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Superconductors and Superfluids
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Unit 3
Superconductors and Superfluids
The Nobel Prize in Physics 2003 — Popular Information
October 7, 2003
The quantum physics that controls the micro-world has a wide range of spectacular effects that do not normally occur in our ordinary macro-world. There are, however, certain situations in which quantum phenomena are visible. This year's Nobel Prize in Physics is awarded for work concerning two of these situations: superconductivity and superfluidity. Alexei Abrikosov and Vitaly Ginzburg have developed theories for superconductivity and Anthony Leggett has explained one type of superfluidity. Both superconductivity and superfluidity occur at very low temperatures.
Flow without resistance
An unexpected cold effect
1. Problems with conducting electricity
When investigations were first carried out into the nature of electricity in the 19th century, it was evident that metals and certain alloys conduct electricity by allowing electrons to move between the atoms. But the disorganised way in which the electrons move causes the atoms to vibrate, so heat is generated. If the current is too strong, the heat can be so great that the conductor melts. In addition it was found that an electric current through a conductor creates a magnetic field, which in turn generates current in the opposite direction. Electricity and magnetism interact and can thus counteract each other.
2. Superconductivity as a phenomenon which could solve the problem.
In 1911 the Dutch physicist Heike Kamerlingh Onnes made a remarkable discovery. He was particularly interested in the properties of substances at low temperatures and had succeeded in producing liquid helium, which has an extremely low temperature. When Onnes investigated the electric conductivity of mercury, he found that when the metal was cooled by means of liquid helium to a few degrees above absolute zero, its electric resistance vanished. He named this phenomenon superconductivity. Although no theoretical explanation could be found for this phenomenon, it was evident that it could have far-reaching significance in a modern society that was becoming more and more dependent on electricity. Onnes was awarded the Nobel Prize in Physics in 1913 for this work.
Superconductors of two types
1. A long way to the theoretical explanation.
Almost 50 years passed before the physicists John Bardeen, Leon Cooper and Robert Schrieffer (Nobel Prize in Physics, 1972) were able to present a theory (the BCS theory, named after the initials of their surnames) that explained the phenomenon. This theory shows that some of the negatively-charged electrons in a superconductor form pairs, called Cooper pairs. These pairs of electrons flow along attracting channels formed by the regular structure of the positively-charged metal atoms in the material. As a result of this combination and interaction the current can flow evenly and superconductivity occurs. The paired electrons are usually thought of as a condensate, similar to the drops of liquid that form in a cooled gas. Unlike an ordinary liquid this “electronic liquid” is superconductive.
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2. Type-1 superconductors counteracting a magnetic field.
These superconductors are called type-I. They are metals and are characterised by the Meissner effect, that is, in the superconductive state they actively counteract a surrounding magnetic field as long as its strength does not exceed a certain limit (fig. 1). If the surrounding magnetic field becomes too strong, the superconductive property disappears.
| Fig. 1. Type-I superconductors repel a magnetic field (the Meissner effect). If the strength of the magnetic field increases, they lose their superconductivity. This does not happen with type-II superconductors, which accomodate strong magnetic fields by letting the magnetic field in | 
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| 3. Peculiar type-2 superconductors. |
But it is known that there are superconductors that lack or show only a partial Meissner effect. These are in general alloys of various metals or compounds consisting of non-metals and copper. These retain their superconductive property even in a strong magnetic field. Experiments show that the properties of these so-called type-II superconductors cannot be described by the BCS theory.
4. Abricosov’s new theory for explaining the effect of a magnetic field on superconductivity.
Alexei Abrikosov, working at the Kapitsa Institute for Physical Problems in Moscow, succeeded in formulating a new theory to describe the phenomenon. His starting point was a description of superconductivity in which the density of the superconductive condensate is taken into account with the aid of an order parameter (a wave function). Abrikosov was able to show mathematically how the order parameter can describe vortices and how the external magnetic field can penetrate the material along the channels in these vortices (fig. 2)
| Fig. 2. This image is of an Abrikosov lattice of vortices in the electron fluid in a type-II superconductor. The magnetic field passes through these vortices |
Abrikosov was also able to predict in detail how the number of vortices can grow as the magnetic field increases in strength and how the superconductive property in the material is lost if the cores of the vortices overlap. This description was a breakthrough in the study of new superconducting materials and is still used in the development and analysis of new superconductors and magnets. His papers from the late 1950s have been quoted more and more frequently during the past ten years.
5. Theoretical base for the new approach.
The theory Abrikosov's argument was based on was formulated in the early 1950s by Vitaly Ginzburg and Lev Landau (the latter was awarded the Nobel Prize in Physics in 1962 for other work, see below). This theory was intended to describe superconductivity and critical magnetic field strengths in the superconductors that were known at that time. Ginzburg and Landau realised that an order parameter (wave function) describing the density of the superconductive condensate in the material had to be introduced if the interaction between the superconductor and magnetism was to be explained. When this parameter was introduced, it was evident that there was a breakpoint when a characteristic value approximately 0. 71 was reached and that in principle there were two types of superconductor. For mercury the value is approximately 0. 16 and other superconductors known at the time have values close to this. There was therefore, at that time, no reason to consider values above the breakpoint. Abrikosov was able to tie up the theory by showing that type-II superconductors had precisely these values.
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6. Achievements in the development of high temperature superconductors.
Our knowledge of superconductivity has led to revolutionary applications (fig. 3). New compounds with superconductive properties are being discovered all the time. In the past few decades a large number of high-temperature superconductors have been developed. The first one was produced by Georg Bednorz and Alex Mü ller, who were awarded the Nobel Prize in Physics in 1987. All high-temperature superconductors are type-II. Cooling is a critical factor for the utilisation of superconductors. An important limit is 77 K (-196°C), the boiling point of liquid nitrogen, which is cheaper and more manageable than liquid helium.
| Fig. 3. An MRI image of a human brain. The resolution in the magnetic resonance camera is dependent partly on the strength of the magnetic field. Today strong superconducting magnets are used, all of them type-II | 
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