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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We present measurements of the shot noise of vertically coupled self-assembled InAs quantum dots (QDs) and find a surprising enhancement with a distinct double-peak structure. 1By applying an external bias voltage we observe pronounced peaks in the dc current -voltage characteristics that correspond to electron transport through a stack of vertically coupled QDs. At these peaks we find enhanced shot noise in contrast to suppressed shot noise at a single layer of QDs. To characterize the measured shot noise S the so-called Fano factor α is introduced. At both sides of the current peak the Fano factor α rises to values up to 1.4, whereas the noise is reduced on top of the peak ( 1 α < ). We discuss qualitative agreement with a theoretical model which predict such a super-Poissonian shot noise.Shot noise is a frequency independent fluctuation of current and has its origin in the discreteness of the charge carrying the current [1]. Its average noise power S is proportional to the average current I, 2 S eI = . Investigations of the shot noise properties on tunneling barrier devices have shown that measuring shot noise grants access to further information than the DC current provides [2][3][4][5].In this paper we will discuss the shot noise properties of transport through vertically coupled zerodimensional systems, so-called quantum dots (QDs).The active part of our sample consists of a GaAs-AlAs heterostructure with embedded InAs quantum dots in vertical direction. The GaAs electrodes consist of a 15 nm undoped spacer layer followed by a GaAs buffer with graded doping ( Fig. 1. Due to the remaining strain the QDs in the second layer are vertically aligned to the dots in the first layer [7]. The dots in the first layer are slightly smaller than the corresponding QDs in the second layer which leads to a higher ground state energy. The QDs are 10 to 15 nm in lateral dimension and about 3 to 4 nm high. Below the etched diode structure of 40 m 40 ¥ m about one million QDs are randomly placed. For similar samples with a single layer of QDs it has been shown that only a small fraction of the selfassembled QDs participates in the electronic transport [8].The transport measurements were carried out in a 4He bath cryostat allowing temperatures down to 1.4 K. By applying an external bias voltage SD V we drive a current through the device which was amplified by a low-noise current amplifier. The setup was tested by measuring the thermal noise of film resis-