Eightfold shell filling in a double-wall carbon nanotube quantum dot.

We fabricated a quantum-dot device consisting of an individual double-wall carbon nanotube and studied its electrical transport properties at low temperatures. In the negative bias region, the gate modulation curve exhibited quasiperiodic current oscillations, attributed to the Coulomb blockade of single-electron tunneling. We observed both four- and eightfold shell filling in the Coulomb diamond structures. The observation implies an eightfold degeneracy in the single-particle energy levels, which is higher than the fourfold degeneracy of a single-wall carbon nanotube. We show that the observed eightfold shell filling is a unique characteristic of a double-wall carbon nanotube quantum-dot device.