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The differential resistance of NbN/two-dimensional electron gas ͑2DEG͒ contact is measured at high magnetic fields. In zero magnetic field the contact shows a pronounced resistance peak at zero bias due to a high barrier at the NbN/2DEG interface, which decreases if a magnetic field is applied in the plane of the 2DEG. For a magnetic field oriented perpendicular to the plane of the 2DEG, not only the zero-bias resistance decreases but so does the normal-state resistance which drops to vanishingly small values. A pronounced substructure due to a splitting of the resistance peak into a three-peak structure is observed at high perpendicular fields. We suggest that the appearance of this substructure can be explained by multiple Andreev reflections due to skipping orbits of electrons and holes accompanied by inelastic scattering in the 2DEG near the interface.

We studied the behavior of Josephson junctions in a two-dimensional array under the influence of microwave irradiation. We are able to distinguish between junctions that are phase locked to the external microwave and unlocked junctions by means of spatially resolving measurements. The degree of coherence in the array can be influenced by tuning the external rf power. Analogies to the coherent behavior of atoms in a laser are pointed out.

We investigated niobium-AlGaSb/InAs-niobium hybrid structures using a high mobility two-dimensional electron gas as a weak link. The Josephson current observed in this structure was suppressed by an injection current driven into the weak link via an additional normal lead. Using a four-terminal configuration the supercurrent is suppressed all over the weak link. In a three-terminal configuration it was possible to suppress the supercurrent locally. ͓S0163-1829͑99͒06217-7͔The semiconductor/superconductor hybrid structure has been the subject of extensive research during the last years, to prove its suitableness as an interface between superconductor and semiconductor electronics. Especially the control of the supercurrent in a weak-link structure has attracted considerable interest in recent years. 1 One approach was to transfer the well-known semiconductor field effect transistor to superconductor/two-dimensional electron gas ͑2DEG͒ structures as proposed by Clark et al. 2 The supercurrrent flowing through the 2DEG was switched off by reducing the coherence length in the channel by applying a negative gate voltage. 3,4 Since the voltage necessary for depleting the electrons in the 2DEG channel is typically of the order of the band gap of the semiconductor and the output signal of the order of the superconducting gap of the electrodes, a voltage gain is hard to achieve. An alternative concept to avoid this problem is controlling the supercurrent by the injection of electrons via an additional nonsuperconductive contact into the weak link. [5][6][7][8] The energy of these carriers should be only of the order of the superconducting gap of the electrodes. Very recently, the control of the supercurrent by an injection current was experimentally succeeded for a Nb/Au weak link structure. 9 This system was in the diffusive regime with inelastic scattering processes thermalizing the injected carriers.In our current-controlled structure the superconductor Nb was contacted to a high-mobility two-dimensional electron gas ͑2DEG͒ in an Al 0.2 Ga 0.8 Sb/InAs heterostructure. Due to the high electron mobility the carrier transport from one Nb electrode to the opposite one can be considered to be ballistic. In addition inelastic scattering should become relevant only for higher temperatures. For weak links without inelastic scattering an even more efficient tuning of the Josephson current was predicted compared with the case of inelastic scattering present. 10 It will be shown that if a current is injected via Ohmic contacts into the 2DEG in a four-terminal configuration the supercurrent can be suppressed completely. In a three-terminal configuration it was possible to suppress the supercurrent locally, which was proven by measuring and fitting the interference patterns of the Josephson current appearing when a magnetic field is applied to the weak link.The heterostructure which forms the weak link in our structure was grown by molecular-beam epitaxy on a semiinsulating GaAs substrate. After a 1100 nm Al-Ga-Sb buffer layer, a 5...