Superconductivity-enhanced bias spectroscopy in carbon nanotube quantum   dots
  We study low-temperature transport through carbon nanotube quantum dots in the Coulomb blockade regime coupled to niobium-based superconducting leads. We observe pronounced conductance peaks at finite source-drain bias, which we ascribe to elastic and inelastic cotunneling processes enhanced by the coherence peaks in the density of states of the superconducting leads. The inelastic cotunneling lines display a marked dependence on the applied gate voltage which we relate to different tunneling-renormalizations of the two subbands in the nanotube. Finally, we discuss the origin of an especially pronounced sub-gap structure observed in every fourth Coulomb diamond. 
K. Grove-Rasmussen, H. I. J{\o}rgensen, B. M. Andersen, J. Paaske, T.   S. Jespersen, J. Nyg{\aa}rd, K. Flensberg, and P. E....