The tunneling conductance between a superconducting STM tip and an out-of-equilibrium carbon nanotube

TL;DR

This paper investigates how superconducting STM tips interact with out-of-equilibrium carbon nanotubes, revealing how interactions affect tunneling conductance features and their experimental implications.

Contribution

It provides a detailed analysis of tunneling conductance between superconducting tips and Luttinger liquids, including effects of interactions, finite size, and non-equilibrium conditions.

Findings

Superconducting coherence peaks are preserved for weak interactions.

Strong interactions suppress peaks, replaced by cusp-like features.

Finite-size effects restore peaks even under strong interactions.

Abstract

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.