Iryna Yakymenko

Effects of geometry and magnetic field for quantum wires injecting electrons into a two-dimensional electron reservoir

Project goal was to investigate a spin polarized current and its dependence on applied magnetic field, spin and geometry of the system consisting of two connected quantum point contacts placed in the same semiconductor GaAs/AlGaAs heterostructure. One of the quantum point contact serves as a spin injector and another one as a spin detector as it is used in the magnetic focusing experiments.

We have studied theoretically the injection of  electrons from a short quantum wire into an open two-dimensional reservoir. The transport was considered for non-interacting electrons at different transmission regimes by using the mode-matching technique numerically. Electron flow through the quantum wire with rectangular, conical and rounded openings has been studied. When a perpendicular magnetic field is applied the electron paths in the two-dimensional reservoir are curved. The effect of spin-splitting due to exchange interactions is present for realistic choices of device parameters and consistent with observations.

The results of this study may be applied in designing magnetic focusing devices.

Electronic properties of semiconductor quantum wires for shallow symmetric and asymmetric confinements

The project has been focused on theoretical analysis of electron localization in a quantum point contact that is made by split and top gates between two large electron reservoirs.

The project has been motivated by experiments with a quantum wire in shallow symmetric and asymmetric confinements that have shown additional conductance anomalies at zero magnetic field.

We have studied electron transport and conductance anomalies of a wide top-gated quantum point contacts for experimental confinement strength and low electron densities within density functional theory. The electron–electron interaction and shallow confinement make the splitting of the conduction channel into two channels possible. Each of them becomes spin-polarized at certain split and top gates voltages and may contribute to conductance giving rise to additional conductance anomalies.

The spin-polarized states in the quantum point contacts with shallow symmetric and asymmetric confinements tuned by electric means may be used for the purposes of quantum technology.

Spin polarization and spin-related transport in quantum point contacts coupled through a two-dimensional electron reservoir

This project presents a theoretical study of electron transport in a device composed of two quantum point contacts, that serve as injector and detector of electrons, connected via a wider two-dimensional region, that can be electrostatically tuned by a top gate. Electron transport is modeled using density functional theory.

Simulations for both symmetric and asymmetric injector quantum point contacts indicate that the width of the current distribution varies with the current through injector quantum point contact. This variation is consistent with the presence of spin-related effects, such as the 0.7-conductance anomaly observed in the injector QPC.

These results may be useful for the future design of semiconductor structures for spintronics and quantum device applications.