Coexistence of full-gap superconductivity and pseudogap in two-dimensional fullerides

TL;DR

This study reveals that trilayer K3C60 exhibits fully gapped s-wave superconductivity coexisting with a pseudogap state, influenced by electronic correlations and doping, suggesting phonon-mediated pairing in alkali fullerides.

Contribution

It demonstrates the coexistence of superconductivity and pseudogap in 2D fullerides and clarifies the role of phonons and electronic correlations in their superconducting mechanism.

Findings

Superconductivity in trilayer K3C60 is fully gapped s-wave.

Superconductivity coexists with a pseudogap state above Tc.

Maximum Tc occurs near a superconductor-Mott insulator transition.

Abstract

Alkali-fulleride superconductors with a maximum critical temperature Tc of 40 K exhibit similar electronic phase diagram with unconventional high-Tc superconductors where the superconductivity resides proximate to a magnetic Mott-insulating state. However, distinct from cuprate compounds, which superconduct through two-dimensional (2D) CuO2 planes, alkali fullerides are attributed to the three-dimensional (3D) members of high-Tc family. Here, we employ scanning tunneling microscopy to show that trilayer K3C60 displays fully gapped strong coupling s-wave superconductivity that coexists spatially with a cuprate-like pseudogap state above Tc = 22 K and within vortices. A precise control of electronic correlations and doping reveals that superconductivity occurs near a superconductor-Mott insulator transition and reaches maximum at half-filling. The s-wave symmetry retains over the entire phase diagram, which, in conjunction with an abrupt decline of superconductivity below half-filling, demonstrates that alkali fullerides are predominantly phonon-mediated superconductors, although the multiorbital electronic correlations also come into play.