K. Blüthner

Josephson voltage standard at low drive frequencies

The authors report on the design and operation of a superconducting microstrip resonator containing four series-connected highly hysteretic Josephson junctions in the top layer. This arrangement generates constant-voltage steps at X-band drive frequencies useful for a Josephson voltage standard. At 12 GHz, steps up to 5 mV have been obtained with step amplitudes of about 100 mu A. The resonator principle can be used to save microwave power and to simplify the microwave circuit. To obtain sufficiently large and stable steps the critical current density and the junction length must be matched carefully with the order of maximum step used for the operation of the Josephson voltage standard. The X-band microwave drive frequencies offer substantial advantages in terms of costs and handling of microwave equipment.<<ETX>>

Preparation of self‐aligned in‐line tunnel junctions for applications in single‐charge electronics

The self‐aligned in‐line technique has been applied to the preparation of ultrasmall low‐capacitance metallic tunnel junctions. By using e‐beam lithography the area of Al/AlOx/Al contacts has so far been reduced to less than 0.005 μm2. At low temperatures high‐ohmic double junctions with a small metallic island between them show the Coulomb blockade effect. The current through such a device could be modulated by a voltage applied to a gate electrode capacitively coupled to the island (single‐electron transistor). Both single‐charge phenomena have been observed at temperatures up to 1 K.

Single-electron transistors based on Al/AlO/sub x//Al and Nb/AlO/sub x//Nb tunnel junctions

As an alternative to the shadow evaporation method for the preparation of ultrasmall tunnel junctions we have established the so-called self-aligned in-line technique. It was applied to the fabrication of common Al/AlO/sub x//Al-type and, for the first time, Nb/AlO/sub x//Nb-based single-electron transistors. The characterization of the samples at temperatures in the range of a few hundred millikelvins reveals charging effects (Coulomb blockade and gate modulation) of the quasiparticle current.

A 1-V series-array Josephson voltage standard operated at 35 GHz

Josephson voltage standards utilize microwave‐induced constant voltage steps in the dc characteristic of Josephson tunnel junctions. This paper describes the design and operation of array circuits with 108 and 2000 junctions connected in series. In contrast with similar realizations, simple Q‐band equipment is used for the microwave supply. The microwave attenuation of 1000 junctions was about 1 dB. The version with 2000 junctions generated Josephson voltages up to 1.2 V when operated at 35 GHz. The stability times of the quantized levels were, under normal laboratory conditions (unshielded room), better than 10 min.

Application of high performance DC SQUIDs in precision measurement technique

In this review the performance of a DC SQUID system with Nb-NbO/sub x/-Pb/In/Au window-type Josephson junctions is described. The SQUIDs and the SQUID control units were developed for universal applications in precision measurement technique. Under optimum conditions a flux noise level of 2/spl times/10/sup -6/ /spl Phi//sub 0///spl radic/Hz was achieved corresponding to an energy resolution of 3/spl times/10/sup -31/ J/Hz. The contribution gives the design of the DC SQUID device and summarizes recent results of its application in precision measurement technique. The applications include a cryogenic current comparator for non-destructive measurement of the beam intensity produced in a particle accelerator, a high-sensitive SQUID voltmeter for investigations of the low-field Hall coefficient of high purity metals, the upgrade of a multichannel RF SQUID neuromagnetometer for biomagnetic research, and a planned high-precision experiment in fundamental physics.<<ETX>>

Microwave circuit for the generation of Josephson voltages at low frequencies

Using a double-resonator microwave circuit with 216 Josephson junctions driven at a frequency of 11.1 GHz we have obtained overlapping Shapiro steps up to 0.16 V with step widths of about 25 mu A. Chaotic instabilities can be tuned out by the microwave power.