arXiv2017
Two classes of topological superconductors and Majorana modes in condensed matter systems are known to date: one, in which impurity disorder strongly suppresses topological superconducting gap and is detrimental to Majorana modes, and the other, where Majorana fermions are protected by disorder-robust superconductor gap. In this work we predict a third class of topological superconductivity and Majorana modes, in which they appear exclusively in the presence of impurity disorder. Observation and control of Majorana fermions and other non-Abelions often requires a symmetry leading to a gap in a single-particle spectra. Disorder introduces states into the gap and enables conductance and proximity-induced superconductivity via the in-gap states. We show that disorder-enabled topological superconductivity can be realized in a quantum Hall ferromagnet, when helical domain walls are coupled to an s-wave superconductor. Solving a general quantum mechanical problem of impurity bound states in a system of spin-orbit coupled Landau levels, we show that disorder-induced Majorana modes emerge in a setting of the quantum Hall ferromagnetic transition in a CdMnTe quantum wells at a filling factor $ν=2$. Recent experiments on transport through electrostatically controlled single domain wall in this system indicated the vital role of disorder in conductance, but left an unresolved question whether this could intrinsically preclude generation of Majorana fermions. The proposed resolution of the problem, demonstrating emergence of Majorana fermions exclusively due to impurity disorder, opens a path forward. We show that electrostatic control of domain walls in an integer quantum Hall ferromagnet allows manipulation of Majorana modes. Similar physics can emerge for ferromagnetic transitions in the fractional quantum Hall regime leading to the formation and control of higher order non-Abelian excitations.