Large hole spin anticrossings in InAs/GaAs double quantum dots

Abstract

We show that hole states in InAs/GaAs double quantum dots can exhibit spin anticrossings of up to 1 meV, according to simulations with a three dimensional Burt-Foreman Hamiltonian including strain and piezoelectric fields. The spin mixing originates in the valence band spin-orbit interaction plus the spatial symmetry breaking arising from misalignment between the dots and piezoelectric potential. The values we report are in better agreement with experiments than previous theoretical estimates and yield good prospects for efficient hole spin control. There is current interest in using the spin of carri-ers confined in semiconductor quantum dots (QDs) for single spintronic, optoelectronic and quantum informa-tion research.[1–4] Self-assembled InAs/GaAs QDs have been particularly successful at this regard because they combine high optical activity, which enables precise op-tical preparation and read-out of the spin degrees of freedom,[5, 6] with moderately strong spin-orbit inter-action (SOI), which provides an additional knob for spin control. In general, the spin of electrons and holes in InAs QDs is a fairly good quantum number except in the vicinity of level crossings between states with orthogonal spins,[7] where SOI or hyperfine interaction with the lattice nuclei mix the two states, lifting the degeneracy and forming a spin anticrossing. The importance of spin anticrossings, also referred to as spin hot spots[3, 8, 9], lies in the fact that they lead to fast spin flips. For this reason they have been proposed and used for spin manipulation protocols (see e.g. Refs.10–12). A strong SOI is desirable to ob-tain large spin anticrossings, thereby enabling faster op-erations. In some protocols, large anticrossings are also convenient to enhance the fidelity of the operations.[12] For electrons, spin anticrossings are mainly due to Rashba and Dresselhaus SOI. Takahashi et al. reported gaps of 70−160 µeV between the s and p − orbitals of sin-gle InAs/GaAs QDs.[13] Greilich et al. investigated spin anticrossings between s-shell singlet and triplet states of two electrons in vertically stacked double quantum dots (DQDs), obtaining gaps under 10 µeV .[14] In the same work, it was observed that the corresponding gap for holes was 36 µeV –four times greater–. This is due to the inherent valence band SOI, which is generally stronger than that of the conduction band. Indeed, Doty et al. observed spin anticrossings as large as 200 µeV for holes tunneling in the neutral exciton states of some self-assembled DQD structures.[15] Soon after, the same au-thors reported a gap of 400 µeV on a similar system.[12] This is the largest spin anticrossing observed in the s-shell of InAs QDs so far.