The Disorder-Free Non-BCS Superconductor Cs ₃ C ₆₀ Emerges from an Antiferromagnetic Insulator Parent State

Abstract

The body-centered cubic A15-structured cesium fulleride Cs3C60 is not superconducting at ambient pressure and is free from disorder, unlike the well-studied face-centered cubic A3C60 alkali metal fulleride superconductors. We found that in Cs3C60, where the molecular valences are precisely assigned, the superconducting state at 38 kelvin emerges directly from a localized electron antiferromagnetic insulating state with the application of pressure. This transition maintains the threefold degeneracy of the active orbitals in both competing electronic states; it is thus a purely electronic transition to a superconducting state, with a dependence of the transition temperature on pressure-induced changes of anion packing density that is not explicable by Bardeen-Cooper-Schrieffer (BCS) theory.

Superconductivity at 5 K in alkali-metal-doped phenanthrene

“…This behaviour is expected by the Bardeen, Cooper and Schrieffer (BCS) theory, because the expansion of lattice leads to an enhancement of the density of states on the Fermi surface. However, the body-centered-cubic fulleride Cs 3 C 60 is found to be a true Mott-Jahn-Teller insulator, and it possesses both the ingredients required by the strongly correlated theory 11,12 . The insulator Cs 3 C 60 can be turned into superconductor by pressure 13 .…”

Non-linear optical properties of molecular systems under high static pressures

“…The role of pressure in achieving superconductivity or increasing the T c is a major subject in high-pressure physics [18,19], and K 3 -picene itself has a large positive pressure dependence of T c [20]. Pressure itself induces important modifications on the electronic structure of molecular crystals, as testified by the examples above, and in this framework an investigation on pristine picene was undertaken [6].…”

Theory of Superconductivity in Graphite Intercalation Compounds

“…Incidentally, in some recently discovered superconductors in the phonon mechanism such as the alkali-doped fullerenes with T c = 18 − 38K [42][43][44][45] , the condition of E F ≫ ω 0 is violated, necessitating to include the vertex corrections in calculating the phonon-mediated attractive interaction 46 . Then, it is by no means clear to treat the overall effect of various phonons in terms of the sum of the contribution from each phonon, implying that the Eliashberg function α 2 F (ω) is not enough to properly describe the attraction because of possible interference effects among virtually-excited phonons.…”