Various types of thin-film dc gradiometer SQUIDs with integrated pickup loops were investigated. The SQUIDs were prepared on a 5 mm × 10 mm chip in the well developed Nb-NbO x -Pb/Au/In technology. It was shown that the SQUIDs work in an unshielded environment. Our best SQUIDs have a field gradient sensitivity better then 30 fT cm −1 Hz −1/2 . All gradiometers have some response to a homogeneous field, i.e., a parasitic area. The effective pickup area, the field gradient sensitivity, and the components of the parasitic area for the field perpendicular and parallel to the gradiometer plane were measured for all types of gradiometer. The calculations of flux coupling efficiency are compared with experimental results. The origins of parasitic area are discussed. A further reduction of external disturbances was achieved by using an electronic second-order gradiometer. Examples of magnetocardiographic measurement in a physical laboratory without any shielding with first and second-order gradiometers are shown.
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,/Al-type and, for the first time, Nb/AIO,/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.
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 I 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 mm.
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