To realize the full potential of spin-based devices, ways must be found to inject, manipulate, and detect the spin of the electron by purely electrical means. Previously, our group has shown that a quantum point contact (QPC) with lateral spin orbit coupling (LSOC) can be used to create a strongly spin-polarized current by purely electrical means. The LSOC results from the lateral in-plane electric field created by the confining potential in QPCs with in-plane side gates (SGs). Strongly spin-polarized currents can be generated by tuning the asymmetric bias voltages on the side gates. A conductance anomaly in the form of a plateau at conductance G ≅ 0.5G0 (where G 0 = 2e2/h) was observed in the ballistic conductance of a QPC based in the absence of magnetic field - which was established to be a signature of complete spin polarization. A Non-Equilibrium Green's Function (NEGF) analysis was used to model a small QPC and three ingredients were found to be essential to generate a strong spin polarization: (1) LSOC, (2) an asymmetric lateral confinement, and (3) a strong electron-electron (e-e) interaction. We have also shown that all-electric control of spin polarization can be achieved for different materials, electron mobility, heterostructure design, QPC dimensions and strength of LSOC.;Our previous experimental and theoretical results have also found the presence of other conductance anomalies (i.e., at values different from 0.5 G0 ) and the main reason for these occurrences was shown to be due to the influence of surface roughness scattering. In this thesis, we address the important technological challenge to better control the location of the conductance anomalies in QPCs and create a tunable all-electric spin polarizer based on a QPC with four gates, i.e., with two in-plane SGs in series. Here, the first pair of SGs, near the source, is asymmetrically biased to create spin polarization in the QPC channel. The second set of gates, near the drain, is symmetrically biased and that bias is varied to maximize the amount of spin polarization in the channel. We have fabricated several InAs based QPCs with four SGs and have shown that the experimental results were in qualitative agreement with our NEGF simulations. Our main finding is that the range of common mode bias on the first set of gates over which maximum spin polarization can be achieved is much broader for the four gate structure compared to the case of a single pair of in-plane SGs.;In addition, we have observed both hysteresis and negative differential regions in the conductance for specific biasing conditions. We believe these are evidence of Coulomb and Spin Blockade effects on the conductance of these devices and cannot be explained within the context of a NEGF approach and require a many-body approach to the description of carrier transport. Our studies suggest that the study of spin valve structures composed of a quantum dot or wire coupled to the source and drain via asymmetrically biased QPCs should open a new area in the field of spintronics.
展开▼
