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The excess current of a point contact on a Ag-Pb bilayer has been measured for several thicknesses of the Ag film. The excess current is due to Andreev reflection and contains information about the position dependence of the superconducting order parameter near the interface. If the Ag film is very thin, the excess current is that of a normal-metal-superconducting point contact, though slightly changed because of the depression of the order parameter at the surface of the bilayer. For larger thicknesses, the combination of the proximity effect and the limited mean free path of the electrons yields very different current-voltage characteristics.PACS numbers: 74.50.+r If a normal metal TV is in good electrical contact with a superconductor S, the Cooper pairs of S leak into TV and the pair amplitude varies gradually with position near the TV-5* interface. The length scale of this proximity effect 1 is determined by the coherence length, which is typically of the order of 0.1 jum. The pair potential A is the product of the pair amplitude and the BCS coupling constant, the latter being smaller in TV than in S. The position dependence of A near the TV -S interface is shown schematically in Fig. 1. Experimentally, the proximity effect can be studied for instance by the measurement of the screening properties of an N-S bilayer in a magnetic field. 1 ' 2 Another approach employs a tunnel junction on the TV side of an N-S bilayer. 1,3 If the TV layer is thin compared with the coherence length, the pair potential at the tunnel-junction interface in TV is finite and this is reflected in the differential resistance of the junction.We employed a point contact on the TV side of an TV-5 bilayer to study the probability of Andreev reflection 4 of quasiparticles incident on an N-S interface like that in Fig. 1. An electron at an TV -S interface in TV can condense into a Cooper pair together with a second electron from N with opposite momentum and spin. The hole (or missing electron) that is created in TV moves back in the direction from which the incident electron came (retroreflection). If it returns through the point contact, the hole gives rise to an excess current that can be measured. The Andreev-reflection process has been studied FIG. 1. Position dependence of A near an N-S interface. The thickness of the TV layer is d, while d' is an effective thickness of the nonsuperconducting layer. both theoretically 5 and experimentally 6 for N-S point contacts. Then, however, the order parameter varies on the scale of the point-contact diameter, which is much smaller than the coherence length. Therefore, in Ref. 5 a step function was used for the position dependence of A. The theory has been extended to include a gradual variation of A near the N-S interface and then yields slightly different results for the probability of Andreev reflection. 7 Measurements have been taken of the excess current of a point contact on a Ag single crystal backed by a Pb film. 8 In these samples, the thickness of the TV layer (200 and 20 jam, respective...

A new method for determination of the inelastic scattering rate for electrons in thin films is presented, using the rninimurn frequency for microwave enhancement of the critical pair-breaking current. Measurements on clean aluminum films with thickness between 144 and 4 nm yield excellent agreement with predictions of Abrahams et al. for electron-electron scattering.Inelastic scattering of electrons at the Fermi surface plays a very important role in phenomena such as weak localization and nonequilibrium superconductivity. Although in both cases theory is well established and in good agreement with experiments, it is still an open question whether electron-phonon or electron-electron scattering is the predominant energy-relaxation mechanism.The methods that have been developed to measure the inelastic scattering time 7","make use of the same phenomena. In particular, the magnetoresistance' of thin films provides a value for 7-;", as does charge imbalance in superconducting films, induced by tunnel injection' or phase-slip centers. ' Each method calls for a rather involved analysis and independent checks are highly desirable. We have developed a new method to determine 7;", based on the enhancement of the critical pair-breaking current of superconducting films by microwaves. Because enhancement is caused by a nonequilibrium distribution of quasiparticles over the energies, it is a direct measure of energy relaxation and is not sensitive to spin-flip or spin-orbit scattering. To obtain v;"no assumptions for uF or pl are required. The method is limited in that~;"can only be determined near the critical temperature T"and that heating effects can only be neglected for materials with large 7;". The method has been applied to thin aluminum films of different thickness d. A strong dependence on d is found, where for thinner films the scattering rate is proportional to the sheet resistance 8&, in excellent agreement with predictions of Abrahams, Anderson, Lee, and Ramakrishnan' for electron-electron scattering.Microwave enhancement of superconductivity is connected with a stationary nonequilibrium distribution of quasiparticles. It increases strongly with frequency until tee exceeds 24. In practice, spurious effects make enhancement of the gap difficult to measure. The critical pair-breaking current I, of one-dimensional strips can be measured easily and accurately. When the frequency is decreased, a clear change from enhancement to suppression of I, is obtained, providing excellent experimental definition of a minimum frequency m;". Calculation of this eo;" for I, enhancement is more complicated than that for gap enhancement.A full discussion of the theoretical aspects will be published elsewhere. Here, only a short account is given. Our treatment is based on approximations requiring b/ksT to be small.We need the equation for the gap in the presence of both a microwave field and a dc current: T, T-0.106 -Q +II (la) kg Tc Tc The dc current is represented by the normalized superfluid momentum Q =p,/(SksT, t/mD)'i'. The mic...

