Merohedral disorder and impurity impacts on superconductivity of fullerenes

Abstract

Local quasiparticle states around impurities provide essential insight into the mechanism of unconventional superconductivity, especially when the candidate materials are proximate to an antiferromagnetic Mott-insulating phase. While such states have been reported in atom-based cuprates and iron-based compounds, they are unexplored in organic superconductors which feature tunable molecular orientation. Here we employ scanning tunneling microscopy and spectroscopy to reveal multiple forms of robustness of an exotic s-wave superconductivity in epitaxial Rb3C60 films against merohedral disorder, non-magnetic single impurities and step edges at the atomic scale. Yu-Shiba-Rusinov (YSR) states, induced by deliberately incurred Fe adatoms that act as magnetic scatterers, have also been observed. The YSR bound states show abrupt spatial decay and vary in energy with the Fe adatom registry. These results and a doping-dependent study of superconductivity point towards local electron pairing in which the multiorbital electronic correlations and intramolecular phonons together drive the high-temperature superconductivity of doped fullerenes.

Tuning the electronic states and superconductivity in alkali fulleride films

“…The s-wave superconductivity is also confirmed by the observation of the isotropic vortices under magnetic fields (see Fig. 10a) [45,46] By fitting the radial dependence of the normalized zero-energy conductance, we deduce the coherence length ξ and the angle dependence of ξ in the case of K 3 C 60 trilayer is plotted in Fig. 10c.…”

“…The preparation of A x C 60 films (see Fig. 2b) using molecular beam epitaxy (MBE) includes two steps [44][45][46]. Firstly, C 60 molecules were evaporated onto the bilayer-graphene dominated SiC surfaces with well-controlled film thickness.…”

“…Experimentally, the C 60 molecules in A 3 C 60 films grown on graphene/SiC are universally H-orientated, but their orderings are strongly fluctuated by the film thickness and alkali-metal species [44][45][46]. The STM topographies of A 3 C 60 thick films (~9 ML) are shown in Fig.…”

“…In 9 ML Rb 3 C 60 and 9 ML RbCs 2 C 60 , the superconducting gap shows a spatial inhomogeneity in both the coherence peak amplitude and the gap size Δ [46]. Such inhomogeneity is proved to be irrelevant to the local merohedral disorder, since apparent coherence peaks can also exist on the regions between nearest neighbor merohedral domains [46]. Moreover, a careful examination of the superconducting Rb 3 C 60 films reveals that the coherence peak amplitude scales inversely with the Δ [46].…”

“…What is more, the phase diagram in A 3 C 60 is commonly documented as a function of pressure or C 60 3− packing volume, in contrast to the doping-controlled phase diagram in cuprates. Therefore, the successful preparation of high-quality superconducting A 3 C 60 films with precisely controlled film thickness and electron doping is crucial to clarify these issues [44][45][46].…”