Higher-Order Topological Superconductivity in Twisted Bilayer Graphene

We show that introducing s-wave superconductivity into twisted bilayer graphene induces higher-order topological superconductivity: corner states are bound to domain walls of opposite signs of pairing. These corner states are protected by a combination of $C_{2z}T$, time-reversal, $U(1)$-valley, the approximate particle-hole symmetry ${\cal P}$, and $SU(2)$-spin. Gapless modes also exist along $C_{2x}$-symmetric domain walls, and bound in Abrikosov vortices. We demonstrate that in the presence of these symmetries, any pairing term must drive the system into a higher-order topological superconductor phase, using the Wilson loop formalism, Dirac Hamiltonians, and numerics.<br/>Finally, we suggest experimental signatures and possible extensions to twisted trilayer graphene.