Hole Spins in GaAs Quantum Dots

Abstract

In this thesis, we report a new architecture for making lateral hole quantum dots based on shallow accumulation-mode AlGaAs/GaAs heterostructures. Utilizing a double-level-gate architecture, we demonstrate the operation of ultra-small single and double quantum dots in the few-hole regime using electrical measurements. Devices with different dimensions and layouts are tested to reach the single-hole limit. With the flexibility of the double-level-gate architecture, both single and double quantum dots can be formed within the same device with good tunability. With the ability to isolate a single heavy-hole spin, we study the Zeeman splitting of the orbital states in different field orientations via magnetospectroscopy measurements. The extracted value of the hole effective g-factor is found to strongly depend on the orbital state, and be highly anisotropic with respect to the magnetic field direction. We show that these peculiar behaviors of the heavy-hole spins can be qualitatively explained by the effects of strong spin-orbit coupling and strong Coulomb interactions in the hole system. By varying the dot size in situ, we also demonstrate the tuning of the g-factor anisotropy, and estimate the shape of electrically-defined quantum dot. With the few-hole double quantum dot, we present the first observation of Pauli spin blockade in GaAs hole quantum dots. Utilizing a vector field magnet system, we study the lifting of spin blockade due to the spin-orbit interaction. We found that the effect of spin-orbit coupling on spin blockade to be highly anisotropic in different magnetic field orientations, which agrees with previous theoretical predictions on systems with strong spin-orbit coupling. From the anisotropic lifting of spin blockade, we identify the orientation of the effective spin-orbit field to be along the transport direction, which is very different from experimental results on electron quantum dots and highlights the uniqueness of hole systems.