Tunneling conductance between a superconducting STM tip and an out-of-equilibrium Luttinger liquid

We calculate the current and differential conductance for the junction between a superconducting (SC) scanning tunneling spectroscopy 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 cusplike 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.