Superconductivity at 52 K in hole-doped C60

Abstract

Superconductivity in electron-doped C60 was first observed almost ten years ago. The metallic state and superconductivity result from the transfer of electrons from alkaline or alkaline-earth ions to the C60 molecule, which is known to be a strong electron acceptor. For this reason, it is very difficult to remove electrons from C60--yet one might expect to see superconductivity at higher temperatures in hole-doped than in electron-doped C60, because of the higher density of electronic states in the valence band than in the conduction band. We have used the technique of gate-induced doping in a field-effect transistor configuration to introduce significant densities of holes into C60. We observe superconductivity over an extended range of hole density, with a smoothly varying transition temperature Tc that peaks at 52 K. By comparison with the well established dependence of Tc on the lattice parameter in electron-doped C60, we anticipate that Tc values significantly in excess of 100 K should be achievable in a suitably expanded, hole-doped C60 lattice.

Molecular terms, magnetic moments, and optical transitions of molecular ions C60m±

“…The number of states in a small energy interval △ε ∼ 0.03 eV near the ground state is 30, 40, 25, 6 for hole configurations (h + u ) m with m =2, 3, 4, 5, respectively. This suggests that the configuration of m = 3 holes is most susceptible for Jahn-Teller distortions of the C 60 molecule and possibly for hole-phonon coupling which causes superconductivity [2].…”

“…Another important observation is that the number of states in a small energy interval △ε ∼ 0.03 eV near the ground state is 30, 40, 25, 6 for m =2, 3, 4, 5, respectively. This suggests that the configuration of m = 3 holes is most susceptible for Jahn-Teller distortions of the C 60 molecule and hence for hole-phonon coupling which causes superconductivity [2]. The results of calculations of excitonic configurations (h + u t − 1u ) and (h + u t − 1g ) are quoted in Table XIII.…”

“…After almost ten years of intensive theoretical and experimental work, unexpected discoveries of ferromagnetic polymerized C 60 [1] as well as superconductivity in hole doped pristine C 60 [2] and in lattice-expanded C 60 [3] raise new questions about the electronic structure of the C 60 molecule and its molecular ions. Using the field-effect doping techniques, it has been shown that high transition temperatures are achieved for the case of three holes per the C 60 molecule [2], when many electron effects come into play. Motivated by these new experimental findings, here we present a calculation of the electronic structure of few holes of C m+ 60 .…”

On interface dipole layers between C 60 and Ag or Au

“…In PVDs, C 60 acts as a very efficient electron acceptor, stabilizing the meta-stable chargeseparated state after the fast (sub-picosecond) photoinduced electron transfer from the electron donor [5][6][7]. In FETs based on C 60 , switching is possible from an insulating to a superconducting state, with a transition temperature, T c , peaking at 52 K [8,9].…”

Nonadiabatic Pauli susceptibility in fullerene compounds

“…7 Nonzero isotope effects and other properties strongly indicates that superconductivity in fullerides is driven by electron-phonon interaction. 6 However, the description of superconductivity of Rb 3 C 60 within the traditional Migdal-Eliashberg (ME) theory is found to be inconsistent with respect to the adiabatic hypothesis λω ph /E F ≪ 1 which is at the basis of the ME theory itself.…”