  Low-temperature transport measurements have been carried out on single-wall
carbon nanotube quantum dots in a weakly coupled regime in magnetic fields up
to 8 Tesla. Four-electron shell filling was observed, and the magnetic field
evolution of each Coulomb peak was investigated, in which magnetic field
induced spin flip and resulting spin polarization were observed. Excitation
spectroscopy measurements have revealed Zeeman splitting of single particle
states for one electron in the shell, and demonstrated singlet and triplet
states with direct observation of the exchange splitting at zero-magnetic field
for two electrons in the shell, the simplest example of the Hund's rule. The
latter indicates the direct analogy to an artificial He atom.
