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

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.

43 pages, 12 figures

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• Planar graphene-NbSe${}_2$ Josephson junctions in a parallel magnetic field
• Strong-coupling superconductivity induced by calcium intercalation in bilayer transition-metal dichalcogenides
• Quantifying the charge carrier interaction in metallic twisted graphene superlattices
• Comparison of highly-compressed C2/m-SnH12 superhydride with conventional superconductors
• $d$-wave superconductivity as a model for diborides apart MgB${}_2$
• Method to extracting the penetration field in superconductors from DC magnetization data
• Thermodynamic parameters of atomically thin superconductors derived from the upper critical field
• Diamond-shaped evolution of the superconducting interference pattern in NbTiN weak-link Josephson junctions