Fully gapped s-wave superconductivity enhanced by magnetic criticality in heavy-fermion systems

In heavy-fermion systems, higher-rank multipole operators are active thanks to the strong spin-orbit interaction (SOI), and the role of diverse multipole fluctuations on the pairing mechanism attracts a lot of attention. Here, we study a mechanism of superconductivity in heavy-fermion systems, by focusing on the impact of vertex corrections (VCs) for the pairing interaction going beyond the Migdal approximation. In heavy-fermion systems, strong interference between multipole fluctuations cause significant VCs that represent many-body effects beyond mean-field-type approximations. Especially, the coupling constants between electrons and charged bosons, including the electron-phonon coupling constant, are strongly magnified by the VCs. For this reason, moderate even-rank (=electric) multipole fluctuations give large attractive interaction, and therefore s-wave superconductivity can emerge in heavy-fermion systems. In particular, phonon-mediated superconductivity is expected to be realized near the magnetic criticality, thanks to the VCs due to magnetic multipole fluctuations. The present mechanism may be responsible for the fully gapped s-wave superconducting state realized in CeCu2Si2.( )