On the origin of critical temperature enhancement in atomically-thin superconductors

TL;DR

Thinning atomically-thin superconductors enhances their critical temperature due to the emergence of an additional superconducting gap, likely caused by a new superconducting band forming in ultrathin films.

Contribution

This study provides experimental evidence that critical temperature enhancement in atomically-thin superconductors is linked to a second superconducting gap.

Findings

Enhanced T_c linked to a second superconducting gap

Ultrathin niobium films also develop a second gap

Emergence of a new superconducting band below the out-of-plane coherence length

Abstract

Recent experiments showed that thinning gallium, iron selenide and 2H tantalum disulfide to single/several monoatomic layer(s) enhances their superconducting critical temperatures. Here, we characterize these superconductors by extracting the absolute values of the London penetration depth, the superconducting energy gap, and the relative jump in specific heat at the transition temperature from their self-field critical currents. Our central finding is that the enhancement in transition temperature for these materials arises from the opening of an additional superconducting gap, while retaining a largely unchanged bulk superconducting gap. Literature data reveals that ultrathin niobium films similarly develop a second superconducting gap. Based on the available data, it seems that, for type-II superconductors, a new superconducting band appears when the film thickness becomes smaller than the out-of-plane coherence length. The same mechanism may also be the cause of enhanced interface superconductivity.