No abstract

Josephson junctions have been realized in which two superconducting magnesium-diboride (MgB2) layers are separated by a thin MgO barrier layer, using the ramp-type configuration. Their current–voltage characteristics follow the behavior described by the resistively shunted junction model, with an excess current of about 30% of the critical current Ic. A suppression of 70% of Ic was achieved in applied magnetic fields. Shapiro steps were observed by irradiating the junctions with 10.0 GHz microwaves, and the dependence of the step height on applied rf current is well described by a current–source model. Reference samples prepared without the MgO layer showed strong-link behavior with large Ic values.

The remarkably high critical temperature at which magnesium diboride (MgB 2) undergoes transition to the superconducting state, T c ≈ 40 K, has aroused great interest and has encouraged many groups to explore the properties and application potential of this novel superconductor. For many electronic applications and further basic studies, the availability of superconducting thin films is of great importance. Several groups have succeeded in fabricating superconducting MgB 2 films. An overview of the deposition techniques for MgB 2 thin film growth will be given, with a special focus on the in situ two-step process. Although, meanwhile, many problems to obtain suitable films have been solved, such as oxygen impurities and magnesium volatility, the question of how single-phase epitaxial films can be grown still remains. The possibility of growing single-crystalline epitaxial films will be discussed from the deposition conditions' point of view as well as substrate choice. Necessary conditions are discussed and possible routes are reviewed. The applicability of MgB 2 in superconducting electronic devices depends on the possibility of making well-controlled, i.e., reproducible and stable, Josephson junctions. The first attempts to make MgB 2-MgO-MgB 2 ramp-type junctions and SQUIDs from MgB 2 nanobridges are discussed.

Since the recent discovery of superconductivity in MgB 2 , various groups worldwide have been actively studying the growth of thin films based on this material. Impressive progress has been made, but various materials science challenges are still left to be solved. Guided by our own activities in this field, and reports presented in the literature, we reflect here on those challenges and possible ways for further improvement. Being important ingredients for many electronic applications, fabrication of Josephson devices and nano-structures in MgB 2 thin films will furthermore be described.

Abstract-we have successfully fabricated and tested a high temperature superconducting sigma-delta A/D converter. The quantizer is a balanced comparator that has been characterized separately in two temperature regimes. The circuits have been fabricated with ramp edge junctions with a PrBaCuGaO-barrier on a buried ground plane. For the current to voltage conversion in the sigma-delta converter we fabricated a 50 mOhm resistor with an in-situ gold layer. The sigma-delta converter has been tested at an internal clock of up to 174 GHz. The signal-to-noise ratio has been measured at a relatively low frequency of 3.4 kHz and was at least 63 dB but most likely higher since the measurement was limited by the noise in the amplifiers. As a first attempt towards the development of a decimation filter we have fabricated and tested a toggle flip-flop. The toggle flip-flop has been tested successfully at 40 K up to a frequency of 33 GHz, which corresponds to 70 % of the Isn-product of a reference junction.h d e x Terms-Balanced comparator, oversampling, AD converter, toggle flip-flop.