Direct observation of Josephson vortex cores

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Abstract

Superconducting correlations may propagate between two superconductors separated by a tiny insulating or metallic barrier, allowing a dissipationless electric current to flow1,2. In the presence of a magnetic field, the maximum supercurrent oscillates3 and each oscillation corresponding to the entry of one Josephson vortex into the barrier4. Josephson vortices are conceptual blocks of advanced quantum devices such as coherent terahertz generators5 or qubits for quantum computing6, in which on-demand generation and control is crucial. Here, we map superconducting correlations inside proximity Josephson junctions7 using scanning tunnelling microscopy. Unexpectedly, we find that such Josephson vortices have real cores, in which the proximity gap is locally suppressed and the normal state recovered. By following the Josephson vortex formation and evolution we demonstrate that they originate from quantum interference of Andreev quasiparticles8, and that the phase portraits of the two superconducting quantum condensates at edges of the junction decide their generation, shape, spatial extent and arrangement. Our observation opens a pathway towards the generation and control of Josephson vortices by applying supercurrents through the superconducting leads of the junctions, that is, by purely electrical means without any need for a magnetic field, which is a crucial step towards high-density on-chip integration of superconducting quantum devices.

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Figure 1: Josephson vortices imaged by scanning tunnelling spectroscopy at 0.3 K.
Figure 2: Josephson vortex formation and evolution with magnetic field.
Figure 3: Simulation of Josephson vortex maps.
Figure 4: Josephson vortex core: density of states and principle of generation by edge currents.

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Acknowledgements

T.C., C.B., F.D., V.S. and D.R. acknowledge financial support from the French ANR project ELECTROVORTEX and the French-Russian program PICS-CNRS/RAS. The authors also thank V. Cherkez for assistance during experiments and V. Vinokur (Argonne National Laboratory, Illinois USA) and A. Buzdin (University of Bordeaux 1, France) for stimulating discussions. J.C.C. acknowledges financial support from the Spanish MICINN (Contract No. FIS2011-28851-C1). V.H.L.B. acknowledges support from CNPq Brazil and productive discussions with Prof. A. Chaves (UFC, Brazil). M.V.M. acknowledges support from Research Foundation Flanders (FWO-Vlaanderen) and CAPES Brazil (PVE project BEX1392/11-5).

Author information

The experiments were conceived by D.R., T.C., C.B. and F.D. The experiments were performed by C.B., D.R., L.S-G., T.C., V.S. and F.D. Theoretical support was provided by J.C.C., V.H.L.B. and M.V.M. Superconducting correlation maps were calculated by T.C. The manuscript was written by D.R., J.C.C. and T.C. with comments and input from all authors.

Correspondence to Tristan Cren.

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Roditchev, D., Brun, C., Serrier-Garcia, L. et al. Direct observation of Josephson vortex cores. Nature Phys 11, 332–337 (2015) doi:10.1038/nphys3240

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