Josephson voltage standards with series arrays of up to 20 000 tunnel junctions can generate stable reference voltages at arbitrary values between - 15 V and + 15 V. These standards, which are now installed in many national and industrial laboratories, are used to calibrate Weston cells or Zener reference standards, to measure the linearity of digital voltmeters, and as precise potentiometers for the measurement of voltage and resistance ratios. The fundamental principles, the historical development, the design, the fabrication and the operation of these voltage standards are reviewed.
Josephson arrays containing 2000 and 20 160 Nbl AI2 0 3 INb tunnel junctions produce stable quantized voltage steps up to 1.S and 14.5 V, respectively, with current amplitudes on the order of 100 !tA. In precision measurements these voltage steps proved to be stable for typically 30 min. Fabricating the voltage standards with the help of the Nbl All OJ INb technology results in a long durability.
A complete solution to the Mattis–Bardeen equations of the anomalous skin effect in superconductors [Phys. Rev. 111, 412 (1958)] is presented in the case of plane, bulk conductors. This solution shows good agreement with existing solutions in the microwave region, and for the first time, it correctly describes measurements in the far-infrared region. It turns out that the solution to the Mattis–Bardeen equations for the extreme anomalous limit cannot be used for a correct description of experimental results. In addition, our exact solution is also applicable to strong-coupling superconductors.
By scanning superconducting Nb/Al&03/Nb tunnel junctions of various geometries with an electron beam at low temperatures we have obtained spatially resolved two-dimensional images related with the dynamics of the junctions. In a current-biased junction the local thermal perturbation effected by the beam results in a voltage change hV representing a convenient imaging-response signal. The spatial resolution is limited by a characteristic thermal healing length with a value of about 2 pm in our experiments. We analyze our results in terms of the sine-Gordon equation. For the interpretation of the spatially resolved voltage signal 5 V we extend the energetic analysis of the perturbed sine-Gordon system of McLaughlin and Scott by including the beam-induced local thermal perturbation. Our model allows the quantitative comparison of our experimental two-dimensional images with the local dynamics expected from the perturbed sine-Gordon equation. Our experiments include the observation of single-mode and multiple-mode cavity resonances, soliton oscillations, and Aux-How behavior.
At PTB, a fabrication process has been developed in SNS Nb/PdAu/Nb technology for the verification of small Josephson junctions (JJs) in the deep sub-micron range to enable the implementation of JJs as active elements in highly integrated superconducting circuits. Two steps of this technological development are described with regard to appropriately designed circuit layouts of JJ series arrays (JJAs), the first one in a conventional window type junction (WTJ) configuration and the second one in a ramp type junction (RTJ) configuration. Test circuits of JJAs containing up to 10 000 JJs have been fabricated and experimentally tested. In WTJ configuration, the circuits proved to be sensitive to external perturbing effects affecting the stability of circuit operation. In contrast to that, in RTJ configuration, the circuits realized showed correct function and a high grade of reliability of operation. To produce RTJ circuits, the technology parameters have been set to realize JJs with contact areas of A = 0.25 µm × 1.3 µm. At a thickness of the PdAu normal metal layer of d = 40 nm, the values achieved for the critical current density and for the product of critical current and normal state resistance are about j c = 200 kA cm −2 and about I c R N = 21 µV.
We have investigated the dependence of the critical current I C on the value and orientation of an externally applied magnetic field H for interface-engineered YBa 2 Cu 3 O 7Ϫx ramp-type Josephson junctions. The results are compared with measurements of Nb ramp-type junctions with a PdAu interlayer. The I C versus H dependences are similar to Fraunhofer patterns and their modulation period changes several orders of magnitude with the orientation of the magnetic field. For both junction types, the dependence of the modulation period on the orientation of the magnetic field can be well described by the change of the relevant projection of the junction area and the influence of flux-focusing. Therefore the features of the I C (H) curves have to be attributed to the ramp geometry and not to specific properties of the superconducting material.
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