Engineered magnetoconductance in InAs nanowire quantum dots

Magnetoconductance is the change in the electrical transport properties of a solid state system in response to an applied magnetic field, determined by both its magnitude and orientation. Such effects arise in many shapes and forms, depending on how the magnetic field couples to the charge carriers and thus affect the conductance.
In this thesis, we investigate various aspects of magnetoconductance arising in quantum dots defined in semiconducting InAs nanowires. Quantum dots - nanostructures which confine charge carriers in all three spatial dimensions – are a well-controlled system for the study and manipulation of single electron spins. InAs nanowires provide a versatile platform for such devices, well-suited to examine different aspects of magnetoconductance. This is due to the relative ease of achieving Ohmic contacts and the strong coupling of the electron spins to magnetic fields, as formulated in a large g factor. Furthermore, the presence of strong spin–orbit interaction carries the possibility of controlling electron spins and magnetoconductance using electric fields.
The direct coupling of the spins and the external magnetic field is addressed by examining the g factor in InAs nanowire quantum dots. The orientation dependence of the g factor is mapped out using two methods for an InAs nanowire quantum dot, revealing considerable anisotropy of similar magnitude for the two studied charge states, between of bulk-like values of 15 and quenched down to 5. However, no correlation was found between the anisotropy axes and the symmetries of the device, nor the charge states.
Nanowire-based quantum dot spin-valve devices were also studied, which offer a straightforward approach to investigate the spin physics in solid state systems through spin-polarized currents. In such devices, the magnetic field affects the conductance through the magnetization of the ferromagnetic leads, making high-quality contacts essential. Our discussion focuses on the fabrication aspects of the novel approach of quasi-suspended ferromagnetic contacts developed for nanowire devices.
The main focus of the thesis is the in-depth investigation of the combined effect of local electric and magnetic fields for devices with ferromagnetic split-gate geometries. This approach utilizes the locally strong magnetic stray fields of ferromagnetic side gate pairs near a semiconducting InAs nanowire. These nanomagnetic structures consist of two long ferromagnetic strips, whose magnetization is reversed at a characteristic external magnetic field, the coercive field. These ferromagnetic side-gates have a controlled and spatially confined magnetic field and can also be used as local electrical gates.
We present proof-of-principle magnetoconductance experiments, in which we find state-dependent and hysteretic features. The conductance depends on both the external and the local magnetic field of the side gates. For most resonances, a sharp change is encountered in the magnetoconductance at the 35 mT coercive field of the nanomagnets, consistent with a stray field offset of about 50 mT. These features are strongly state-dependent and gate tunable with relative changes up to ±50% at the coercive field. More intriguingly, we also find more complex features, reminiscent of tunneling magnetoresistance (TMR) between two ferromagnets, though our device is not in direct contact with a ferromagnet.
We account for most magnetoconductance features using intuitive single- and double-dot models, which qualitatively reproduce our experimental findings. Our model shows that the TMR-like characteristics could stem from a transition between singlet and triplet ground state of a double quantum dot.
The approach of ferromagnetic side gates also opens an avenue for conceptually new experiments: for entanglement detection exploiting non-collinear spin projection axes in the two branches of a Cooper pair splitter, or the creation of fractional fermions by inducing a spatially periodic magnetic field using a superstructure of such nanomagnets.