Controlling hole spin in quantum dots: Rashba or not Rashba

Hole spins in semiconductor quantum dots (QD) are promising qubits. Zeeman-split states form two-level systems with splitting determined by the physical spin of the hole. Due to strong spin-orbit coupling, hole spin orientation is locked to the QD axis for magnetic fields B away from the Voigt configuration. However, in Voigt configuration, the hole spin displays significant texture across the dot but is weakly polarized. Application of an electric field parallel or antiparallel to B in the Voigt configuration restores the hole spin. This spin control can be related to the QD geometry. The question remains whether this can be explained as a Rashba effect originating from interface fields or is inherent to an atomistic description of hole spins in QDs. Tight-binding theory is used to study GaAs/AlAs QDs with a graded alloy describing the QD interface to minimize Rashba effects of sharp interfaces. The results are compared with results for QDs with sharp interfaces. Several examples illustrate how a graded interface influences the spin locking seen for QDs with sharp interfaces and how this changes spin texture and spin polarization in the Voigt configuration. The results are used to assess the contribution of Rashba effects.