In superconductors,fermionic electrons are paired into bosons,and condensed into the ground state.An impurity can break the electron pairs into quasiparticles with energy states inside the superconducting gap.The characteristics of such in-gap states reflect accordingly the properties of the superconducting ground state.A zeroenergy in-gap state is particularly interesting,because it can In superconductors, fermionic electrons are paired into bosons, and condensed into the ground state. An impurity can break the electron pairs into quasiparticles with energy states inside the superconducting gap. These characteristics of such in-gap states reflect accordingly the properties of the superconducting ground state.A zeroenergy in-gap state is quite that because it can