Topological superconductivity of adatom chains as probe for unconventional pairing

Chains of magnetic atoms on the surface of s-wave superconductors, have been established as a laboratory for the study of Majorana bound states. In such systems, the breaking of time reversal due to magnetic moments makes the system a one-dimensional topological superconductor. However, in unconventional superconductors even non-magnetic impurities can induce in-gap states since scattering of Cooper pairs changes their momentum but not their phase. Here, we propose a novel paradigm for creating topological superconductivity, which is based on an unconventional superconductor with a chain of non-magnetic impurities on its surface. The system can be driven into the topological phase by tuning magnitude and direction of an external Zeeman field. We show that the topological energy gap can approach the minimum of the bulk energy gap. We develop a general mapping of films with impurity chains in multiband superconductors to one-dimensional lattice Hamiltonians. This allows us to illustrate the feasibility of our proposal in the case of the material Sr₂RuO₄, and to demonstrate that the study of impurity chains can be employed as a diagnostic tool for the nature of the pairing symmetry. This enables to distinguish competing proposals for the pairing symmetry in Sr₂RuO₄.