Momentum-resolved magnetotunneling spectroscopy is performed at a single sharp quantum Hall (QH) edge to probe the structure of integer QH edge modes. An epitaxially overgrown cleaved edge is shown to realize the sharp-edge limit with interchannel distances smaller than both the magnetic length and the Bohr radius where the Chklovskii soft-edge picture is no longer valid. The line shape of principal conductance peaks is explained, and an edge filling factor is determined from the peak position. A step in the dispersion is attributed to fluctuations in the QH ground energy.

Periodic modulation of a free two-dimensional electron system changes the bandstructure in which the electrons move. A weak modulation opens up small energy gaps in the free electron bandstructure while a strong, short-period modulation leads to the formation of well separated cosine-like minibands. We study transport in both cases using structures grown with the Cleaved-Edge-Overgrowth method. It allows to fabricate high quality two-dimensional electron systems with arbitrary modulation periods which are precise on an atomic scale. Perturbing the system only weakly with a long-period superlattice that is located some distance away from the electron layer leads to magneto-transport characteristics which exhibit each a number of different 1/B-periodic oscillations. These can be very well explained by semiclassical theory and allow the extraction of the Fermi surface of the system. The strong modulation with a short-period superlattice leads to the two-dimensional equivalent of a conventional superlattice. One very important difference lies in the stability of the expected negative differential conductance regime with respect to electric field instabilities. Geometrical considerations show that the observation of stable Bloch oscillations is possible in these systems. We will show that density dependent transport studies confirm the formation of a two-dimensional superlattice. Although negative diffential conductance, also observed in those samples, must be attributed to an inhomogeneous density distribution. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

This article highlights the history of the quantum Hall edge, focusing on the distinction between sharp and smooth edges. We then introduce an edge-tunneling geometry consisting of two orthogonal quantum wells wherein magneto-tunneling measurements at a cleaved-edge overgrown quantum Hall edge are made. Peaks in the tunnel conductance are shown to arise from momentum conserved tunneling into individual quantum Hall edge states. Knowledge of the Fermi momentum of the tunneling electrons determines the real-space distance of the edge states from the tunnel barrier. The results can be interpreted as the first quantifiable evidence of edge states in the sharp quantum Hall edge limit.

A quantum cascade emitter structure is suggested that is based on electronic transitions in an artificial band structure of coupled quantum wires in contrast to the conventional quantum layer systems. The electron transport is normal to the wires in a GaAs/AlxGa1-xAs heterostructure produced by the cleaved-edge overgrowth method. For a realistic system of this type, the relevant rates of radiative transitions and of nonradiative transitions mediated by longitudinal optical phonons have been calculated and have been compared with corresponding data for quantum cascade emitters based on layered structures.

We present experimental investigations of tunneling between two quantum wells forming a T-shaped structure. At zero magnetic field we observe a nonlinear tunnel characteristic with clearly pronounced negative differential resistance. With magnetic field B, both quantum wells can independently be set in a quantum Hall state. We demonstrate spectroscopy of Quantum Hall bulk states in one B field orientation, and spectroscopy of edge states with the orthogonal orientation. The observed features can be explained assuming that transverse momentum is conserved during tunneling.

The purpose of this paper is twofold. First we introduce a system that gives independent control over the strength and period of an atomically precise one-dimensional potential modulation imposed on a two-dimensional electron system. Second, along with low temperature magnetotransport experiments on such a system, we employ a semiclassical framework to explain all observed magnetoresistance oscillation frequencies. In particular two distinct quantum interference processes are recognized. a) self-interference along closed orbits, partly rendered possible by magnetic breakdown b) mutual interference between open electron orbits. The notion of the latter mechanism bridges the gap between the known commensurability oscillations and the artificial bandstructure. We suggest a close relation also to quantum-interference effects observed in magnesium [1] and organic conductors [2].

The non-equilibrium transport of electrons confined at an atomically sharp interface and subject to a periodic potential is studied. We find a pronounced negative differential resistance, the magnitude of which increases with increasing modulation strength. The data is qualitatively consistent with the Esaki–Tsu transport model. We emphasize the significance of the two-dimensionality of the electron system and the gate to inhibit domain formation. Additional features in the source–drain current are attributed to Bloch-phonon resonances.

Magnetotransport experiments on two-dimensional electron systems with an atomically precise, one-dimensional potential modulation reveal striking quantum interference oscillations. Within a semiclassical framework, they are recognized either as self-interference along closed orbits, many of them rendered possible by magnetic breakdown between Fermi contour segments of the artificial band structure, or as interference-enhanced backscattering. The known commensurability oscillations appear as a special case of the latter mechanism.

