  We calculate the current and differential conductance for the junction
between a superconducting (SC) STM tip and a Luttinger liquid (LL). For an
infinite single-channel LL, the SC coherence peaks are preserved in the
tunneling conductance for interactions weaker than a critical value, while for
strong interactions (g <0.38), they disappear and are replaced by cusp-like
features. For a finite-size wire in contact with non-interacting leads, we find
however that the peaks are restored even for extremely strong interactions. In
the presence of a source-drain voltage the peaks/cusps split, and the split is
equal to the voltage. At zero temperature, even very strong interactions do not
smear the two peaks into a broader one; this implies that the recent
experiments of Y.-F. Chen et. al. (Phys. Rev. Lett. 102, 036804 (2009)) do not
rule out the existence of strong interactions in carbon nanotubes.
