Sammanfattning
We calculate the phase property of the reflection coefficient in twoterminal structures using a lattice tightbinding 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 twoterminal structures coupled to a third lead. The systems are effectivley threeterminal and current conservation is broken with regard to the original twoterminal systems. Two structures, a waveguide with an attached stub quantum dot and a waveguide with an inline, doublebarrier 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 currentconserved twoterminal 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 diatomic asymmetric artificial molecule (double quantum dot) in the nonlinear response regime is studied by means of the same model, but now including selfconsistent electronelectron interactions in the HartreeFock approximation. This approach takes into account the delocalized quantum states of the two coupled quantum dots. The currentvoltage characteristic is found to be strongly nonlinear 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 spinpolarized 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.
Next, we study the reflection and transmission phase properties of twoterminal structures coupled to a third lead. The systems are effectivley threeterminal and current conservation is broken with regard to the original twoterminal systems. Two structures, a waveguide with an attached stub quantum dot and a waveguide with an inline, doublebarrier 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 currentconserved twoterminal 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 diatomic asymmetric artificial molecule (double quantum dot) in the nonlinear response regime is studied by means of the same model, but now including selfconsistent electronelectron interactions in the HartreeFock approximation. This approach takes into account the delocalized quantum states of the two coupled quantum dots. The currentvoltage characteristic is found to be strongly nonlinear 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 spinpolarized 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.
Originalspråk  engelska 

Kvalifikation  Licentiat 
Tilldelande institution 

Handledare 

Status  Published  2004 
Ämnesklassifikation (UKÄ)
 Den kondenserade materiens fysik