Shell Filling in Closed Single-Wall Carbon Nanotube Quantum Dots

Abstract

We observe twofold shell filling in the spectra of closed one-dimensional quantum dots formed in single-wall carbon nanotubes. Its signatures include a bimodal distribution of addition energies, correlations in the excitation spectra for different electron number, and alternation of the spins of the added electrons. This provides a contrast with quantum dots in higher dimensions, where such spin pairing is absent. We also see indications of an additional fourfold periodicity indicative of $K-K^{\prime }$ subband shells. Our results suggest that the absence of shell filling in most isolated nanotube dots results from disorder or nonuniformity.