Tuning the electronic states and superconductivity in alkali fulleride films

TL;DR

This study uses advanced molecular beam epitaxy and cryogenic STM to explore and tune superconductivity in alkali fulleride films, revealing robust s-wave pairing despite electronic correlations and pseudogaps.

Contribution

It provides the first direct molecular-scale investigation of superconductivity in alkali fulleride films and maps the phase diagram relating doping, correlation, and superconducting gap.

Findings

Superconductivity is robust against pseudogap and impurities.

Tuning alkali species and doping systematically controls superconductivity.

A unified phase diagram of gap size versus correlation and doping is established.

Abstract

The successful preparation of superconducting alkali fulleride (A$_x$C$_{60}$, 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 A$_x$C$_{60}$, 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 A$_x$C$_{60}$ 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 C$_{60}^{x-}$ orientational orderings and superconductivity in A$_x$C$_{60}$ 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.