Superconductivity with On-Site Attractive Interaction in Heavy-Fermion CeCu₂Si₂

Abstract

In exotic superconductors including high-T_c cuprates, the interactions mediating electron pairs are widely considered to have a magnetic rather than the conventional electron-phonon origin. Interest in such exotic pairing was initiated by the 1979 discovery of heavy-fermion superconductivity in CeCu₂Si₂ . A hallmark of unconventional pairing is that the superconducting energy gap changes sign as a function of electron momentum, often leading to gap nodes. Here we report low-temperature specific heat, thermal conductivity, and magnetic penetration depth measurements in CeCu₂Si₂ , demonstrating the absence of gap nodes at any point on the Fermi surface. Moreover, electron-irradiation experiments reveal that the full-gap superconductivity is extremely robust against impurities, implying that there is no sign change in the gap function. Our findings reveal that, contrary to the long-standing belief, heavy electrons with extremely strong Coulomb repulsion can condense into non sign-changing and fully gapped s-wave superconducting state, which has an on-site attractive pairing interaction.