Fermionic scenario for the destruction of superconductivity in ultrathin MoC films evidenced by STM measurements

TL;DR

This study uses low-temperature STM to explore how ultrathin MoC films lose superconductivity, revealing a fermionic transition involving a metallic state rather than a direct bosonic insulator transition.

Contribution

It provides evidence for a two-stage fermionic superconductor-insulator transition in ultrathin MoC films, challenging the bosonic scenario.

Findings

Superconductivity remains spatially homogeneous down to 3 nm thickness.

Superconducting energy gap scales with critical temperature during thinning.

Transition involves a metallic state, supporting a fermionic SIT model.

Abstract

We use sub-Kelvin scanning tunneling spectroscopy to investigate the suppression of superconductivity in homogeneously disordered ultrathin MoC films. We observe that the superconducting state remains spatially homogeneous even on the films of 3 nm thickness. The vortex imaging suggests the global phase coherence in our films. Upon decreasing thickness, when the superconducting transition drops from 8.5 to 1.2 K, the superconducting energy gap follows perfectly Tc. All this is pointing to a two-stage fermionic scenario of the superconductor-insulator transition (SIT) via a metallic state as an alternative to the direct bosonic SIT scenario with a Cooper-pair insulating state evidenced by the last decade STM experiments.