Once again we visit the topic of a carbon nanotube quantum dot with superconducting contacts, and again we use niobium for these contacts. Only, this time the connection between the nanotube and the superconductor is pretty bad, i.e., very low electronic tunnel rates. In the end this means that the superconductor does not influence the localized electronic system very much. However, in the metallic contacts we still have a superconductor, meaning electrons pair up into Cooper pairs, and for free quasiparticles carrying only one electron charge an energy gap evolves (the so-called BCS density of states).
Since we're using niobium, we can see superconducting effects over a fairly large temperature range. If we increase the temperature enough, thermal quasiparticles are excited over this energy gap. This precisely is what we observe in our experiment, as additional discrete lines in the transport spectrum. A detailed theoretical analysis of single electron tunneling, in a close cooperation with the research group Prof. Dr. M. Grifoni, confirms our results very well, especially also the temperature dependence of the features visible in the measurements.
In addition there is an interesting bonus to be had here. The thermally activated processes lead to a distinct double-peak of the conductance at zero bias, and the relative height of the two maxima is controlled by the degeneracy of the quantum dot ground states involved in tunneling. This means that looking at the thermally activated current provides additional information to identify the carbon nanotube level spectrum, even if it is not immediately clear from the usual "Coulomb diamond spectroscopy".
"Sub-gap spectroscopy of thermally excited quasiparticles in a Nb contacted carbon nanotube quantum dot"
M. Gaass, S. Pfaller, T. Geiger, A. Donarini, M. Grifoni, A. K. Hüttel, and Ch. Strunk
Phys. Rev. B 89, 241405(R) (2014), arXiv:1403.4456 (PDF)
