SQUIDs, the Josephson Effects and Superconducting Electronics

J.C Gallop

• Publisher: CRC Press
• Year: 2017
• ISBN:
  9781351414944
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

The science of superconducting electronics was first developed over forty years ago, fifty years after the discovery of superconductivity. Since then, a wide range of applications has emerged, and more are envisaged within this ever expanding and exciting field. SQUIDs, the Josephson Effects and Superconducting Electronics chronicles this development from fundamental principles to the present work with high-temperature superconductors. The book discusses superconductivity, Josephson effects, and detectors of unparalleled sensitivity such as SQUIDs. It punctuates theory with practical discussions on how to harness this new science. This complete guide to the subject is an invaluable resource for graduate students and researchers with a specific interest in this field. It also provides guidance to those working in areas of industry where superconducting electronics could be applied.

1

Introduction to Superconductivity

1.1 Basic Properties

The universally known fact about superconductivity is that below some characteristic transition temperature (T_c) many metals exhibit a complete lack of electrical resistance to a flow of direct current. Such a lossless ‘supercurrent’ can be set up around a closed loop of superconducting wire, there being no detectable decay with time (with a half-life shown experimentally to be at least 10⁶ years).

This complete loss of resistance was first observed by Onnes (1911) for the metal mercury for which T_c = 4.19 K, close to the normal boiling point of liquid helium. The latter had been liquefied for the first time only shortly before in the same laboratory, thus opening up a new field of cryogenics.

1.1.1 Materials exhibiting superconductivity

Up to the present some 28 out of the 75 metallic elements have been shown to exhibit zero resistance with transition temperatures ranging from 0.005 K for rhodium up to 9.2 K for niobium. There are a vast number of alloy and compound superconductors. At the time of writing the highest known T_c is 125 K for a perovskite structured ternary oxide of thallium, calcium, barium and copper (see Chapter 9). The current frenetic activity to raise T_c to as high a value as possible is linked to the reduced cost of refrigeration which this would incur, and hence to a far greater application of superconducting devices in the world outside of research laboratories. Many other materials also exhibit superconductivity, including a number of non-metallic compounds and some organic salts, a few of which have T_c’s which only a few years ago would have been regarded as remarkable. At the time of writing these have no apparent practical potential, although they reflect clearly the increasingly widespread but complex nature of the superconducting state.

1.1.2 Critical magnetic fields

From the earliest years the remarkable properties of superconductors have led to many proposed applications of the effects. Onnes himself suggested that high-field magnets having zero electrical losses could be made using superconductors. However the loss-free current-carrying capability of all these materials is very dependent on magnetic field, as well as on temperature. Above some critical field H_c, characteristic of the material and temperature, superconductivity is destroyed. The field generated by a current flowing in the superconductor itself must also be considered, so that even in zero external magnetic field there is a maximum supercurrent which can flow. This is a function of geometry as well as temperature and material. Onnes’ original hopes for high-field solenoids took many years to realise, as the critical fields of elemental superconductors are too low to be useful. Amongst these the highest occurs for the element niobium, with a value of around 0.2 T, a factor of at least ten less than can be achieved with iron-cored solenoids.

1.1.3 Large-scale and high magnetic field applications

Some 50 years after Onnes’ discovery a number of practically useful alloy superconductors, mainly based on niobium, began to be developed. These alloys have critical fields as great as 40 T, sufficient to replace high-field conventional solenoids, since the extra cost of refrigeration etc is far outweighed by the cost of power consumption through ohmic losses in normal metal coils.

As well as providing a reduction of power consumption, superconducting magnets are also able to provide very spatially and temporally stable magnetic fields. Superconducting solenoids may be operated in the ‘persistent mode’, in which the ends of the windings are joined so that no external current source is required. The stability of high magnetic fields produced by the best persistent supercurrent solenoids has been shown to be better than 1 part in 10¹⁰ per hour (Petley 1989). The ability to use much higher current densities in superconducting magnets means that solenoid windings may be tailored to produce fields which are extremely spatially uniform (to better than 1 in 10⁸ over a volume of 10⁻⁶ m³).

Apart from magnets, superconducting windings may also be used to improve the efficiency of motors and generators. A limited number of pilot applications of this kind have already been demonstrated. A further expansion of these ‘electrical engineering’ applications will probably not happen unless and until there is a new round of construction of even larger electrical generating sets. The subject of this book is ‘superconducting electronics’, dealing with devices that rely entirely on the extreme sensitivity of superconducting circuits to very low magnetic fields. Such applications do not require vast capital investment and hence may be expected to proceed to expand in a steady way.

Superconductors are not truly lossless in applied alternating electromagnetic fields. In addition to being a strong function of temperature, particularly in the region from T_c down to roughly T_c/3, the losses are also a rapidly increasing function of frequency of the applied oscillating field. ‘Passive’ AC applications of superconductivity, which rely only on low-loss properties, have so far been limited to high-Q microwave cavities. These have been used as frequency standards, narrow band filters and, in higher-power operations, as components in particle accelerators.

1.2 The Meissner Effect

Applications of superconductivity of the electrical engineering type outlined above depend wholly on the property of zero electrical resistance. In fact, superconductivity ...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Introduction
• 1 Introduction to Superconductivity
• 2 The Josephson Effects
• 3 Superconducting Quantum Interference Devices
• 4 DC SQUIDS
• 5 Applications of SQUIDS to Analogue Measurements
• 6 High-frequency and Digital Applications of the Josephson Effects
• 7 A Practical Guide to Using SQUIDS
• 8 Fundamental Physics with SQUIDS
• 9 High-temperature Superconductors and SQUIDS
• Appendix A: Electrodynamics of Superconductors
• Appendix B: Mechanical Analogue of the Classical Josephson Effects
• References
• Index