In 1911, while studying the properties of matter at very low temperature, the Dutch physicist Heike Kamerlingh Onnes and his team discovered that the electrical resistance of mercury goes to zero below 4.2 K (-269°C). This was the very first observation of the phenomenon of superconductivity. The majority of chemical elements become superconducting at sufficiently low temperature.
Superconducting heroes despite the zeroes
Below a certain “critical” temperature, materials undergo transition into the superconducting state, characterized by two basic properties: firstly, they offer no resistance to the passage of electrical current. When resistance falls to zero, a current can circulate inside the material without any dissipation of energy. Secondly, provided they are sufficiently weak, external magnetic fields will not penetrate the superconductor, but remain at its surface. This field expulsion phenomenon is known as the Meissner effect, after the physicist who first observed it in 1933.
Three names, three letters and an incomplete theory
Conventional physics does not adequately explain the superconducting state and neither does the elementary quantum theory of the solid state, which treats the behaviour of the electrons separately from that of the ions in the crystalline lattice. It was only in 1957 that three American researchers - John Bardeen, Leon Cooper and John Schrieffer - established the microscopic theory of superconductivity. According to their “BCS” theory, electrons group into pairs through interaction with vibrations of the lattice (so-called “phonons”), thus forming “Cooper pairs” which move around inside the solid without friction. The solid can be seen as a lattice of positive ions immersed in a cloud of electrons. As an electron passes through this lattice, the ions move slightly, attracted by the electron’s negative charge. This movement generates an electrically positive area which, in turn, attracts another electron. The energy of the electron interaction is quite weak and the pairs can be easily broken up by thermal energy – this is why superconductivity usually occurs at very low temperature. However, the BCS theory offers no explanation for the existence of “high-temperature” superconductors around 80 K (-193°C) and above, for which other electron coupling mechanisms must be invoked.
Type-I or Type-II, different states
The superconducting state can be destroyed by a rise in temperature or in the applied magnetic field, which then penetrates the material and suppresses the Meissner effect. From this perspective, a distinction is made between two types of superconductors. Type-I materials remain in the superconducting state only for relatively weak applied magnetic fields. Above a given threshold, the field abruptly penetrates into the material, shattering the superconducting state. Conversely, Type-II superconductors tolerate local penetration of the magnetic field, which enables them to preserve their superconducting properties in the presence of intense applied magnetic fields. This behaviour is explained by the existence of a mixed state where superconducting and non-superconducting areas coexist within the material. Type-II superconductors have made it possible to use superconductivity in high magnetic fields, leading to the development, among other things, of magnets for particle accelerators.