Impurity effect as a probe for the pairing symmetry of graphene-based superconductors

We study theoretically the single impurity effect on graphene-based superconductors. Four different pairing symmetries are discussed. Sharp in-gap resonant peaks are found near the impurity site for the d + i d pairing symmetry and the p + i p pairing symmetry when the chemical potential is large. As the chemical potential decreases, the in-gap states are robust for the d + i d pairing symmetry while they disappear for the p + i p pairing symmetry. Such in-gap peaks are absent for the fully gapped extended s -wave pairing symmetry and the nodal f -wave pairing symmetry. The existence of the ingap resonant peaks can be explained well based on the sign-reversal of the superconducting gap along different Fermi pockets and by analyzing the denominator of the T -matrix. All of the features may be checked by the experiments, providing a useful probe for the pairing symmetry of graphene-based superconductors.