Direct observation of Josephson vortex cores

Abstract

Josephson vortices are circulating supercurrents with an inner structure that is challenging to probe experimentally. Scanning tunnelling microscopy now shows that such vortices contain non-superconducting cores. 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.