In a high-mobility two-dimensional electron gas (2DEG) in a GaAs/Al 0.3 Ga 0.7 As quantum well we observe a strong magnetoresistance. In lowering the electron density, the magnetoresistance gets more pronounced and reaches values of more than 300%. We observe that the huge magnetoresistance vanishes when increasing the temperature. An additional density-dependent factor is introduced to be able to fit the parabolic magnetoresistance to the electron-electron interaction correction. Since the first observation of the fractional quantum Hall effect (FQHE) 1,2 the quality and the mobility of the two-dimensional electron gas (2DEG) increased by more than two orders of magnitude. The increased mobility has allowed not only the observation of the FQHE at many different filling factors and smaller magnetic fields but also many new effects. So, microwave-induced oscillations were observed, which were up to now not fully understood. [3][4][5] In weak magnetic fields the increased mobility enabled also the observation of phonon-induced resistance oscillations, which are caused by inelastic scattering between electrons and three-dimensional acoustic phonons. 6,7 The period of phonon-induced oscillations is tunable by an additional dc electric field. 8,9 Also a new type of QHE was enabled in high mobility 2DEGs, the re-entrant integer quantum Hall effect (RIQHE). 10,11 In the regime of the RIQHE the longitudinal resistance between integer filling factors decreases to zero suggesting fractional filling factors, but the corresponding Hall plateaus are quantized at integer values.Here we will present the observation of a huge magnetoresistance in a high mobility 2DEG which depends strongly on electron density and temperature.Our samples were cleaved from a wafer of a high-mobility GaAs/Al 0.3 Ga 0.7 As quantum well grown by molecular-beam epitaxy. The quantum well has a width of 30 nm and is Si-doped from both sides. The 2DEG is located 150 nm beneath the surface and has an electron density of n e ≈ 3.1 × 10 11 cm −2 and a mobility of μ ≈ 11.9 × 10 6 cm 2 /Vs in the dark. The specimens are Hall bars with a total length of 1.2 μm, a width of w = 200 μm and a potential probe spacing of l = 275 μm [see Fig. 1(a)]. The Hall bars were defined by photolithography and wet etching. Different ungated and gated samples were used for the magnetotransport measurements. In the case of the gated sample there is an additional layer of 600 nm PMMA between the Hall bar and the metallic top-gate to avoid leakage current. We apply top-gate voltages up to −6 V to manipulate the electron density. Our measurements were performed in a dilution refrigerator with a base temperature of 20 mK. The measurements were carried out by using low-frequency (13 Hz) lock-in technique.Figure 1(a) shows the longitudinal resistance R xx and the Hall resistance R xy vs. magnetic field B to demonstrate the quality of our samples. A series of different fractional quantum Hall states appears for filling factor ν < 2. We observe also the filling factor ν = 5/2. Over the ...

The use of two truly two-dimensional gapless semiconductors, monolayer and bilayer graphene, as current-carrying components in field-effect transistors (FET) gives access to new types of nanoelectronic devices. Here, we report on the development of graphene-based FETs containing two decoupled graphene monolayers manufactured from a single one folded during the exfoliation process. The transport characteristics of these newly-developed devices differ markedly from those manufactured from a single-crystal bilayer. By analyzing Shubnikov-de Haas oscillations, we demonstrate the possibility to independently control the carrier densities in both layers using top and bottom gates, despite there being only a nano-meter scale separation between them.

We have investigated the noise properties of the tunneling current through vertically coupled self-assembled InAs quantum dots. We observe super-Poissonian shot noise at low temperatures. For increased temperature this effect is suppressed. The super-Poissonian noise is explained by capacitive coupling between different stacks of quantum dots.

Folded single-layer graphene forms a system of two decoupled monolayers being only a few angstroms apart. Using magnetotransport measurements we investigate the electronic properties of the two layers conducting in parallel. We show a method to obtain the mobilities for the individual layers despite them being jointly contacted. The mobilities in the upper layer are significantly larger than in the bottom one indicating weaker substrate influence. This is confirmed by larger transport and quantum scattering times in the top layer. Analyzing the temperature dependence of the Shubnikov-de Haas oscillations, effective masses and corresponding Fermi velocities are obtained yielding reduced values down to 66% in comparison to monolayers.

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