We have fabricated and studied bismuth quantum dot devices, which consist of a bismuth ball of {dollar}sim{dollar}500A diameter connected to two {dollar}sim{dollar}200A wide copper wires via two copper oxide tunnel junctions. Using silicon nitride substrate for our devices, we have successfully removed most of the jumping noise existing in previous devices and collected much better and more reliable data. This allowed us to distinguish the real and bismuth-related effects from those caused by other sources. We confirmed with more authority the existence of a gap zone in the Coulomb blockade oscillation spectrum, which we tend to associate with the quantum confinement effect in bismuth. We also observed the migration of this gap zone with bias voltage and magnetic field. Several possible causes were discussed. By building low-temperature microwave filters in all wires that lead to our devices, we were able to eliminate the S-shape current flow at zero bias and further identify the energy source behind it. Experimental results and computer simulation confirmed our model. However, we did not observe in any of our Bi quantum dot devices the periodic kinks reported earlier as the evidence for discrete energy levels in bismuth. Possible explanations for this are given. We also fabricated aluminum "quantum" dot devices for comparison experiments. The results are helpful in identifying the several real bismuth-related effects.
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