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. V C 2013 AIP Publishing LLC.[http://dx.doi.org/10.1063/1.4823458]There is current interest in using the spin of carriers confined in semiconductor quantum dots (QDs) for single spintronic, optoelectronic, and quantum information research. 1-4 Self-assembled InAs/GaAs QDs have been particularly successful at this regard because they combine high optical activity, which enables precise optical preparation and read-out of the spin degrees of freedom, 5,6 with moderately strong spin-orbit interaction (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 mixes 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 obtain large spin anticrossings, thereby enabling faster operations. 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 leV between the s and p -orbitals of single 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 leV. 14 In the same work, it was observed that the corresponding gap for holes was 36 leV, 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 leV for holes tunneling in the neutral exciton states of some self-assembled DQD structures. 15 Soon after, the same authors reported a gap of 400 leV on a similar system. 12 This is the largest spin anticrossing observed in the s-shell of InAs QDs so far.The origin of these large spin anticrossings was investigated by some of us in Ref. 15. It arises from a combination of valence band SOI plus strong spatial asymmetries. Separately, the QDs can be modeled as circular structures.The SOI then couples the Bloch and envelope angular momenta, giving ri...

Self‐Assembled InAs/GaAs Coupled Quantum Dots for Photonic Quantum Technologies

“…As discussed in Section 8, hole spin mixing originates in the spin–orbit interactions within the valence band and requires symmetry breaking. In a vertically coupled CQD, this symmetry breaking arises naturally from lateral misalignment between the two QDs; the magnitude of the hole spin mixing parameter is proportional to the magnitude of the misalignment . As reported by Doty et al., randomly grown CQDs often have a lateral misalignment between 0 and 4 nm.…”

“…Hole spin mixing arises due to spin–orbit interactions within the valence band and requires symmetry breaking. Detailed descriptions of the origin of hole spin mixing and its magnitude under varying conditions can be found in the literature . The key concepts are that hole spins in QDs must be considered as spinors comprising both heavy and light components with and spin projections, respectively .…”

“…Detailed descriptions of the origin of hole spin mixing and its magnitude under varying conditions can be found in the literature. [193][194][195] The key concepts are that hole spins in QDs must be considered as spinors comprising both heavy and light components with ±3∕2 and ±1∕2 spin projections, respectively. [94] Every hole state contains some contribution from all four of these hole spin projections, but the spinors describing the two ground states of the hole in a QD are dominated by the contributions from the ±3∕2 components.…”

Hole spins in an InAs/GaAs quantum dot molecule subject to lateral electric fields

“…The symmetry of piezoelectric potentials has been found to depend on a variety of parameters. 25,44,45 The rotated symmetry we find occurs when QDs are close together and act as a strongly coupled entity. 44 Under a constant lateral electric field, we observe Stark shifts and Zeeman splitting reduction similar to QDMs without piezoelectric fields with only small shifts in the resonant electric fields at which molecular states are formed.…”

“…22 The symmetry breaking provided by the lateral offset allows light-hole components of the hole spinor to mediate a spin-orbit interaction that creates the hole spin mixing. 22,[24][25][26] Larger offsets (∆x) create stronger spin mixing interactions that are advantageous for all-optical information processing schemes and manifest as larger spin mixing anticrossings (∆ sm ). Although lateral offsets frequently occur in QDMs grown by molecular beam epitaxy, the typical offset distance is relatively small and cannot be controlled by growth techniques.…”