The search for superconductivity at van Hove singularities in carbon nanotubes

Measurements of intrinsic superconductivity in ropes and arrays of carbon nanotubes, as well as multi-walled carbon nanotubes (carbon nanotubes nested into each other) and films of boron-doped carbon nanotubes, show a wide range of critical temperatures, from 500 mK to 12 K. One possible cause for the large spread in critical temperature is the fact that samples with multiple nanotubes may contain tubes with different chiralities and thus different electronic properties. Here we discuss samples made of isolated single-walled carbon nanotubes to relate the occurrence of superconductivity to their density of states. We measure anomalous transport features indicating that nanotubes may become superconducting when the gate voltage shifts the Fermi energy into van Hove singularities of the electronic density of states. In this scenario, the transport features are caused by the proximity effect at the interface between the superconducting nanotube and the normal electrode and the superconducting properties can be tuned by chemical doping or by applying an electric field.