The discovery that magnesium diboride is a superconductor with a transition temperature of 39 K is currently the ‘hottest’ paper in physics according to the Institute for Scientific Information in the US. Although 39 K does not qualify magnesium diboride for status as a high-temperature superconductor, the transition temperature of this relatively simple material is much higher than predicted by naive theories. Now theorists in the US have proposed that the anomalous superconducting properties of magnesium diboride can be explained by the presence of two rather than one superconducting energy gaps in the material (H J Choi et al 2002 Nature 418 758).
Superconductivity occurs in a material when electrons bind together to form Cooper pairs, which can travel through the materials without any resistance. Electrons normally repel each other but in low-temperature superconductors they can overcome their mutual repulsion by interacting with lattice vibrations known as phonons. (The binding mechanism in high-temperature superconductors is still a mystery). The energy gap is essentially the energy needed to break the pairs apart: it also determines the thermodynamic properties of the material and is directly related to the superconducting transition temperature.
Most superconductors have just one energy gap but experiments suggested that magnesium diboride might have more than one. Now Steven Louie, Marvin Cohen and co-workers at the University of California at Berkeley and the Lawrence Berkeley National Laboratory have performed ab initio calculations which strongly suggest that magnesium diboride does indeed have two energy gaps. The gaps correspond to transition temperatures of 15 K and 45 K, and combine to give an overall transition temperature of 39 K. Their model can also explain the unusual variation of specific heat capacity with temperature that has been measured in magnesium diboride.
Magnesium diboride is a metal with a layered structure in which the boron atoms form hexagonal layers and the magnesium atoms are located in between the boron layers, above and below the centres of the hexagons. The unusual properties of the material are thought to result from strong interactions between phonons and electron orbitals in the boron layer. The Berkeley team predicts that other layered materials based on boron, carbon and nitrogen should also display similar or higher transition temperatures.