Diverse multipole fluctuations inherent in heavy-fermion systems cause an amazing variety of superconducting states. Here, the authors discover theoretically that the electron-phonon coupling is strongly dressed and magnified owing to interference between multipole fluctuations. They achieve this by going beyond the conventional Migdal approximation. For this reason, even if the original electron-phonon interaction is small, phonon-mediated superconductivity can emerge near the magnetic criticality, contrary to expectations. This mechanism is responsible for the fully gapped s-wave superconductivity in CeCu₂Si₂.

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