Superconductors have two special properties that allow them to do things that normal conductors cannot do. Both result from phenomena that can only be explained through quantum mechanics. One relates to a quantization of magnetic fields. The second is responsible for the name “superconductors,” for these materials have the capability of carrying electrical current totally without any electrical resistance.

SUPERCONDUCTIVITY

When cooled to sufficiently low temperatures, some metals, alloys of metals, and compounds suddenly lose all trace of electrical resistance, each at its own specific low critical temperature (T_c). This phenomenon, called superconductivity, was discovered by the Dutch scientist Kamerling Onnes in 1911 in mercury cooled in liquid helium. Physicists John Bardeen, Leon Cooper, and Robert Schrieffer, in 1972, received the Nobel Prize in Physics for work done in 1957: “For their jointly developed theory of superconductivity, usually called the BCS theory.”1 The theory has been of considerable help in guiding the search for superconductors with higher critical temperatures, though new high-temperature² superconductors (to be described) have shown a need for additional theory.

Below the critical temperature, each electron of plus ½ spin is found (in a complex way) to “pair up” with another electron of minus ½ spin to form a quasiparticle with a net spin of zero. Particles or quasiparticles with zero spin, unlike spin ½ particles, experience no exclusion. They tend to collectively occupy a single lowest-energy superconducting state. But the entire sea of electrons must be in this state at the same time. And any collision (even of one quasiparticle with an impurity that distorts the lattice) has to be of sufficient energy to break apart the entire collection of paired particles. (It's as if an army of policemen with locked arms was to march down a street. It would be a difficult thing to break through all of those locked arms simultaneously to disrupt the march.) And so the entire collection of electrons remains impervious to collisions with impurities—and impervious to the lattice oscillations that normally contribute to electrical resistance, so long, of course, as the superconductor is kept below its critical temperature and below a certain critical current level.3 In this superconducting state there is no resistance, that is, no heat is generated, and only a relatively small amount of refrigeration is needed to remove any heat that leaks into the cold region from the outside world (through a Thermos-bottle-like type of thermal insulation).

MAGLEV—MAGNETICALLY LEVITATED TRAINS

One of the more exciting commercial developments of superconductivity is magnetically levitated trains. Trains using permanent magnets and more conventional electromagnet technology have also been under development, and some are already operational.

The Japanese have been developing the world's most advanced superconducting maglev train as a successor to their famous Shinkansen (bullet train). One of the maglev trains is shown in Figure 19.1 on its Yamanashi test track. Another of their trains (in 2003) set a world's record for high-speed trains at 361 miles per hour (581 km/h).

The “track” is not really a track in the conventional sense. The train has wheels for operation at slow speeds. But once the train gets moving, the strong constant magnetic fields of superconducting magnets under the cars of the train induce electrical currents in an electrically conducting guideway. These induced currents in turn produce magnetic fields that repel the magnets on the train, and at a sufficient speed this repulsion is strong enough to lift the train, wheels-free, above the floor of the guideway. Similar forces keep it centered laterally within the guideway, and a linear electric motor couples magnetically to the guideway to propel the train.

Fig. 19.1. Superconducting magnetically levitated (maglev) five-car train on the test “track” in Yamanashi, Japan, September 5, 2013. (Image from Wikipedia Creative Commons; file: SCMaglev Series L0.jpg; author: Saruno Hirobano. Licensed under CC BY-SA 3.0.)

Both the magnetic levitation and magnetic propulsion allow for a smoother, quieter ride, avoiding the sensitivity to conventional track misalignment experienced with more conventional trains. Alignment, and associated track maintenance and cost, would be a major issue were conventional trains to run at 300 miles per hour.

In 2011, the Japanese gave the go-ahead4 to begin construction in 2014 on a line initially between Tokyo and Nagoya and to enter commercial maglev operation in 2025. Later extensions to Osaka are to be completed in 2045. The estimated travel time for the 272-mile (438-km) Tokyo to Osaka run is just 67 minutes, making it faster by 44 percent than the rapid runs of the present Japanese bullet trains.

MRI—MAGNETIC RESONANCE IMAGING

As Rosenblum and Kuttner nicely summarize, MRI (for which the Nobel Prize in Physics was awarded in 2009) “is made possible by the coming together of the quantum phenomena responsible for nuclear magnetic resonance (NMR), superconductivity, and the transistor.”5 NMR is involved because the quantum spin states of the protons in the nucleus of certain atoms in our bodies are sensitive to the conditions of surrounding tissue, and these are probed using resonant radio waves. The fundamentals of semiconductor operation, and the powerful computers that are involved in producing images from these measurements, are described in Chapters 17 and 23. And the role of superconductivity in providing the rock-solid high steady magnetic fields needed for MRI is described here.

