Investigations of a new artificial atom: The bismuth quantum dot

We have observed new phenomena in low-temperature transport measurements of a small ball of bismuth (Bi) in a double junction configuration. We have fabricated devices consisting of a 500 A diameter ball of Bi contacting two copper wires of width 200 A through copper-oxide tunnel junctions. We have successfully demonstrated the existence of a discrete density of states in the Bi ball. The level spacing was determined to be $\delta=.22\pm.02$ mV. There also appears to be much larger jumps in the density of states, with gap-like behavior. The combination of these two phenomena with the Coulomb Blockade result in a change in the position of the gap with the addition of electrons to the ball of Bi. The Coulomb Blockade we observed was large enough to operate the device above liquid helium temperatures, which is a realization of a highly sensitive electrometer. Our devices further exhibit a coupling to ambient microwave radiation which manifests itself as photo-current, pumping charge through the device. The discovery of this phenomenon provides a path to follow that may aid in the development of practical opto-electronic mesoscopic devices. We have developed theories to qualitatively explain the major features of the data, yet we are left with many intriguing questions and exciting avenues for further research in these new artificial atoms