventional Hall-bar, (2)Corbino geometry samples without sample edge, and (3)side gated Hall-bar whose confinement potential at the sample boundary is tunable by the gate voltage. Both linear and sub-linear types of the GaAs/AlGaAs two-dimensional electron systems(2DES) are used to compare the experimental results. In the linear type samples, the observed critical Hall electric field in Hall-bar shaped sample is in good agreement with the critical electric field measured in Corbino samples. It demonstrates that the current distribution is uniform in the linear type samples. On the other hand, the critical electric field in the sub-linear type sample is much smaller than those in the linear type samples, and the width dependence of critical current in Hall-bar and the critical voltage in Corbino shows sub-linear feature. Moreover, the value of breakdown current is scarcely affected by changing side gate voltage, which suggests that the QHE breakdown is independent of the confinement potential at the sample boundary.Those experimental results support that the sub-linear type QHE breakdown is not "edge effect" but the properties of bulk 2DES. We also have measured the current dependence of activation energy of residual bulk conductivity at QHE plateau. The possible origin of sub-linear type QHE breakdown ; the localized current distribution in the bulk 2DES and current (electric field) effect on the activation energy in 2DES with small potential fluctuation, is discussed.