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Localization of preformed Cooper pairs in disordered superconductors

Abstract

The most profound effect of disorder on electronic systems is the localization of the electrons transforming an otherwise metallic system into an insulator. If the metal is also a superconductor then, at low temperatures, disorder can induce a pronounced transition from a superconducting into an insulating state. An outstanding question is whether the route to insulating behaviour proceeds through the direct localization of Cooper pairs or, alternatively, by a two-step process in which the Cooper pairing is first destroyed followed by the standard localization of single electrons. Here we address this question by studying the local superconducting gap of a highly disordered amorphous superconductor by means of scanning tunnelling spectroscopy. Our measurements reveal that, in the vicinity of the superconductor–insulator transition, the coherence peaks in the one-particle density of states disappear whereas the superconducting gap remains intact, indicating the presence of localized Cooper pairs. Our results provide the first direct evidence that the superconductor–insulator transition in some homogeneously disordered materials is driven by Cooper-pair localization.

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Figure 1: Coherent versus incoherent Cooper pairing revealed by local tunnelling spectroscopy.
Figure 2: Disorder-induced inhomogeneity and phase incoherence.
Figure 3: Local pairing field.
Figure 4: Onset of the superconducting phase coherence.

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Acknowledgements

We thank A. Finkel’stein, V. Kravtsov, M. Mézard, Z. Ovadyahu and N. Trivedi for valuable discussions. D.S. and M.O. acknowledge the Israeli Science Foundation and the Minerva Fund.

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Contributions

B.S., T.D. and C.C. carried out the experiments. B.S., T.D., C.C. and D.S. analysed the data. M.O. and B.S. prepared the InO film samples. B.S., T.D., C.C., D.S., M.F. and L.I. wrote the paper. M.F. and L.I. carried out the theoretical analysis and the numerical simulations. M.S., C.C. and D.S. initiated this work. All of the authors discussed the results and commented on the manuscript.

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Correspondence to Benjamin Sacépé.

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Sacépé, B., Dubouchet, T., Chapelier, C. et al. Localization of preformed Cooper pairs in disordered superconductors. Nature Phys 7, 239–244 (2011). https://doi.org/10.1038/nphys1892

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