  Interest in superconducting proximity effect has been revived by the
exploitation of Andreev states and by the possible emergence of Majorana bound
states at the interface. Spectroscopy of these states has been so far
restricted to just a handful of superconductor-metal systems in the diffusion
regime, whereas reports in otherwise clean superconductor-superconductor
heterojunctions are scarce. Here, we realize molecular beam epitaxy growth of
atomically sharp planar heterojunctions between Pb and a topological
superconductor candidate RhPb2 that allows us to spectroscopically image the
proximity effect in the clean limit. The measured energy spectra of RhPb2 vary
with the spatial separation from proximal Pb, and exhibit unusual modifications
in the pairing gap structure and size that extend over a distance far beyond
the coherence length. This anomalously long-range proximity (LRP) effect breaks
the rotational symmetry of Cooper pair potential in real space and largely
deforms the Abrikosov vortex cores. Our work opens promising avenues for
fundamental studies of the Andreev physics and extraordinary states in clean
superconducting heterojunctions.
