Magnesitum diborode (MgB₂): The surprising superconductor of 2001

• The recent surprise discovery of superconductivity in magnesium diboride (MgB₂) at 39 K has led researchers to wonder whether the origin of the high T_c is an electron-phonon or other exotic mechanism? By combining first-principles calculations and inelastic neutron scattering measurements of the phonons in MgB₂, we offered the first direct answer to this fundamental question. The calculations reveal that the in-plane boron phonons (with E_2g symmetry) near the zone-center are very anharmonic. Moreover these modes strongly couple to the partially occupied planar B-sigma electronic bands near the Fermi level. We showed that this giant anharmonicity and non-linear electron-phonon coupling is the key to quantitatively explaining not only the observed high T_c and boron isotope effect in MgB₂ but also the pressure dependence of the T_c. The excellent agreement between theory and our inelastic neutron scattering measurements of the phonon density of states gives confidence that the calculations provide a sound description of the physical properties of the system. This work (Phys. Rev. Lett. 87, 37001 (2001)) has revived a great deal of interest within the physics community as evidenced by recent articles in Physical Review Focus (July 2001), in September 2001 issue of Materials Today. I have also been invited to present this work at the upcoming APS March Meeting, the Workshop on Spectroscopies of Novel Superconductors (SNS2001) held in Chicago, the International Symposium on Superstripes held in Rome, and Argonne National Lab.

  More information can be found at http://www.ncnr.nist.gov/staff/taner/mgb2 (opens a new window). This website has been visited over 2500 times, another indication of the impact of our work in this area.

Fullerene Superconductors:

• The discovery of superconductivity in A3C₆₀ compounds where A is K, Rb, or Cs sparked a great deal of interest in the condensed matter physics community. It was quickly realized that the superconducting transition temperature depends strongly on the lattice constant which could be easily controlled by changing the alkali metal or by applying external pressure. This dependence was attributed to the changing density of electronic states at the Fermi level. To explore this idea further, one would like to adjust the carrier concentration in these systems. Unfortunately this was fixed at 3 electrons per C₆₀due to the narrow solubility range in the phase diagram of these compounds. We overcame this problem by synthesizing two novel families of fullerides: Na₂Cs_x C₆₀ and M _3-x Ba_xC₆₀, thereby allowing the first studies of the correlation between the superconducting transition temperature and the carrier concentration (see Physical Review Letters 77, 167 (1996)). This work clearly demonstrated that the mechanism of superconductivity in these systems is unique and quite different than that in the high T_c cuprates. I have made invited presentations on this subject at the Euroconference on Novel Superconducting Materials in Pisa, Italy (Jan. 1996), at the Electrochemical Society Meeting in Los Angeles (May 1996) and at many universities and research centers.