Superconducting FeSe monolayer with milli-electron volt Fermi energy

TL;DR

This paper reports on the discovery of superconductivity in FeSe monolayers with extremely small Fermi energies around a few meV, enabling exploration of the crossover between BCS and BEC regimes.

Contribution

It demonstrates that FeSe monolayers on graphene have ultra-small Fermi pockets and tunable Fermi energy, revealing new physics in the low carrier density regime.

Findings

FeSe monolayers have Fermi energies of a few meV.

Superconductivity persists despite extremely low carrier density.

Fermi energy can be tuned by graphene layer thickness.

Abstract

Iron selenide (FeSe) is an iron-based superconductor which shows unique properties, including strongly anisotropic superconducting gap, paramagnetism in undoped compound and extremely small Fermi pocket size. In this work, we demonstrate that the sizes of electron and hole pockets in FeSe monolayer become much smaller than those in bulk. The Fermi energy is in the order of a few meV and can be fine-tuned by the thickness of graphene layers underneath. Despite the low carrier density, the FeSe monolayers grown on trilayer or multi-layer graphene are superconducting. The superconducting gap size is sensitive to the Fermi energy of the hole band. Remarkably, the FeSe monolayer provides the opportunity to study the physics in the crossover regime where the Fermi energy and superconducting gap are comparable to each other.