The nonlinearities in the IV characteristics have been studied of high-mobility Si metal oxide semiconductor field-effect transistors in the quantum Hall regime. The breakdown curves were measured with different sets of voltage contacts and for different directions of magnetic field and current. Comparison of these curves shows that the breakdown of the quantum Hall effect (@HE) in these samples is an intrinsic effect that starts at the current contact where the electrons are injected into the two-dimensional electron gas {2DEG). This fundamental asymmetry and the crucial role of the current contact are explained using the Biittiker-Landauer approach to the QHE and its recent extension to the nonlinear regime. The electron-injection process contains two mechanisms that lead to breakdown voltages in the 2DEG. We have identified both experimentally by comparing the critical currents of different configurations of current and voltage contacts. In one of the mechanisms, the nonequilibrium distribution of electrons that is injected into the 2DEG extends to the voltage contacts. This means that the equilibration length of the 2D electrons is at least of the order of 100 p, m. For currents far beyond breakdown and for voltage contacts that are further from the electron-injection contact, the breakdown characteristics are harder to understand. The variation of the electron density of the 2DEG due to the large Hall voltage has to be taken into account as well as the equilibration induced by additional voltage contacts.

A novel method is presented to study the dynamics of vortices in superconducting films at fields close to the upper critical magnetic field. It is shown that moving flux lines in the gate of a superconductoroxide-semiconductor field-effect transistor are magnetically coupled to a two-dimensional electron gas, leading to an induced voltage. The major part of this voltage is proportional to the magnetoresistance, which is varied by changing the Landau-level filling. A second part is independent of the electron density and is tentatively attributed to the Hall component of the resistivity tensor. PACS numbers: 74.60. Ge, 73.40.Qv, 73.50.Jt The recent discovery of high-critical-temperature superconductors has led to a dramatic increase of interest in vortex dynamics. Various models have been developed to understand the collective behavior of a vortex lattice under the influence of external driving forces and/or in the presence of thermal activation [1], For temperatures close to the critical temperature (7V), or magnetic fields close to the upper critical field (B C 2), possible transitions from the well-known hexagonal lattice to a glasslike or liquidlike state have been predicted. Although these concepts strongly influence our understanding of the vortex dynamics, few techniques are available to resolve these issues experimentally. Most researchers focus on a study of the magnetization supplemented with a careful study of current-voltage characteristics. Further progress in the understanding of the dynamics of flux lines, either collectively or individually, may greatly benefit from new experimental information.The most convincing proof of flux flow has been provided by Giaever [2], who studied flux flow in two superposed superconducting films. He showed that currentinduced flux flow in one of the films (primary) induces a voltage in the magnetically coupled secondary film. In this Letter we describe a related system (inset, Fig. 1) in which the secondary superconducting film is replaced by a two-dimensional electron gas (2DEG). The 2DEG is formed at the interface between silicon and silicon dioxide by applying a voltage between the superconducting gate and the silicon. Compared to the original system studied by Giaever this has two important advantages. First, the magnetically coupled films are different in nature and the viscous forces on the vortices in the primary film are independent of the dissipation in the secondary. Second, the dependence of an induced voltage in the secondary on the electronic properties of the 2DEG can be studied by varying the voltage between the gate and the silicon. As expected [3,4], we find that moving flux lines in the (low-7V) superconducting film induce a voltage in the 2DEG. Two contributions to this voltage are found, one proportional to the magnetoresistance, which is varied through the electron density, and one roughly independent of electron density. Apart from their use in the study of vortex dynamics, these observations will also raise interesting new questions with resp...