Studies of two-dimensional (2D) superconductivities have been generally limited to the regime where the superconducting order parameter behaves as a 2D wave function but the underline electrons are still three dimensional. Recent advancements in materials synthesis have enabled the growth of epitaxial superconductor thin films with unprecedented control in crystallinity, atomic smoothness, and film thickness, thus opening up new opportunities in investigations of 2D superconductivity. 

Recently, we have studied superconductivity in the extreme two-dimensional limit on ultrathin lead films down to two atomic layers (see figs.), where only a single channel of quantum well states exists. Scanning tunneling spectroscopy reveals that local superconducting order remains robust until two atomic layers, where the transition temperature abruptly plunges to a lower value, depending sensitively on the exact atomic structure of the film. Our result shows that Cooper pairs can still form in the last two dimensional channel of electron states, although their binding is strongly affected by the substrate.

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