  We consider a model of ballistic quasi-one dimensional semiconducting wire
with intrinsic spin-orbit interaction placed on the surface of a bulk s-wave
superconductor (SC), in the presence of an external magnetic field. This setup
has been shown to give rise to a topological superconducting state in the wire,
characterized by a pair of Majorana-fermion (MF) bound states formed at the two
ends of the wire. Here we demonstrate that, besides the well-known direct
overlap-induced energy splitting, the two MF bound states may hybridize via
elastic correlated tunneling processes through virtual quasiparticles states in
the SC, giving rise to an additional energy splitting between MF states from
the same as well as from different wires.
