The nonlinearities in the IV characteristics have been studied of high-mobility Si metal oxide semiconductor field-effect transistors in the quantum Hall regime. The breakdown curves were measured with different sets of voltage contacts and for different directions of magnetic field and current. Comparison of these curves shows that the breakdown of the quantum Hall effect (@HE) in these samples is an intrinsic effect that starts at the current contact where the electrons are injected into the two-dimensional electron gas {2DEG). This fundamental asymmetry and the crucial role of the current contact are explained using the Biittiker-Landauer approach to the QHE and its recent extension to the nonlinear regime. The electron-injection process contains two mechanisms that lead to breakdown voltages in the 2DEG. We have identified both experimentally by comparing the critical currents of different configurations of current and voltage contacts. In one of the mechanisms, the nonequilibrium distribution of electrons that is injected into the 2DEG extends to the voltage contacts. This means that the equilibration length of the 2D electrons is at least of the order of 100 p, m. For currents far beyond breakdown and for voltage contacts that are further from the electron-injection contact, the breakdown characteristics are harder to understand. The variation of the electron density of the 2DEG due to the large Hall voltage has to be taken into account as well as the equilibration induced by additional voltage contacts.
A novel method is presented to study the dynamics of vortices in superconducting films at fields close to the upper critical magnetic field. It is shown that moving flux lines in the gate of a superconductoroxide-semiconductor field-effect transistor are magnetically coupled to a two-dimensional electron gas, leading to an induced voltage. The major part of this voltage is proportional to the magnetoresistance, which is varied by changing the Landau-level filling. A second part is independent of the electron density and is tentatively attributed to the Hall component of the resistivity tensor. PACS numbers: 74.60. Ge, 73.40.Qv, 73.50.Jt The recent discovery of high-critical-temperature superconductors has led to a dramatic increase of interest in vortex dynamics. Various models have been developed to understand the collective behavior of a vortex lattice under the influence of external driving forces and/or in the presence of thermal activation [1], For temperatures close to the critical temperature (7V), or magnetic fields close to the upper critical field (B C 2), possible transitions from the well-known hexagonal lattice to a glasslike or liquidlike state have been predicted. Although these concepts strongly influence our understanding of the vortex dynamics, few techniques are available to resolve these issues experimentally. Most researchers focus on a study of the magnetization supplemented with a careful study of current-voltage characteristics. Further progress in the understanding of the dynamics of flux lines, either collectively or individually, may greatly benefit from new experimental information.The most convincing proof of flux flow has been provided by Giaever [2], who studied flux flow in two superposed superconducting films. He showed that currentinduced flux flow in one of the films (primary) induces a voltage in the magnetically coupled secondary film. In this Letter we describe a related system (inset, Fig. 1) in which the secondary superconducting film is replaced by a two-dimensional electron gas (2DEG). The 2DEG is formed at the interface between silicon and silicon dioxide by applying a voltage between the superconducting gate and the silicon. Compared to the original system studied by Giaever this has two important advantages. First, the magnetically coupled films are different in nature and the viscous forces on the vortices in the primary film are independent of the dissipation in the secondary. Second, the dependence of an induced voltage in the secondary on the electronic properties of the 2DEG can be studied by varying the voltage between the gate and the silicon. As expected [3,4], we find that moving flux lines in the (low-7V) superconducting film induce a voltage in the 2DEG. Two contributions to this voltage are found, one proportional to the magnetoresistance, which is varied through the electron density, and one roughly independent of electron density. Apart from their use in the study of vortex dynamics, these observations will also raise interesting new questions with resp...
We have investigated electron transport in a Si(100) inversion layer at high carrier densities and temperature T & 4.2 K. From magnetoresistance oscillations and Hall measurements we And that at electron densities larger than 5x10' m a second subband is populated. In this regime the conductivity sho~s a linear temperature dependence similar to the one-subband conductivity in high-mobility inversion layers at low electron density. This effect is tentatively ascribed to the temperature broadening of the electron distribution in the second subband. %e have determined the mobility in, and the population of, the two subbands as a function of gate voltage. Since the mobility in the second subband is relatively low, the strong T dependence is an indication that we
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