In a zero-dimensional superconductor, quantum size effects1,2 (QSE) not only set the limit to superconductivity, but are also at the heart of new phenomena such as shell effects, which have been predicted to result in large enhancements of the superconducting energy gap3,4,5,6. Here, we experimentally demonstrate these QSE through measurements on single, isolated Pb and Sn nanoparticles. In both systems superconductivity is ultimately quenched at sizes governed by the dominance of the quantum fluctuations of the order parameter. However, before the destruction of superconductivity, in Sn nanoparticles we observe giant oscillations in the superconducting energy gap with particle size leading to enhancements as large as 60%. These oscillations are the first experimental proof of coherent shell effects in nanoscale superconductors. Contrarily, we observe no such oscillations in the gap for Pb nanoparticles, which is ascribed to the suppression of shell effects for shorter coherence lengths. Our study paves the way to exploit QSE in boosting superconductivity in low-dimensional systems.
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We thank K. Richter and M. Ternes for critically reading the manuscript. S.B. thanks the Alexander von Humboldt foundation and I.B. the Marie Curie action for support. A.M.G.G. acknowledges financial support from the Spanish DGI through project no. FIS2007-62238.
The authors declare no competing financial interests.
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Bose, S., García-García, A., Ugeda, M. et al. Observation of shell effects in superconducting nanoparticles of Sn. Nature Mater 9, 550–554 (2010). https://doi.org/10.1038/nmat2768
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