Multifold Majorana corner modes arising from multiple pairs of helical edge states

Quantum spin Hall insulators with a pair of helical edge states and proximity-induced superconductivity have been shown to support second-order topological superconductors with Majorana corner modes. As the Majorana corner modes originate from the helical edge states of the quantum spin Hall insulators, the question of whether quantum spin Hall insulators with multiple pairs of helical edge states and proximity-induced superconductivity can give rise to second-order topological superconductors with multifold Majorana corner modes is an interesting one to address. In this work, we consider a quantum spin Hall insulator with two pairs of helical edge states. We find that robust twofold Majorana corner modes can be achieved when the helical edge states are gapped by a combined action of a magnetic exchange field and an $s$-wave pairing or an $s+p$ mixed-parity pairing. The stability of two Majorana zero modes per corner under the action of magnetic exchange fields is attributed to the protection from the chiral symmetry. Our study reveals that heterostructures composed of superconductors and quantum spin Hall insulators with multiple pairs of helical edge states could serve as a platform to pursue multifold Majorana corner modes.