Abstract
When a low-energy electron is incident on an interface between a metal and superconductor, it causes the injection of a Cooper pair into the superconductor and the generation of a hole that reflects back into the metal—a process known as Andreev reflection. In confined geometries, this process can give rise to discrete Andreev bound states (ABS), which can enable transport of supercurrents through non-superconducting materials and have recently been proposed as a means of realizing solid-state qubits1,2,3. Here, we report transport measurements of sharp, gate-tunable ABS formed in a superconductor–quantum dot (QD)–normal system realized on an exfoliated graphene sheet. The QD is formed in graphene beneath a superconducting contact as a result of a work-function mismatch4,5. Individual ABS form when the discrete QD levels are proximity-coupled to the superconducting contact. Owing to the low density of states of graphene and the sensitivity of the QD levels to an applied gate voltage, the ABS spectra are narrow and can be continuously tuned down to zero energy by the gate voltage.
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Acknowledgements
This research was supported by the Department of Energy Division of Materials Science under grant DE-FG02-07ER46453 through the Frederick Seitz Materials Research Laboratory at the University of Illinois at Urbana-Champaign, and partly carried out in the Materials Research Laboratory Central Facilities (partially supported by the Department of Energy under DE-FG02-07ER46453 and DE-FG02-07ER46471). T.L.H. acknowledges the National Science Foundation under grant DMR-0758462 and the Institute for Condensed Matter Theory at University of Illinois at Urbana-Champaign, and B.U. acknowledges the Department of Energy under grant DE-FG02-91ER45439. We acknowledge conversations with J. Maciejko.
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T.D. and Y-F.C. carried out the experiments. C.C. helped fabricate the samples. T.D., T.L.H., S.L., B.U., P.M.G. and N.M. analysed the data and wrote the main paper. T.L.H., B.U. and P.M.G. wrote Supplementary Information.
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Dirks, T., Hughes, T., Lal, S. et al. Transport through Andreev bound states in a graphene quantum dot. Nature Phys 7, 386–390 (2011). https://doi.org/10.1038/nphys1911
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DOI: https://doi.org/10.1038/nphys1911
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