An international research team, led by Professor Kosmas Prassides of Tohoku University, has investigated the electronic properties of the family of unconventional superconductors based on fullerenes*1 which have the highest known superconducting critical temperature (Tc) among molecular superconductors*2.
In results published in the American scientific journal Science Advances, the team was able to demonstrate the guiding influence of the molecular electronic structure in controlling superconductivity and achieving the maximum Tc, opening the way to new routes in the search of new molecular superconductors with enhanced figures of merit.
Metals are used for electricity transmission, but energy is lost as heat because of electrical resistance. Superconductors have no electrical resistance and can carry electricity without losing energy, so it is important to find superconductors which can work at the highest possible temperature.
Work by members of the team on molecular fulleride-based systems has previously led to the discovery of the highest working temperature (at 38 K) for a molecular superconductor (Nature Materials 7, p. 367, 2008).
The electronic ground state, which is in competition with superconductivity, was found to be magnetically ordered (Science 323, p. 1585, 2009). And the zero-resistance superconducting state could be switched on by tuning the exact arrangement of the C60 molecules in the solid by external pressure (Nature 466, p. 221, 2010).
The controlling role of the molecular electronic structure was then identified by demonstrating that the parent insulating state involves Jahn-Teller distortion*3 of the molecular anions that produces the magnetism from which the superconductivity emerges (Nature Communications 3, 912, 2012).
The research team has addressed for the first time the relationship between the parent insulator, the normal metallic state above Tc and the superconducting pairing mechanism in a new family of chemically-pressurized fullerene materials. This is a key question in understanding all unconventional superconductors including the high-Tc cuprates, the iron pnictides and the heavy fermion systems.
Their work unveiled a new state of matter -- the Jahn-Teller metal -- and showed that when the balance between molecular and extended lattice characteristics of the electrons at the Fermi level is optimized, the highest achievable temperature for the onset of superconductivity is attained.
As synthetic chemistry allows the creation of new molecular electronic structures distinct from those in the atoms and ions that dominate most known superconductors, there is now strong motivation to search for new molecular superconducting materials.
Fullerenes are molecules consisting of an even number of carbon atoms arranged over the surface of a closed hollow cage. C60 (buckminsterfullerene) which has a soccer-ball shape is the archetypal member of the fullerene family and can be considered as the third allotrope of carbon after graphite and diamond. British and American scientists won the Nobel Prize in Chemistry in 1996 for their discovery of the fullerenes.
Superconductors have no electrical resistance and can carry electricity without losing energy. The temperature at which the resistance becomes zero is called the critical temperature for the onset of superconductivity, Tc. In superconducting materials, a strong attractive force acts between the electrons, which pair up and can move throughout the material without resistance.
(*3) Jahn-Teller effect
The Jahn-Teller theorem states that for any degenerate electronic state associated with a molecular electronic configuration, there will be some electron-vibrational interaction which lifts the electronic degeneracy and leads to a molecular distortion. A negatively-charged C60 molecular ion can undergo a Jahn-Teller distortion by reshaping its molecular structure away from perfect icosahedral symmetry.
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