The most successful present commercial use of superconductors is for the MRI scans used in medical diagnostic imaging.⁶ Patients slide into large tubes surrounded by high-field solenoidal magnets within thermally insulating vacuum jackets. These magnets produce a highly uniform base field in the region of the body to be imaged. The magnets are wound from miles of niobium-titanium alloy superconductor wire, and they operate immersed in liquid helium at –452 degrees Fahrenheit (–269 degrees Celsius). At this temperature, air and all other substances are frozen solid.

(The Celsius scale is particularly convenient for the temperatures that we normally experience, with zero degrees set as the melting point of ice and 100 degrees set as the boiling point of water. When considering very low temperatures, however, a Kelvin scale is used. Each degree of temperature is the same size as for the Celsius scale, but the absolute zero of temperature, the lowest temperature theoretically possible, which occurs at –273 degrees Celsius, is defined as the beginning of the Kelvin scale, zero degrees Kelvin. So, one measures temperature upward in Kelvin degrees from absolute zero. The critical temperature of niobium-titanium is at about 9 Kelvin, and magnets made using this alloy are usually operated immersed in liquid helium, which boils at 4.2 Kelvin at standard atmospheric pressure. On the Kelvin scale, ice melts at 273 degrees, so you get the idea: 4.2 Kelvin is pretty cold, everything is frozen solid there except liquid helium!)

An MRI magnet is energized by applying a voltage that drives the increase in the current through its windings to create a uniform, high (~2 tesla) magnetic field7 in the bore (patient region) within the vacuum-jacketed magnet. Then a heated piece of superconductor across the terminals of the magnet is allowed to cool, forming a superconducting short, and all external electrical connections are removed. Because the magnet and its short are entirely superconducting, with almost-perfect superconducting joints, there is essentially no resistance in the electrical circuit, and the current continues to circulate in a very close approximation to perpetual motion. The magnet thus maintains an essentially fixed precise magnetic field. (The field will actually drop very slightly, by less than a millionth of a percent per year. This is because of the small resistance in the joints at the short and between wire sections of the magnet which convert some of the energy stored in the magnetic field to heat.)

Except for this very small amount of heating produced by resistance in the joints between sections of superconductor and connections to the short, no heat is produced in the superconducting winding. But heat does try to leak through the vacuum jacket from the outside. Most of this heat is intercepted using a cryocooler, a small refrigerator that you may hear making a continuous clicking, thudding, or pulsed hissing sound in the background.⁸ The small amount of heat that does get through the vacuum jacket and past the refrigeration slowly boils away the liquid helium (which, like water when it is boiled, holds temperature at its boiling point until it is entirely boiled away). In modern MRI systems, the refill period is once every three years.

OTHER MEDICAL, SCIENTIFIC, AND COMMERCIAL APPLICATIONS

To date, superconductors have found major commercial application only in MRI magnet systems, as just described. However, they have also been used to create highly attracting magnetic fields for the removal of (dark-colored) magnetic impurities in the purification of clays for the manufacture of white fine china dishware. Superconducting quantum interference device (SQUID) magnetometers⁹ have been developed for the ultra-precise measurement of magnetic fields toward both scientific and commercial applications. Though the sale of magnetometers is a relatively small business, the value of ores discovered using SQUID magnetometers is easily in the range of tens of billions of dollars.

Biomedical applications include neuroscience10 and cancer detection.¹¹

One recent development involving SQUIDs is worthy of special mention. A safe ten-minute, noninvasive method, much more accurate than and superior to electrocardiograms, for the quick early detection of heart problems has been developed by Cardiomag Imaging in Latham, New York. The technique uses an array of SQUIDs to measure the very weak magnetic fields produced by the small electrical currents that drive the muscles of the heart. This method has FDA clearance and other regulatory approvals, but not yet the necessary payment reimbursement codes from insurance companies. Cardiomag is arranging the necessary funding to obtain such reimbursement and launch into the large-scale manufacture and marketing of their imaging systems. Once the necessary financing is received for this purpose, the broadly based use of this innovative advancement is projected to save many hundreds of thousands of lives and avoid expenditures of tens of billions of dollars in annual healthcare costs.¹²

Superconducting electronics have aided our search of the cosmos, both in support of the Atacama Pathfinder Experiment and a the Atacama Large Millimeter Array radio telescope activities in the high Atacama desert of Chile.¹³ Finally, I note that developments have been underway off and on since the 1960s to examine the possible uses of superconductors to minimize heating as a major obstacle in making faster and/or more compact computers. I link to one update in August 2015¹⁴ and further note the December 2014 announcement (of the Intelligence Advanced Projects Activity) of a multiyear project to develop a superconducting computer.¹⁵ The most recent and most exciting of these developments involves quantum computing, using Josephson junctions (the key superconducting element involved in SQUIDS) to produce quantum-linked storage bits called qubits. I report more on this development in Chapter 8, on quantum computing, and in its related Appendix C.