  The coherence length of two Majorana zero energy modes in a p-wave
topological superconductor is inversely proportional to the superconducting
order parameter. We studied the finite size effect of the Majorana zero modes
at vortices in a topological insulator/superconductor heterostructure in the
presence of a vortex and found that the the coherence length of the two zero
energy modes at the terminals of a vortex line is independent of
superconducting order parameter, and determined by the intrinsic properties of
the topological insulator. This anomalous property illustrates that the
superconducting topological insulator is topologically distinct, contrary to a
$p$-wave topological superconductor.
