Theoretical Studies of Electron Transport in Quantum Dot Structures

Abstract: We calculate the phase property of the reflection coefficient in two-terminal structures using a lattice tight-binding model. It is seen that, provided that there exist two coherent reflection paths, the reflection probability can be zero for certain electron energies. At these energies, the phase of the reflection coefficient shift abruptly by $pi$. Next, we study the reflection and transmission phase properties of two-terminal structures coupled to a third lead. The systems are effectivley three-terminal and current conservation is broken with regard to the original two-terminal systems. Two structures, a waveguide with an attached stub quantum dot and a waveguide with an inline, double-barrier confined quantum dot, are considered. The transmission and reflection phase properties are calculated for these systems with different couplings to the third lead. The results show that the discontinuous phase shifts seen in the current-conserved two-terminal systems are removed when the third lead is attached. However, as long as the coupling between the quantum systems and the additional lead is weak, sharp but continuous phase drops within narrow energy ranges can still be clearly identified. Finally, transport through a di-atomic asymmetric artificial molecule (double quantum dot) in the non-linear response regime is studied by means of the same model, but now including self-consistent electron-electron interactions in the Hartree-Fock approximation. This approach takes into account the delocalized quantum states of the two coupled quantum dots. The current-voltage characteristic is found to be strongly non-linear and strikingly different for opposite bias polarities, indicating a possibility for the structure to be utilized as a current rectifier. We also find that it is possible to obtain spin-polarized currents. The observed features are found to result from an interplay between Pauli spin blockade and transmission through molecular states, the localizations of which are sensitive to the applied bias.