Superconductors of two types

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.

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

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.

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.

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.

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 Müller, 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