Superconductivity in monolayer and few-layer graphene. III. Impurity-induced subgap states and quasiparticle interference patterns

We consider the most energetically favorable symmetry-allowed spin-singlet and spin-triplet superconducting pairing symmetries in monolayer and few-layer graphene and for each calculate the energy spectrum in the presence of a scalar or magnetic impurity. We find that two doubly degenerate subgap states exist for scalar impurities for all types of pairing, except for the spin-singlet $s$-wave state. For magnetic impurities, two or four subgap states may form depending on the order parameter symmetry. We find that the spin polarization of these states allows one to distinguish between spin-singlet and -triplet pairing; for example, only the spin-triplet states show opposite-energy subgap states with the same spin. We also calculate the quasiparticle interference patterns associated with the subgap states and find that they exhibit features that could distinguish between different types of pairing symmetries, especially a breaking of rotational symmetry for nodal states, which is stronger for the spin-singlet ${d}_{xy}$ and ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ states than for the spin-triplet ${p}_{x}$ and ${p}_{y}$ states.