  Majorana zero modes are fractional quantum excitations appearing in pairs,
each pair being a building block for quantum computation . Some possible
signatures of these excitations have been reported as zero bias peaks at
endpoints of one-dimensional semiconducting wires and magnetic chains. However,
1D systems are by nature fragile to a small amount of disorder that induces
low-energy excitations, hence obtaining Majorana zero modes well isolated in a
hard gap requires extremely clean systems. Two-dimensional systems offer an
alternative route to get robust Majorana zero modes. Indeed, it was shown
recently that Pb/Co/Si(111) could be used as a platform for generating 2D
topological superconductivity with a strong immunity to local disorder. While
2D systems exhibit dispersive chiral edge states, they can also host Majorana
zero modes located on local topological defects. According to predictions, if
an odd number of zero modes are located in a topological domain an additional
zero mode should appear all around the domain's edge. Here we use scanning
tunneling spectroscopy to characterize a disordered superconducting monolayer
of Pb coupled to underlying Co-Si magnetic islands meant to induce a
topological transition. We show that pairs of zero modes are stabilized: one
zero mode positioned at a point in the middle of the magnetic domain and its
zero mode partner extended all around the domain. The zero mode pair is
remarkably robust, it is isolated within a hard superconducting energy gap and
it appears totally immune to the strong disorder present in the Pb monolayer.
Our theoretical scenario supports the protected Majorana nature of this zero
mode pair, highlighting the role of magnetic or spin-orbit coupling textures.
This robust pair of Majorana zero modes offers a new platform for theoretical
and experimental study of quantum computing.
