  We theoretically study bilayer superconducting topological insulator film, in
which superconductivity exists for both top and bottom surface states. We show
that an in-plane magnetic field can drive the system into Larkin-Ovchinnikov
(LO) phase, where electrons are paired with finite momenta. The LO phase is
topologically non-trivial and characterized by a Z 2 topological invariant,
leading to a Majorana zero mode chain along the edge perpendicular to in-plane
magnetic fields.
