Abstract
The successful preparation of superconducting alkali fulleride (AxC60, A = K, Rb, Cs) films using state-of-the-art molecular beam epitaxy overcomes the disadvantages of the air-sensitivity and phase separation in bulk AxC60, enabling for the first time a direct investigation of the superconductivity in alkali fullerides on the molecular scale. In this paper, we briefly review recent cryogenic scanning tunneling microscopy results of the structural, electronic, and superconducting properties of the fcc AxC60 films grown on graphitized SiC substrates. Robust s-wave superconductivity is revealed against the pseudogap, electronic correlation, non-magnetic impurities, and merohedral disorder. By controlling the alkali-metal species, film thickness, and electron doping, we systematically tune the C60x− orientational orderings and superconductivity in AxC60 films and then complete a unified phase diagram of superconducting gap size vs electronic correlation and doping. These investigations are conclusive and elucidated that the s-wave superconductivity retains in alkali fullerides despite of the electronic correlation and presence of pseudogap.
Highlights
Alkali fullerides (A3C60, A = K, Rb, Cs) have been intensively studied since the first discovery of superconductivity in K3C60 in 1991 [1]
The superconductivity in alkali fullerides seemly falls into the conventional Bardeen-Cooper-Schrieffer (BCS) regime where the intramolecular phonon plays a decisive role and the Tc is mainly controlled by the density of states (DOS) at the Fermi level (EF) [12, 13]
4.6 Tuning the electronic states and superconductivity via electron doping Electron doping provides another way to tune the electronic states of AxC60, because itinerant carriers can significantly screen the on-molecular electronic correlation U [42]
Summary
The larger energy of JT effect (JJT > JH) renders a low-spin state (S = 1/2) of Cs3C60 and an orbital disproportionation of filled electrons in fullerides [22, 24, 27, 32–35] From this point of view, Cs3C60 (fcc and A15) polymorphs have been classified as magnetic Mott–Jahn–Teller insulators (MJTIs) [8, 10, 21–26]. The complexity arising from the air sensitivity and phase separation of A3C60 polymorphs, limits the experimental techniques primarily to NMR and magnetization measurements [10, 21–28] Local measuring probes, such as STM, may circumvent the above issues but far have been limited to several mechanically cleaved A3C60 crystals under the atmospheric or argon environment [14, 15, 37–39] and a few non-superconducting KxC60 films on metal substrates [40–42]. The submolecular structures and the 2 × 2 superstructure have been routinely observed in C60 films grown on either the graphene substrate or metal substrates, and will not be discussed here [52–55]
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