Local spectrum of a superconductor as a probe of interactions between magnetic impurities

Qualitative differences in the spectrum of a superconductor near magnetic impurity pairs with moments aligned parallel and antiparallel are derived. A proposal is made for a nonmagnetic scanning tunneling spectroscopy of magnetic impurity interactions based on these differences. Near parallel impurity pairs the midgap localized spin-polarized states associated with each impurity hybridize and form bonding and antibonding molecular states with different energies. For antiparallel impurity moments the states do not hybridize; they are degenerate.