  By an exact formulation of tunnelling into a multi-band superconductor in
terms of Green's functions, we demonstrate that the multi-band feature of the
iron-based superconductors can lead to novel interference between Andreev
reflections and decoherence effect of quasi-particles to Andreev reflections in
a tunnelling junction. These effects depend on the relative sign of the gaps
for s-wave pairing, and the sign of the Fermi velocities also matters for
tunnelling along the nodal direction of d-wave gaps. Experimental probe of such
effects could determine both the pairing symmetry and the relative sign of the
gaps on different bands.
