Single electron transport and charge quantization in coupled quantum dots.

摘要

This thesis presents results on the breakdown of charge quantization in single and coupled quantum dots as the strength of tunneling between dots or between a dot and its environment is increased. Ordinarily, single-electron transport is studied in systems in which the number of electrons is well-defined. However, increasing the strength of tunneling between two dots or between a dot and its environment allows an electron to be shared between the two locations in the same fashion as electrons are shared between atoms in forming molecular bonds. Thus it is no longer meaningful to assign a particular number of electrons to a given dot.; In single quantum dots which are sufficiently small that the energy of adding a single electron is large, there are two signatures of charge quantization in very low temperature transport: sharp peaks in the linear conductance and a region of current suppression in the current-voltage characteristic. In a double quantum dot with adjustable interdot tunneling rate, the linear conductance peaks split into pairs with the separation between the pairs depending on the rate of interdot tunneling. The temperature dependence of these peaks indicates that each member of the pair results from a single electron tunneling into the entire double dot system. The region of current suppression, known as the Coulomb gap, narrows as interdot tunneling increases, and secondary regions grow until the gate voltage period of the Coulomb gap doubles and the size has shrunk to that of one large composite dot. The dependence of these nonlinear transport signatures on interdot tunneling rate is in agreement with theories describing the breakdown of charge quantization by tunneling.; In single quantum dots in which the tunneling rate between the dot and its environment is allowed to grow, we find that oscillations in the conductance which are periodic in the total charge on the dot are visible even in the presence of strong tunneling, except when there is exactly one channel available for transport at the Fermi level. However, increased tunneling decreases the peak-to valley ratio of the oscillations and also decreases the temperature to which these oscillations persist.; Finally, we observe excited states in the nonlinear conductance of a well-defined single quantum dot. These states remain at the same energies even as the total number of electrons in the dot is increased by about twenty out of approximately 750 electrons total. This suggests that the excitation spectrum of the dot is surprisingly independent of the exact shape of the dot and of the number of electrons.