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The Quantum Hall Effect (QHE) and Shubnikov-de Haas effect are investigated experimentally using n type modulation-doped GaAs/GaAlAs quantum wells (QWs) additionally doped in the well with beryllium acceptor atoms. It is presently shown that the localized magneto-acceptor (MA) states which possess discrete energies above the corresponding Landau levels (LLs) lead to two observable effects in magneto-transport: magnetic thaw-down and magnetic boil-off of 2D electrons. Both effects are related to the fact that electrons occupying the localized MA states cannot conduct. Thus in the thaw-down effect the electrons fall down from the MA states to the free Landau states. This leads to a shift of the Hall plateau towards higher magnetic fields as a consequence of an increase of the 2D electron density NS. In the boil-off effect the electrons are pushed from the free Landau states to the empty MA states under high enough Hall electric field. This process has an avalanche character leading to a dramatic increase of magneto-resistance, consequence of a decrease of NS.

The quantum Hall effect (QHE) and the Shubnikov-de Haas effect in the QHE regime are investigated experimentally using modulation doped n-type GaAs/GaAlAs quantum wells additionally doped in the well with beryllium. It is known that acceptor states introduced by Be atoms have a localized character in the conduction band due to a combined effect of the well and a magnetic field parallel to the growth direction and that they possess discrete energies above the corresponding conduction Landau levels. It is presently shown that the localized magnetoacceptor (MA) states lead to two observable effects in magnetotransport in the ultraquantum limit: magnetic thaw down and magnetic boil-off of two-dimensional (2D) electrons. Both effects are related to the fact that electrons occupying localized MA states cannot conduct. Thus in the thaw down effect the electrons fall down from the MA states to the free Landau states, which leads to a shift of the Hall plateau toward higher magnetic fields as a consequence of an increase of the 2D electron density Ns. In the boil-off effect the electrons are pushed from the free Landau states to the MA states which leads to a dramatic increase of resistance, as a consequence of the decrease of Ns. Differences between the above effects and those induced by magnetodonors in 2D systems are emphasized. We analyze the magnetic boil-off effect theoretically assuming that it is caused by the quantum Hall electric field present in our experiments. It is demonstrated that a sufficiently strong electric field in the crossed-field configuration can indeed populate localized MA states above the Landau levels.

Terahertz Detection of quantum cascade laser emission by plasma waves in Nano-transistors

We have processed very small high electron mobility transistor (HEMT) on the two dimensional electron gas of a GaAs/GaInAs heterostructure. The contacts (down to 2 μm wide) connect channels of different lengths and widths. We measure the saturation I(V) curves and obtain the source drain voltage at saturation. Experiments demonstrate a universal linear dependence of the saturation voltage drop with the length of the channel, whatever its width. Our experimental result is understood with basic equations of the 2D channel, taking into account the contact resistivity. The universal law Usat(L) gives a nice way to measure easily and precisely the resistivity of contacts in ultrasmall devices when their width cannot be precisely known, like in narrow two dimensional electron gas.

We report on the resonant detection of a 3.1 THz radiation produced by a quantum cascade laser using a 250 nm gate length GaAs/AlGaAs field effect transistor at liquid nitrogen temperature. We show that the physical mechanism of the detection is related to the plasma waves excited in the transistor channel. The detection is enhanced by increasing the drain current and driving the transistor into saturation regime. These results clearly show that plasma wave nanometer-size transistors can be used as detectors in all-solid-state terahertz systems where quantum cascade lasers act as sources.

We performed a local spectroscopy of the Landau levels density of states using gated mesoscopic Hall bars placed at very low temperature in the integer quantum Hall regime. The transverse and longitudinal conductances were measured while scanning both the two-dimensional electron density and the applied magnetic field. We observe a succession of sharp peaks due to backscattering across the samples caused by tunneling effects. Using temperature as a parameter in the range of 0.1-1 K, we characterize those tunnel processes: a resonant double-barrier tunneling and a single-barrier tunneling which corresponds to the variable range hopping regime. We show that for vanishing temperature and noninteger filling factor nu the conductance sigma(T=0, nu) does not vanish unlike the case of wide samples: instead, it converges to a limit function sigma(S)(nu) that is a noisy image of the Landau levels density of states.