  Contrary to the widespread belief that Majorana zero-energy modes, existing
as bound edge states in 2D topological insulator (TI)-superconductor (SC)
hybrid structures, are unaffected by non-magnetic static disorder by virtue of
Anderson's theorem, we show that such a protection against disorder does not
exist in realistic multi-channel TI/SC/ferromagnetic insulator (FI) sandwich
structures of experimental relevance since the time-reversal symmetry is
explicitly broken locally at the SC/FI interface where the end Majorana mode
(MM) resides. We find that although the MM itself and the \emph{bulk}
topological superconducting phase inside the TI are indeed universally
protected against disorder, disorder-induced subgap states are generically
introduced at the TI edge due to the presence of the FI/SC interface as long as
multiple edge channels are occupied. We discuss the implications of the finding
for the detection and manipulation of the edge MM in realistic TI/SC/FI
experimental systems of current interest.
