The lateral photovoltage (LP) generated in a two-dimensional electron gas (2DEG) by a focussed laser spot has been shown to yield the potential distribution of a current carrying Hall-bar. The width of this distribution, which exhibits an abrupt change at even integer filling factors, is determined by the equilibration between the bulk and the edge states of the 2DEG. The effects of an anisotropic surface morphology and sample dimensions are reported.

Electrons are photoelectrically injected into the two-dimensional electron system (2DES) near a standard AlGaAs–GaAs heterojunction by a focussed light spot. The photovoltage in the plane of the 2DES exhibits quantum oscillations with magnetic field that are basically out of phase with respect to the oscillations of the conductivity σxx. This is evidence for the influence of screening effects on the injected charge. At low filling factors a dip is superimposed on the maxima, which points to a competition between screening properties and the expected diffusion effects.

The lateral photovoltage generated in the plane of a two-dimensional electron system (2DES) by a focused light spot, exhibits a fine-structure in the quantum oscillations in a magnetic field near the quantum Hall conductivity minima. A double peak structure occurs near the minima of the longitudinal conductivity oscillations. This is the characteristic signature of the interplay between screening and Landau quantization.

Partly ballistic propagation of carriers and phonons in single crystals of the metals Ag and W and the semimetal Bi is studied in real space. The carriers are excited by heating a small surface area of the sample by illumination or electron beam irradiation. Alternatively they are injected through point contacts. They spread out from this point-like source setting up a characteristic anisotropic current pattern representing some kind of fingerprint of the material's band structure. If the sample thickness is comparable to the mean free path of the carriers, they produce a potential pattern on the opposite sample surface. We present images of these patterns revealing the phenomenon of electron focusing, i.e. caustics produced by a diverging density of trajectories in specific directions. The influence of magnetic fields is studied. We give some basic theoretical analysis of the data in terms of fully ballistic transport. An outlook into possible future developments and applications of the technique and the completion of the theoretical model is given.

Electron focusing patterns originating from ballistic electron transport in single crystalline materials are observed in transmission. Spatially resolved excitation by scanning electron microscopy and detection by a point contact is employed for imaging. The effects of electron–phonon and electron–electron interactions are demonstrated for bismuth by observation of the thermal dependence of the spatially resolved transport properties of quasiparticles in the range of 5–100K.

We report experiments using a Hall photovoltage imaging technique to investigate the carrier transport in a two-dimensional electron gas (2DEG). A laser-beam is coupled into an optical fibre, which can be scanned by a mechanical cryogenic micropositioning device. This technique allows a resolution better than 5 mum to visualise the potential profile of the 2DEG in a standard Hall bar device at low temperatures and in high magnetic fields. We see flux channels (10 mum in size) due to the edge confining potential and in agreement with the theory of skipping orbits. So-called light-induced Shubnikov-de Haas oscillations have been investigated near the edges.

Ballistic carrier transport is studied in real space in tungsten single crystals using carrier excitation by laser light. So-called electron focusing caustics, singularities of the carder flux in specific directions, have been investigated systematically for crystals in three different orientations. The influence of magnetic fields B on the focusing patterns has been studied for different field orientations. Sondheimer oscillations periodic in B have been observed. The experimental data can be described by a simple model based on fully ballistic transport. Calculated focusing patterns are in satisfactory agreement with the experiments.

A scanning electron microscope with cryogenic sample holder has been employed for spatially resolved excitation of carriers. A point contact at a fixed position on the sample surface opposing the area scanned by the electron beam serves as a local current probe. Ballistic propagation and focusing of electron beam excited carriers is observed in single crystalline high purity samples of bismuth and tungsten over distances of typically 100 μm. Signal contributions from diffusive carrier flow as well as ballistic phonons are observed in addition to the ballistic electron signals.

We report on the two-dimensional observation of electron focusing under the influence of a longitudinal magnetic field B-l using a real space imaging technique. As a function of B-l the focusing patterns are modified and shrink together in good qualitative agreement with calculated focusing patterns of ballistic electrons. At low fields we observed Sondheimer oscillations periodic in B-l. At high fields light-induced magneto-oscillations due to Landau quantization occur.

A technique for studying conduction electron/interface interactions is used to study reflection and refraction of an electron beam at a Bi intercrystalline boundary. The technique involves scanning a small laser spot over the surface of a bicrystal, exciting conduction electrons locally, and using a magnetic field to focus excited electrons onto a fixed point contact (an electron collector), after the electrons have been reflected from, or transmitted through, the intercrystalline boundary. Refraction of conduction electrons, high bicrystal interface transparency, and a correlated electron transmission through the interface have all been observed.

A simple design for a mechanically driven three-dimensional cryogenic micropositioner is presented. The design is based on a parallelogram structure constructed from leaf springs and wires. Actuation is achieved by the elastic deformation of the parallelogram by screws. Positions within a volume of roughly (2 mm)3 are attainable. The precision and reproducibility of positioning are in the μm-range. The deviations from linearity are smaller than 10% for the whole working range and the deviation from orthogonality is smaller than 3°. Calibration measurements performed on a Cu-mesh with a lattice constant of 60 μm are presented. In an experiment investigating the ballistic transport of carriers in the semimetal Bi, two such devices are used. The first one is used as a scanning unit for an optical fiber and the second one is used as micropositioner for a Cu point contact.

The nearly ballistic propagation of electrons in Ag and of holes in W is investigated in real space. The carriers are excited by the illumination of a small area on one surface of the sample, and are detected on the opposite side by a Cu point contact. So-called caustics, singularities of the carrier flux in real space related to regions on the surfaces of constant energy in momentum space with zero Gaussian curvature, have been observed. These experiments will permit evaluation of Fermi-surface models and band-structure calculations near the Fermi level.