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samples 3 and ballistic GaAs structures 4 showed the presence of higher harmonics, but the results were not understood completely. An experimental disadvantage of these systems is the fact that it is very difficult to apply an abrupt voltage drop across the sample. The voltage drop from one lead to the other is determined by the microscopic arrangement of impurities. This problem is circumvented when the wire is interrupted by a tunnel barrier. 5 In this system the voltage drops almost completely over the tunnel barrier, since the resistance of the tunnel junction is much larger than the resistance of the metal leads. The ability to apply an abrupt voltage drop is a necessary condition for a systematic study of higher harmonic generation. In this paper we experimentally explore the generation of higher harmonics in mesoscopic wires and rings. We study both the harmonic content of aperiodic conductance fluctuations and periodic Aharonov-Bohm oscillations and demonstrate the significantly different nonlinear response. The experiments are quantitatively described within a theoretical framework. The higher harmonic generation has been studied in two types of samples: an Aharonov-Bohm ring interrupted by two small tunnel junctions and a metal wire interrupted by one tunnel junction. The mask of the samples was defined by electron beam lithography in a double layer PMMA ~polymethylmethacrylate! resist. Using a shadow evaporation technique the overlap Al-Al 2O 3-Al tunnel junctions were formed. The width of the arms of the aluminum ring is 60 nm and the circumference of the loop is 3.2 mm. The total thickness of the wires is 65 nm. By determining the overlap areas of the two junctions we conclude that the conductances of the two junctions do not differ by more than 10%. This is confirmed by critical current measurements in the superconducting state in which the device operates as a SQUID. The tunnel conductance GT of the two junctions in parallel is 125 mS. The wire interrupted by one tunnel junction is fabricated in the same way as the ring. The width of the left and right parts of the wire are 200 and 400 nm, respectively. The thickness of the wire is 65 nm. The tunnel conductance of the single junction is 57 mS. Both the Aharonov-Bohm loop and the single wire are capacitively coupled on-chip to a low impedance platinum shunt resistor to create a well-defined electromagnetic environment. Measurements were performed in a dilution refrigerator at a base temperature of 20 mK. Electrical leads were carefully filtered by microwave copper powder filters andRC filters at base temperature. At room temperature the leads were additionally filtered byP filters. The sample was mounted in two microwave-tight copper boxes at base temperature and an additional shield at 1 K. A sinusoidal current was injected into the sample at n517 Hz. The voltage response was detected using a lock-in amplifier synchronized to one of the higher harmonics Nn. The samples were biased with a current I5 I

Giant higher harmonic generation in mesoscopic metal wires and rings interrupted by tunnel junctions