[en] Magnetoresistance is used to study localization and interaction in the 2-dimensional electron layer of ⁴He-ion implanted GaAs/AlGaAs modulation doped heterostructures. At very low magnetic fields weak localization magnetoresistance can be fitted to theory, thereby determining the diffusion constant and the phase relaxation rate. An unexpected saturation of the phase relaxation rate at low temperature was found to have an interesting relation to mobility and sample size. In the magnetic field range, where the cyclotron radius becomes of the order of the mean free path, the magnetoresistance was related to the effect of electron-electron interaction. By comparison of the magnetoresistance at different implantation doses, we extracted a remnant quantum correction to the conductivity, which has no earlier been noticed. In samples with two subbands populated interband scattering is observed to cause spin-orbit effects in the weak localization magnetoresistance. (orig.)

[en] The first heterostructure wafer only had one electronic subband at the GaAs/AlGaAs interface populated. Weak localization magnetoresistance was interpreted by a theory valid to relatively high magnetic fields and also valid for electrons with a long mean free path. The adjustable parameter in fitting the magnetoresistance was in each case the phasebreaking relaxation time, which could then subsequently be plotted as a function of temperature. The temperature dependence of the phasebreaking rate could be interpreted on the basic of existing theories, but the residual relaxation rate at the lowest temperature remains so far unexplained. Already at low magnetic fields the weak localization magnetoresistance saturates, indicating a complete quench of weak localization. We find that the value of saturation (i.e. the total weak localization at the appropriate temperature) was smaller than predicted by the existing theories. At magnetic fields of the order of the inverse electron mobility, a quadratic magnetoresistance show up in our experiments. This quadratic magnetoresistance corresponds to corrections to the conductivity of the order of e²/h. Whereas we find that the temperature dependence of this conductivity correction is well in agreement with predicted effects of electron-electron interaction, the dependence on mobility, which we can measure via our ion implantation, is larger than any existing theory predicts, yet still in the ballpark of the conductance quantum. (orig./BHO)

[en] Magnetoresistance with the field perpendicular to the 2-dimensional electron gas in a high mobility GaAs/AlGaAs heterostructure at low temperatures is studied. At the lowest magnetic field we observe the weak localization. At magnetic fields, where the product of the mobility and the magnetic field is of the order of unity, the quantum correction to conductivity due to the electron-electron interaction is as a source of magnetoresistance. A consistent analysis of experiments in this regime is for the first time performed. In addition to the well known electron-electron term with the expected temperature dependence, we find a new type of temperature independent quantum correction, which varies logarithmically with mobility. (orig.)

[en] We have irradiated some GaAs/AlGaAs heterostructures with 80 keV He⁺ ions and studied bombardment-induced changes of the electrical resistivity and the carrier density of a two-dimensional electron layer located at the intersection between a GaAs buffer and an AlGaAs spacer. The whole structure was so thin that the projectiles passed through the two-dimensional electron layer and stopped in the supporting substrate. The resistivity and the carrier density of the samples were determined at temperatures between 1.2 K and 4.2 K before and after the bombardments, which were carried out at room temperature. The carrier density was determined from Shubnikov-de Haas oscillations. An interesting result is that the resistivity depends strongly on the projectile fluence in contrast to the carrier density. As an example, a fluence of 3x10⁹ He⁺ ions per cm² doubles the resistance but reduces the carrier density by 5% only. (orig.)

[en] In this letter the gauge invariance of the integral over time of the electrostatic potential (the electrostatic Aharonov-Bohm effect) is used to elucidate the quantum Hall effect. Our discussion uses the Laughlin geometry. For real samples edge currents as well as currents along internal boundaries play a crucial role. (orig.)

[en] A voltage-dependent current-phase relation for short microbridges, I/sub s/(V,phi) = I₀(V) sin phi + I₁(V) cos phi + I₂(V), is calculated on the basis of time-dependent Ginzburg--Landau theory. Used in the current-controlled resistively shunted Josephson junction model such a current-phase relation enables us to explain a number of features in the I--V characteristics of thin-film microbridges. Experimentally, an estimate can be made of the upper frequency limit of the Josephson effect in thin-film microbridges

[en] Weak localization magnetoresistance is used to separate spin-orbit and the phase relaxation rates. The spin-orbit rate is independent of temperature. By plotting the spin-orbit rate as a function of the inverse thickness of the films, the authors separate the spin-orbit relaxation into two parts: one from the scattering off the bulk imperfections, and one from the scattering against the two surfaces. The ratio between the spin-orbit and the impurity relaxation rates in the bulk was found to be close to 2 times 10^-5 for all the different samples. The influence of implanted heavy ions on the spin-orbit relaxation was also investigated. The dependence of the phase relaxation rate on resistivity, film thickness, and temperature has been studied. Theoretical results for electron-electron and electron-phonon scattering are compared to their data. They consider two novel temperature-independent phase relaxation mechanisms which may explain the residual rate they observe. The influence of a high-frequency electromagnetic field on the phase relaxation was investigated. For small microwave power levels the phase relaxation rate was found to increase linearly with microwave power. In the absence of a magnetic field and for samples having a dominating spin-orbit interaction (antilocalization) the resistance increases with microwave power, but turns into a decrease at high microwave power, but turns into a decrease at high microwave power levels. For those samples having very weak spin-orbit interaction the resistance decreases continuously as they apply microwaves. This is roughly the expected behavior, but the observed change in resistance was larger than that calculated at small microwave power levels

[en] A supercurrent through a superconducting microbridge gives rise to a minimum in the density of Cooper pairs in the middle of the bridge. Application of a microwave electric field creates a non-equilibrium situation and therefore smears the Cooper pair distribution. This allows for a higher critical current and explains the microwave-enhanced supercurrent observed in thin-film microbridges. (author)

[en] This review describes the Josephson behaviour of superconducting microbridges in terms of the resistively shunted junction model and how this behaviour manifests itself in experiment. Most of the observed deviations from this model must be treated on the basis of non-equilibrium theories. The present knowledge of the non-equilibrium properties of superconducting microbridges is reviewed from both an experimental and theoretical point of view. (author)

[en] We have studied the current-voltage characteristics of aluminium microbridges at several different temperatures and in a microwave field, where the microwave frequency and the microwave power are varied over several decades. (orig./BUD)