The discovery of superconductivity in MgB2 raised the following problem: what is the nature of this superconductor? Its critical temperature is close to 40 K, which is quite high (higher than the critical temperature of the cuprate compound discovered by Bednorz and Müller in 1986) whereas its chemical composition tends to look like conventional superconductors, since it contains neither oxygen nor copper. MgB2 seems to lie in a grey area between conventional superconductors, which are well understood, and high-temperature superconductors, for which many questions have not found an answer yet.
The originality and possible industrial potential of MgB2 first aroused the curiosity of many scientists in the world: websites listing the electronic publications have counted hundreds of articles on the matter only a few months after the announcement of the discovery of superconductivity. These websites have enabled different laboratories to communicate quickly and provoked a great effervescence, enabling quick progress.
In order to try and understand high-temperature superconductors, many teams interested in superconductivity have conducted experiments and developed more and more efficient theoretical tools. The scientific community has expended all its energy to study this new and yet simple compound; in less than two years, a consensus on the mechanisms responsible for this superconductivity has appeared.
The structure of MgB2 is quite simple: hexagonal layers made of bore atoms, separated by magnesium atoms lined up on the centres of these hexagons. This simplicity enables reliable theoretical and numerical studies. These studies have proved the existence of two electron families in this compound, with almost no exchange between the two families, which is extremely unusual in metal compounds. This model was called “two-band model”. The idea of a two-band model is actually quite old and had been proposed soon after the BCS theory was formulated. However, the effect of a second band was then considered insignificant on the measurements, because it seemed impossible that two bands with very different superconducting properties could coexist.
What makes MgB2 so unique is that it forms a system in which both these families include about the same amount of electrons, with very different properties; one family is made up of electrons located in the boron atoms layers, while the electrons of the other family can move in every direction. The electrons in the boron layers have a strong interaction with the vibrations of the boron atoms; it is because of these unusual interactions that the standard mechanism creating Cooper pairs can explain why superconductivity appears at such high temperatures according to the BCS theory. The appearance of superconductivity in this electron family leads to the superconductivity of the other electrons, a little reluctantly, because these electrons do not have any particular coupling.
This two-band model appeared very quickly, and was soon confirmed by experiments. Different experimental techniques have proved the presence of these two electron families and their properties: measurements of specific heat and thermal conductivity, experiments with scanning tunneling microscopes… In less than two years, this model has become obvious since the experimental and theoretical indications tallies perfectly.
Measurements of thermodynamic properties depending on the temperature and the magnetic field, and comparison with the classical BCS theory predictions (in red) and the two-band model (in green). The two-band model describes the experimental results better.
The behaviour of this compound in presence of a magnetic field is also unusual: the two electron families react differently to the applied magnetic field. The electron family enabling the existence of superconductivity enables the existence of vortices, small tubes in which the magnetic field can enter, above a critical value (we call it type II). The existence of these vortices is imposed to the electrons of the second band, although these are more type I electrons, which means there should not be any vortex. A complicated compromise is established between the two families; although in a type II superconductor, vortices always expel each other, in MgB2, they group together when the magnetic field is weak enough. We call them type “1.5” superconductors, a term that was invented in this particular case to describe this intermediate behaviour between type I and type II… (cf. this figure, and the corresponding article).
Although MgB2 is a unique compound, the idea that two families could coexist with very different properties has found other examples, the most striking of which being the case of the superconductors based on arsenic, pnictides.
Since superconductivity in the MgB2 has been explained, many groups have started focusing on other problems. Nevertheless, MgB2 remains an important research subject because of its potential applications thanks to its two main advantages: a high critical temperature and a metal structure much simpler than that of cuprates.A few links :
First reaction when superconductivity was discovered:
http://www.fis.unipr.it/news_fisica/download/cava.pdf
The two first years of research and the physicists' frenzy:
http://www.iitap.iastate.edu/htcu/39K.html
Article published in the journal La Recherche n°369 - novembre, Le premier supraconducteur double:
http://www.larecherche.fr/content/recherche/article?id=4347
