Impurity States in Altermagnetic Superconductors

Altermagnetic superconductors hold the potential for novel phenomena, including topological states, finite-momentum superconductivity, as well as promising applications in spintronics. However, an experimental demonstration of the coexistence of these two effects is still lacking. In this work, we propose potential impurities as a way to probe the distinctive properties of altermagnetic superconductors. These impurities induce spin-polarized subgap states that extend along the crystal axes, inheriting the magnetic characteristics of the bulk. When the impurities respect the bulk symmetries, they generate spin-degenerate doublets; otherwise, the degeneracy is lifted. An external magnetic field aligned with the N\'eel vector can tune the energies of the spin components of the subgap states. These components show different spatial extensions that can be measured via local probes, like scanning tunneling microscopy. For certain parameters, the impurity state can induce a local sign change on the order parameter, so-called -transition. Lastly, we investigate the interaction between impurities, revealing a position-dependent effective coupling that facilitates the engineering of spin-dependent tunneling. This tunable coupling introduces a new approach for in-situ control of devices critical for quantum information processing and topological superconductivity.