Alkali-Fulleride Superconductors: Synthesis, Composition, and Diamagnetic Shielding

Abstract

The recent report of a superconductivity onset near the critical temperature T c = 18 K in potassium-doped C 60 raises questions concerning the composition and stability of the superconducting phase. The effects of mixing and heat treatment of K χ C 60 samples prepared over a wide range of initial compositions on the superconducting transition was determined from shielding diamagnetism measurements. A single superconducting phase ( T c = 19.3 K) occurs for which the composition is K 3 C 60 . The shielding reaches a maximum of greater than 40 percent of the perfect diamagnetism, a high value for a powder sample, in samples prepared from 3:1 mixtures. A Rb χ C 60 sample prepared and analyzed in an analogous way exhibited evidence for superconductivity with T c = 30 K and a diamagnetic shielding of 7 percent could be obtained.

Fulereno[C60]: química e aplicações

“…8 Isto foi comprovado por cálculos teóricos, sendo que o caráter eletrônico deficitário do C 60 foi atribuído aos orbitais moleculares LUMO não-ligantes, que estão num patamar de energia muito baixo. Outra propriedade interessante é que a molécula de C 60 pode se tornar supracondutora em espécies do tipo M 3 C 60 (M= metal alcalino) 9 além de possuir propriedades ópticas não lineares 10 e um estado tripleto de longa duração. 11 Muitas das propriedades do C 60 estão relacionadas com sua elevada simetria, na qual os 60 átomos de carbono são equivalentes, resultando em um espectro de RMN de 13 C com um único sinal em 142,68 ppm (Figura 3).…”

Surface Hubbard U of alkali fullerides

“…The JT interaction is stronger than the on-ball exchange coupling [9,17], leading to low-spin ground states and to the stabilization of even-charge anionic states [18,19]. In compounds of even stoichiometry (x = 2, 4) charge fluctuations are hindered both by correlation and JT effects [20][21][22], resulting in insulating and diamagnetic ground states [23][24][25], while in odd stoichiometries of cubic symmetry metallic behaviour and even phonon-mediated superconductivity (for x = 3) are observed [26][27][28]. In the latter systems a symmetry reduction involving the lifting of the LUMO degeneracy or a lattice expansion lead to an insulating state [8,9,29].…”

Superconductivity in Lu3Os4Ge13

“…Understanding the nature and origin of superconductivity in semiconductors and semi-metals is a challenging problem and it is hampered by the fact that only a handful of well characterised compounds have been studied till date [1,2,3,4,5]. The occurrence of superconductivity in these low carrier density compounds is surprising since low carrier density is an unfavourable element for superconductivity within the conventional framework of Bardeen-Cooper-Schrieffer (BCS) [6] or Migdal-Eliashberg [7,8] theories.…”