Discovery of new state of matter could lead to higher temperature superconductors

/ CSA Executive Director

In their paper “Optimized unconventional superconductivity in a molecular Jahn-Teller metal” published in the journal Science Advances, an international team of researchers led by Kosmas Prassides of Tohoku University has reported the discovery of a new state of matter that could lead to higher temperature superconductors. The team 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, and was able to demonstrate the guiding influence of the molecular electronic structure in controlling superconductivity and achieving maximum Tc, potentially opening the way to new routes in search of new molecular superconductors with enhanced figures of merit.

Study background

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 that can work at the highest possible temperature.

Most superconductors have simple structures build from atoms. But recently, superconductors made from molecules arranged in regular solid structures have been found. Work by members of the team on molecular fulleride-based systems has previously led to the discovery of the highest working temperature (at 38K) for a molecular superconductor [2]. The electronic ground state, which is in competition with superconductivity, was found to be magnetically ordered [3], 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 [4].

Change in electron state of the fulleride solids with change in volume per C60. Schematic depictions of the Jahn-Teller molecular distortion of the fullerene units in the Mott-Jahn-Teller insulator (blue molecules) and the Jahn-Teller metal (yellow molecules), their respective molecular electronic structure (lifting of the orbital degeneracy due to the distortion), and the resulting intermolecular hopping of the electrons (prohibited in the insulator, weak hopping in the Jahn-Teller metal). This situation contrasts with the behavior of the conventional metal where hopping is allowed, the orbital degeneracy is retained, and the molecules are undistorted (green molecules). Image: Tohoku University

Change in electron state of the fulleride solids with change in volume per C60. Schematic depictions of the Jahn-Teller molecular distortion of the fullerene units in the Mott-Jahn-Teller insulator (blue molecules) and the Jahn-Teller metal (yellow molecules), their respective molecular electronic structure (lifting of the orbital degeneracy due to the distortion), and the resulting intermolecular hopping of the electrons (prohibited in the insulator, weak hopping in the Jahn-Teller metal). This situation contrasts with the behavior of the conventional metal where hopping is allowed, the orbital degeneracy is retained, and the molecules are undistorted (green molecules). Image: Tohoku University

The controlling role of the molecular electronic structure was then identified by demonstrating that the parent insulating state involves Jahn-Teller distortion [5] of the molecular anions that produces the magnetism from which the superconductivity emerges [6].

Breakthrough

The research team has addressed for the first time the relationship between the parent insulator, the normal metallic state about 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.

Electronic phase diagram of face-centered-cubic (fcc) structured fullerides showing the evolution of the superconducting transition temperature, Tc (superconductivity dome) and the Mott-Jahn-Teller insulator to Jahn-Teller crossover temperature, T' as a function of volume per C60. Within the metallic (superconducting) regime, gradient shading from orange to green schematically illustrates the Jahn-Teller metal to conventional metal (unconventional to weak-coupling BCS conventional superconductor) crossover. The inset shows the crystal structure of ccc A3C60 fullerides (A=alkalai metal, green spheres represent cations on tetrahedral and red on octahedral holes, respectively). Image: Tohoku University

Electronic phase diagram of face-centered-cubic (fcc) structured fullerides showing the evolution of the superconducting transition temperature Tc (superconductivity dome) and the Mott-Jahn-Teller insulator to Jahn-Teller crossover temperature T' as a function of volume per C60. Within the metallic (superconducting) regime, gradient shading from orange to green schematically illustrates the Jahn-Teller metal to conventional metal (unconventional to weak-coupling BCS conventional superconductor) crossover. The inset shows the crystal structure of fcc A3C60 fullerides (A=alkalai metal, green spheres represent cations on tetrahedral and red on octahedral holes, respectively). Image: Tohoku University

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 Fermi level is optimized, the highest achievable temperature for the onset of superconductivity is obtained. 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.

References

  1. 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.
  2. Nature Materials 7, p. 367, 2008
  3. Science 323, p. 1585, 2009
  4. Nature 466, p. 221, 2010
  5. The Jahn-Teller theorem states that for any degenerate electronic state associated with a molecular electronic configuration, there will be some electron-vibrational interaction that 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.
  6. Nature Communications 3, 912, 2012