Anomalous impurity effect in the heavy-fermion superconductor CeCu2Si2

Recent observations of a contradictory impurity effect in the heavy-fermion superconductor CeCu₂Si₂ at ambient pressure have hindered the identification of its pairing symmetry. Here, we perform theoretical analyses with both intraband and interband impurity scatterings for the nodeless s^±-wave pairing, and report an anomalous nonmonotonic variation of its Tc suppression with the scattering strength. Our results reproduce the prominent reduction of Tc in good agreement with earlier experiments by atomic substitution and explains as well its robustness against electron irradiation. We ascribe the latter to the screening of the interband impurity potential in the strong scattering or unitary region. This resolves the seeming contradiction in different experimental probes and provides an important support to the nodeless s^±-wave scenario in CeCu₂Si₂ at ambient pressure. Our theory may be extended to other narrow-band systems such as twisted bilayer graphene and the recently discovered bilayer or trilayer nickelate superconductors, and provide a useful way to distinguish different pairing candidates thereof.