A5 Unwanted Quantum Effects: Coulomb Blocking

Topic

When this nanostructure is sufficiently small and weakly coupled to the surrounding electrodes, the charging energy of a single additional electron—the energy required to add that electron to the nanostructure—ceases to be negligible compared to the available thermal energy and instead becomes significant. While this same single-electron charging energy would be insignificant in a macroscopic structure, in this nanostructure—with its much lower electrical capacitance—that same charge adds a comparatively much larger amount of charging energy. Making the nanostructure even smaller and more weakly coupled to the electrodes renders this single-electron charging energy even more significant.

This significant single-electron charging energy creates an energy barrier that an additional electron must overcome to enter the nanostructure; it is precisely this barrier that gives rise to the Coulomb blockade. As long as the voltage applied to the nanostructure fails to overcome this charging energy barrier, no additional electron can enter, and the electric current flowing through the nanostructure is consequently blocked. A higher single-electron charging energy results in a higher energy barrier, requiring a greater applied voltage to overcome the Coulomb blockade.

Thus, this model—characterized by single-electron charging energy and the Coulomb blockade—disrupts the quantized conductance that, according to another model in this set, ideally describes quantum transport through such a nanostructure. Instead of the ideal quantized conductance predicted by the other model—manifesting as a series of perfectly defined steps as the applied voltage increases—the Coulomb blockade introduces an additional region of blocked current between those steps, a feature not accounted for by that ideal quantum transport model. The fact that this single-electron charging energy—in such a tiny, weakly coupled nanostructure—is significant enough to produce Coulomb blockade renders this anomaly increasingly relevant as the nanostructure's size decreases; that is, precisely within the regime where ideal quantum transport should, in principle, be most clearly manifested.

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