We present low-temperature measurements of the ballistic transport in high-quality quantum wires. As the Fermi energy is varied, the conductance of these wires exhibits quantized plateaus at values lower than integer multiples of 2e2/h. We observe Luttinger liquid power laws in the temperature dependence of the plateau conductances as well as in the non-linear current–voltage characteristics. From these power laws we extract the Luttinger liquid scaling exponent αN as a function of the Fermi energy and the number of occupied subbands.

We experimentally investigate the miniband transport in a novel kind of superlattice fabricated by the “cleaved edge overgrowth” method. The structure represents a field effect transistor, where the channel consists of an MBE-grown superlattice perpendicular to the current flow. By means of the gate the Fermi energy can be adjusted between the bottom of the first miniband and into the minigap. We observe pronounced negative differential resistance at electric fields across the superlattice as low as 160 V/cm. From magnetotransport measurements a relation between the applied gate voltage and the position of the Fermi energy in the artificial band structure is established. Electron mobility depending on the Fermi energy is deduced separately from Shubnikov–de Haas oscillations, from the voltage at the peak current and from the low-field resistance.

Recent progress in the fabrication of GaAs/AlGaAs low-dimensional structures by cleaved edge overgrowth (CEO) - a molecular beam growth technique that involves one or two regrowth steps on the sidewalls of an in situ cleaved layer structure - is reviewed. Ballistic electron transport in modulation-doped quantum wires prepared in this way is characterized by pronounced quantization of the conductance in integer multiples < 2e(2)/h. A magnetic field oriented perpendicular to the quantum wires is found to increase the quantized conductance values. While for a 400 Angstrom wide channel the canonical values of n x 2e(2)/h are almost reached at a magnetic field strength of 4 T the corresponding conductance rise in a 250 Angstrom wide channel is much slower. In addition, the magnetic field dependence of the positions of the conductance steps as a function of an applied gate voltage which controls the electron density is distinctly different for wires of 400 and 250 A width. The optical, properties of atomically precise quantum dots which originate at the right angle intersection of three quantum wells are characterized by narrow "atom-like" emission lines. Two different types of quantum dot structures have been investigated using microscopic photoluminescence spectroscopy: linear arrays of weakly coupled dots, i.e. an one-dimensional quantum dot superlattice and single as well as paired quantum dots. The latter allow the assembly of "artificial molecules') out; of almost identical quantum dots which can be considered as "artificial atoms" as the dot-to-dot distance is varied.

Cleaved edge overgrowth (CEO) has proven to be a powerful technique for the fabrication of atomic scale T-shaped quantum wires (QWRs) which form at the intersection of two quantum wells (QWs). Here we report on the first experimental demonstration of quantum dots (QDs) which result when three QWs intersect each other at right angles. Optical emission from zero-dimensional (0D) states in these QDs which were fabricated by a twofold CEO technique is clearly identified by means of micro-photoluminescence (μPL) and μPL excitation (μPLE) spectroscopy. In contrast to the inhomogeneously broadened QW and QWR signals originating from the complex sample structure, the QD response, which is characterised by sharp lines (FWHM <70 μeV), is strongly spatially localised at a position where the QWs meet.

AlGaAs antidot arrays with about 107 antidots are produced by single-shot interference processing with a pulsed high-power Nd:YAG laser system. We apply magnetotransport experiments and atomic force microscopy (AFM) to explore the electronic and geometric properties of the arrays. The size of the antidot arrays are 3 mm×3 mm and the period varies from 400 to 1000 nm. The dots are elliptic or circular and have diameters ranging from 255 to 690 nm. The magnetotransport experiments are performed at 1.5 K in van der Pauw contact configuration. The laser structuring leaves the two dimensional electron density nearly unchanged but decreases the mobility by a factor of about 30. Several maxima are detected in the low magnetic field magnetoresistivity which are discussed based on the geometric data determined by AFM.

Hall experiments on a series of microcrystalline, microcrystalline amorphous, amorphous and crystalline silicon samples with various defect densities are presented and discussed. Normal signs of the Hall effect in boron- and phosphorus-doped hydrogenated amorphous silicon have been observed. We interpret these results as due to a small volume fraction of nanocrystalline silicon, which falls below the detection limit of Raman experiments. Hydrogenated amorphous silicon, prepared under conditions far from microcrystalline growth, shows the known double-sign anomaly. On the other hand, sign reversals in crystalline silicon in which disorder was increased by silicon implantation up to apparently complete amorphization, were not found.