Abstract
Carbon nanotubes (CNTs) are not intrinsically superconducting but they can carry a supercurrent when connected to superconducting electrodes1,2,3,4. This supercurrent is mainly transmitted by discrete entangled electron–hole states confined to the nanotube, called Andreev bound states (ABS). These states are a key concept in mesoscopic superconductivity as they provide a universal description of Josephson-like effects in quantum-coherent nanostructures (for example molecules, nanowires, magnetic or normal metallic layers) connected to superconducting leads5. We report here the first tunnelling spectroscopy of individually resolved ABS, in a nanotube–superconductor device. Analysing the evolution of the ABS spectrum with a gate voltage, we show that the ABS arise from the discrete electronic levels of the molecule and that they reveal detailed information about the energies of these levels, their relative spin orientation and the coupling to the leads. Such measurements hence constitute a powerful new spectroscopic technique capable of elucidating the electronic structure of CNT-based devices, including those with well-coupled leads. This is relevant for conventional applications (for example, superconducting or normal transistors, superconducting quantum interference devices3 (SQUIDs)) and quantum information processing (for example, entangled electron pair generation6,7, ABS-based qubits8). Finally, our device is a new type of d.c.-measurable SQUID.
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Change history
17 November 2010
In the version of this Letter originally published online, there were several errors in Figure 1 and 'ϕ=(2e/h)Φ' should have read 'ϕ=(2e/ℏ)Φ' in the fourth paragraph of the text. These errors have now been corrected in all versions of the Letter.
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Acknowledgements
This work was partially supported by ANR project ANR-07-BLAN-0240 SEMAFAET, C’Nano project SPLONA and Spanish MICINN under contracts NAN2007-29366 and FIS2008-04209. The authors gratefully acknowledge discussions with the Quantronics group, O. Auslaender, J. C. Cuevas, R. Egger, M. Grifoni, T. Kontos, H. le Sueur, A. Martín-Rodero, P. Simon and C. Strunk.
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J-D.P. and C.Q.H.L. fabricated the sample. J-D.P., C.Q.H.L. and P.J. designed and carried out the experiment. J-D.P., C.Q.H.L., P.J., C.B. and A.L.Y. analysed the data and wrote the paper. P.M. provided the equipment for the nanotube synthesis.
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Pillet, JD., Quay, C., Morfin, P. et al. Andreev bound states in supercurrent-carrying carbon nanotubes revealed. Nature Phys 6, 965–969 (2010). https://doi.org/10.1038/nphys1811
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DOI: https://doi.org/10.1038/nphys1811
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